DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.

�

NONRESIDENTTRAININGCOURSE

�

October 1995

Electronics Technician

Volume 7—Antennas and WavePropagation

NAVEDTRA 14092�

DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.

Although the words “he,” “him,” and“his” are used sparingly in this course toenhance communication, they are notintended to be gender driven or to affront ordiscriminate against anyone.

i

PREFACE

By enrolling in this self-study course, you have demonstrated a desire to improve yourself and the Navy.Remember, however, this self-study course is only one part of the total Navy training program. Practicalexperience, schools, selected reading, and your desire to succeed are also necessary to successfully roundout a fully meaningful training program.

COURSE OVERVIEW: In completing this nonresident training course, you should be able to: discusswave propagation in terms of the effects the earth’s atmosphere has on it and the options available to receiveoptimum performance from equipment; identify communications and radar antennas using physicalcharacteristics and installation location, radiation patterns, and power and frequency-handling capabilities.Be familiar with safety precautions for technicians working aloft; and discuss the different types oftransmission lines in terms of physical structure, frequency limitations, electronic fields, and radiationlosses.

THE COURSE: This self-study course is organized into subject matter areas, each containing learningobjectives to help you determine what you should learn along with text and illustrations to help youunderstand the information. The subject matter reflects day-to-day requirements and experiences ofpersonnel in the rating or skill area. It also reflects guidance provided by Enlisted Community Managers(ECMs) and other senior personnel, technical references, instructions, etc., and either the occupational ornaval standards, which are listed in the Manual of Navy Enlisted Manpower Personnel Classificationsand Occupational Standards, NAVPERS 18068.

THE QUESTIONS: The questions that appear in this course are designed to help you understand thematerial in the text.

VALUE: In completing this course, you will improve your military and professional knowledge.Importantly, it can also help you study for the Navy-wide advancement in rate examination. If you arestudying and discover a reference in the text to another publication for further information, look it up.

1995 Edition Prepared byETC Larry D. Simmons

andETC Floyd L. Ace III

Published byNAVAL EDUCATION AND TRAINING

PROFESSIONAL DEVELOPMENTAND TECHNOLOGY CENTER

NAVSUP Logistics Tracking Number0504-LP-026-7580

ii

Sailor’s Creed

“I am a United States Sailor.

I will support and defend theConstitution of the United States ofAmerica and I will obey the ordersof those appointed over me.

I represent the fighting spirit of theNavy and those who have gonebefore me to defend freedom anddemocracy around the world.

I proudly serve my country’s Navycombat team with honor, courageand commitment.

I am committed to excellence andthe fair treatment of all.”

CONTENTS

1. Wave Propagation

CHAPTER Page

1-1

2. Antennas 2-1

3-13. Introduction to Transmission and Waveguides

APPENDIX

I. Glossary AI-1

II. References AII-1

INDEX Index-1

iii

SUMMARY OF THE ELECTRONICSTRAINING SERIES

TECHNICIAN

This series of training manuals was developed to replace the Electronics Technician3 & 2 TRAMAN. The content is directed to personnel working toward advancement toElectronics Technician Second Class.

The nine volumes in the series are based on major topic areas with which the ET2 shouldbe familiar. Volume 1, Safety, provides an introduction to general safety as it relates tothe ET rating. It also provides both general and specific information on electronic tag-outprocedures, man-aloft procedures, hazardous materials (i.e., solvents, batteries, and vacuumtubes), and radiation hazards. Volume 2, Administration, discusses COSAL updates, 3-Mdocumentation, supply paperwork, and other associated administrative topics. Volume 3,Communication Systems, provides a basic introduction to shipboard and shore-basedcommunication systems. Systems covered include man-pat radios (i.e., PRC-104, PSC-3)in the hf, vhf, uhf, SATCOM, and shf ranges. Also provided is an introduction to theCommunications Link Interoperability System (CLIPS). Volume 4, Radar Systems, is abasic introduction to air search, surface search, ground controlled approach, and carriercontrolled approach radar systems. Volume 5, Navigation Systems, is a basic introductionto navigation systems, such as OMEGA, SATNAV, TACAN, and man-pat systems. Volume6, Digital Data Systems, is a basic introduction to digital data systems and includes discussionsabout SNAP II, laptop computers, and desktop computers. Volume 7, Antennas and WavePropagation, is an introduction to wave propagation, as it pertains to Electronics Technicians,and shipboard and shore-based antennas. Volume 8, Support Systems, discusses systeminterfaces, troubleshooting, sub-systems, dry air, cooling, and power systems. Volume 9,Electro-Optics, is an introduction to night vision equipment, lasers, thermal imaging, andfiber optics.

iv

v

INSTRUCTIONS FOR TAKING THE COURSE

ASSIGNMENTS

The text pages that you are to study are listed atthe beginning of each assignment. Study thesepages carefully before attempting to answer thequestions. Pay close attention to tables andillustrations and read the learning objectives.The learning objectives state what you should beable to do after studying the material. Answeringthe questions correctly helps you accomplish theobjectives.

SELECTING YOUR ANSWERS

Read each question carefully, then select theBEST answer. You may refer freely to the text.The answers must be the result of your ownwork and decisions. You are prohibited fromreferring to or copying the answers of others andfrom giving answers to anyone else taking thecourse.

SUBMITTING YOUR ASSIGNMENTS

To have your assignments graded, you must beenrolled in the course with the NonresidentTraining Course Administration Branch at theNaval Education and Training ProfessionalDevelopment and Technology Center(NETPDTC). Following enrollment, there aretwo ways of having your assignments graded:(1) use the Internet to submit your assignmentsas you complete them, or (2) send all theassignments at one time by mail to NETPDTC.

Grading on the Internet: Advantages toInternet grading are:

• you may submit your answers as soon asyou complete an assignment, and

• you get your results faster; usually by thenext working day (approximately 24 hours).

In addition to receiving grade results for eachassignment, you will receive course completionconfirmation once you have completed all the

assignments. To submit your assignmentanswers via the Internet, go to:

http://courses.cnet.navy.mil

Grading by Mail: When you submit answersheets by mail, send all of your assignments atone time. Do NOT submit individual answersheets for grading. Mail all of your assignmentsin an envelope, which you either provideyourself or obtain from your nearest EducationalServices Officer (ESO). Submit answer sheetsto:

COMMANDING OFFICERNETPDTC N3316490 SAUFLEY FIELD ROADPENSACOLA FL 32559-5000

Answer Sheets: All courses include one“scannable” answer sheet for each assignment.These answer sheets are preprinted with yourSSN, name, assignment number, and coursenumber. Explanations for completing the answersheets are on the answer sheet.

Do not use answer sheet reproductions: Useonly the original answer sheets that weprovide—reproductions will not work with ourscanning equipment and cannot be processed.

Follow the instructions for marking youranswers on the answer sheet. Be sure that blocks1, 2, and 3 are filled in correctly. Thisinformation is necessary for your course to beproperly processed and for you to receive creditfor your work.

COMPLETION TIME

Courses must be completed within 12 monthsfrom the date of enrollment. This includes timerequired to resubmit failed assignments.

vi

PASS/FAIL ASSIGNMENT PROCEDURES

If your overall course score is 3.2 or higher, youwill pass the course and will not be required toresubmit assignments. Once your assignmentshave been graded you will receive coursecompletion confirmation.

If you receive less than a 3.2 on any assignmentand your overall course score is below 3.2, youwill be given the opportunity to resubmit failedassignments. You may resubmit failedassignments only once. Internet students willreceive notification when they have failed anassignment--they may then resubmit failedassignments on the web site. Internet studentsmay view and print results for failedassignments from the web site. Students whosubmit by mail will receive a failing result letterand a new answer sheet for resubmission of eachfailed assignment.

COMPLETION CONFIRMATION

After successfully completing this course, youwill receive a letter of completion.

ERRATA

Errata are used to correct minor errors or deleteobsolete information in a course. Errata mayalso be used to provide instructions to thestudent. If a course has an errata, it will beincluded as the first page(s) after the front cover.Errata for all courses can be accessed andviewed/downloaded at:

http://www.advancement.cnet.navy.mil

STUDENT FEEDBACK QUESTIONS

We value your suggestions, questions, andcriticisms on our courses. If you would like tocommunicate with us regarding this course, weencourage you, if possible, to use e-mail. If youwrite or fax, please use a copy of the StudentComment form that follows this page.

For subject matter questions:

E-mail: n315.products@cnet.navy.milPhone: Comm: (850) 452-1001, Ext. 1713

DSN: 922-1001, Ext. 1713FAX: (850) 452-1370(Do not fax answer sheets.)

Address: COMMANDING OFFICERNETPDTC N3156490 SAUFLEY FIELD ROADPENSACOLA FL 32509-5237

For enrollment, shipping, grading, orcompletion letter questions

E-mail: fleetservices@cnet.navy.milPhone: Toll Free: 877-264-8583

Comm: (850) 452-1511/1181/1859DSN: 922-1511/1181/1859FAX: (850) 452-1370(Do not fax answer sheets.)

Address: COMMANDING OFFICERNETPDTC N3316490 SAUFLEY FIELD ROADPENSACOLA FL 32559-5000

NAVAL RESERVE RETIREMENT CREDIT

If you are a member of the Naval Reserve, youmay earn retirement points for successfullycompleting this course, if authorized undercurrent directives governing retirement of NavalReserve personnel. For Naval Reserve retire-ment, this course is evaluated at 5 points. (Referto Administrative Procedures for NavalReservists on Inactive Duty, BUPERSINST1001.39, for more information about retirementpoints.)

vii

Student Comments

Course Title: Electronics Technician, Volume 7—Antennas and Wave Propagation

NAVEDTRA: 14092 Date:

We need some information about you:

Rate/Rank and Name: SSN: Command/Unit

Street Address: City: State/FPO: Zip

Your comments, suggestions, etc.:

Privacy Act Statement: Under authority of Title 5, USC 301, information regarding your military status isrequested in processing your comments and in preparing a reply. This information will not be divulged withoutwritten authorization to anyone other than those within DOD for official use in determining performance.

NETPDTC 1550/41 (Rev 4-00

CHAPTER 1

WAVE PROPAGATION

The eyes and ears of a ship or shore station dependon sophisticated, highly computerized electronicsystems. The one thing all of these systems have incommon is that they lead to and from antennas. Ship’soperators who must communicate, navigate, and beready to fight the ship 24 hours a day depend on youto keep these emitters and sensors operational.

In this volume, we will review wave propagation,antenna characteristics, shore-based and shipboardcommunications antennas, matching networks, antennatuning, radar antennas, antenna safety, transmissionlines, connector installation and weatherproofing,waveguides, and waveguide couplings. When youhave completed this chapter, you should be able todiscuss the basic principles of wave propagation andthe atmosphere’s effects on wave propagation.

THE EARTH’S ATMOSPHERE

While radio waves traveling in free space havelittle outside influence to affect them, radio wavestraveling in the earth’s atmosphere have manyinfluences that affect them. We have all experiencedproblems with radio waves, caused by certainatmospheric conditions complicating what at firstseemed to be a relatively simple electronic problem.These problem-causing conditions result from a lackof uniformity in the earth’s atmosphere.

Many factors can affect atmospheric conditions,either positively or negatively. Three of these arevariations in geographic height, differences ingeographic location, and changes in time (day, night,season, year).

To understand wave propagation, you must haveat least a basic understanding of the earth’s atmosphere.The earth’s atmosphere is divided into three separateregions, or layers. They are the troposphere, thestratosphere, and the ionosphere. These layers areillustrated in figure 1-1.

TROPOSPHERE

Almost all weather phenomena take place in thetroposphere. The temperature in this region decreasesrapidly with altitude. Clouds form, and there may bea lot of turbulence because of variations in thetemperature, pressure, and density. These conditionshave a profound effect on the propagation of radiowaves, as we will explain later in this chapter.

STRATOSPHERE

The stratosphere is located between the troposphereand the ionosphere. The temperature throughout thisregion is almost constant and there is little water vaporpresent. Because it is a relatively calm region withlittle or no temperature change, the stratosphere hasalmost no effect on radio waves.

IONOSPHERE

This is the most important region of the earth’satmosphere for long distance, point-to-point communi-cations. Because the existence of the ionosphere isdirectly related to radiation emitted from the sun, themovement of the earth about the sun or changes inthe sun’s activity will result in variations in theionosphere. These variations are of two general types:(1) those that more or less occur in cycles and,therefore, can be predicted with reasonable accuracy;and (2) those that are irregular as a result of abnormalbehavior of the sun and, therefore, cannot be predicted.Both regular and irregular variations have importanteffects on radio-wave propagation. Since irregularvariations cannot be predicted, we will concentrateon regular variations.

Regular Variations

The regular variations can be divided into fourmain classes: daily, 27-day, seasonal, and 11-year.We will concentrate our discussion on daily variations,

1-1

Figure 1.1—Atmospheric layers.

since they have the greatest effect on your job. Daily of the ultraviolet energy that initially set them freevariations in the ionosphere produce four cloud-likelayers of electrically-charged gas atoms called ions,which enable radio waves to be propagated greatdistances around the earth. Ions are formed by aprocess called ionization.

Ionization

In ionization, high-energy ultraviolet light wavesfrom the sun periodically enter the ionosphere, strikeneutral gas atoms, and knock one or more electronsfree from each atom. When the electrons are knockedfree, the atoms become positively charged (positiveions) and remain in space, along with the negatively-charged free electrons. The free electrons absorb some

and form an ionized layer.

Since the atmosphere is bombarded by ultravioletwaves of differing frequencies, several ionized layersare formed at different altitudes. Ultraviolet wavesof higher frequencies penetrate the most, so theyproduce ionized layers in the lower portion of theionosphere. Conversely, ultraviolet waves of lowerfrequencies penetrate the least, so they form layersin the upper regions of the ionosphere.

An important factor in determining the densityof these ionized layers is the elevation angle of thesun. Since this angle changes frequently, the heightand thickness of the ionized layers vary, depending

1-2

on the time of day and the season of the year.Another important factor in determining layerdensity is known as recombination.

Recombination

Recombination is the reverse process ofionization. It occurs when free electrons and positiveions collide, combine, and return the positive ions totheir original neutral state.

Like ionization, the recombination processdepends on the time of day. Between early morningand late afternoon, the rate of ionization exceeds therate of recombination. During this period the ionizedlayers reach their greatest density and exertmaximum influence on radio waves. However, duringthe late afternoon and early evening, the rate ofrecombination exceeds the rate of ionization, causingthe densities of the ionized layers to decrease.Throughout the night, density continues to decrease,reaching its lowest point just before sunrise. It isimportant to understand that this ionization andrecombination process varies, depending on theionospheric layer and the time of day. The followingparagraphs provide an explanation of the fourionospheric layers.

Ionospheric Layers

The ionosphere is composed of three distinctlayers, designated from lowest level to highest level(D, E, and F) as shown in figure 1-2. In addition, the

F layer is divided into two layers, designated F1 (thelower level) and F2 (the higher level).

The presence or absence of these layers in theionosphere and their height above the earth varywith the position of the sun. At high noon, radiationin the ionosphere above a given point is greatest,while at night it is minimum. When the radiation isremoved, many of the particles that were ionizedrecombine. During the time between these twoconditions, the position and number of ionized layerswithin the ionosphere change.

Since the position of the sun varies daily,monthly, and yearly with respect to a specific pointon earth, the exact number of layers present isextremely difficult to determine. However, thefollowing general statements about these layers canbe made.

D LAYER.— The D layer ranges from about 30to 55 miles above the earth. Ionization in the D layeris low because less ultraviolet light penetrates to thislevel. At very low frequencies, the D layer and theground act as a huge waveguide, making communica-tion possible only with large antennas and high-power transmitters. At low and medium frequencies,the D layer becomes highly absorptive, which limitsthe effective daytime communication range to about200 miles. At frequencies above about 3 MHz, the Dlayer begins to lose its absorptive qualities.

Figure 1-2.—Layers of the ionosphere.

1-3

Long-distance communication is possible atfrequencies as high as 30 MHz. Waves at frequenciesabove this range pass through the D layer but areattenuated. After sunset. the D layer disappearsbecause of the rapid recombination of ions. Low-frequency and medium-frequency long-distancecommunication becomes possible. This is why AMbehaves so differently at night. Signals passingthrough the D layer normally are not absorbed butare propagated by the E and F layers.

E LAYER.— The E layer ranges from approxi-mately 55 to 90 miles above the earth. The rate ofionospheric recombination in this layer is ratherrapid after sunset, causing it to nearly disappear bymidnight. The E layer permits medium-rangecommunications on the low-frequency through very-high-frequency bands. At frequencies above about 150MHz, radio waves pass through the E layer.

Sometimes a solar flare will cause this layer toionize at night over specific areas. Propagation in thislayer during this time is called SPORADIC-E. Therange of communication in sporadic-E often exceeds1000 miles, but the range is not as great as with Flayer propagation.

F LAYER.— The F layer exists from about 90 to240 miles above the earth. During daylight hours, theF layer separates into two layers, F1 and F2. Duringthe night, the F1 layer usually disappears, The Flayer produces maximum ionization during theafternoon hours, but the effects of the daily cycle arenot as pronounced as in the D and E layers. Atoms inthe F layer stay ionized for a longer time after sunset,and during maximum sunspot activity, they can stayionized all night long.

Since the F layer is the highest of theionospheric layers, it also has the longest propagationcapability. For horizontal waves, the single-hop F2distance can reach 3000 miles. For signals topropagate over greater distances, multiple hops arerequired.

The F layer is responsible for most high-frequency, long-distance communications. Themaximum frequency that the F layer will returndepends on the degree of sunspot activity. Duringmaximum sunspot activity, the F layer can return

signals at frequencies as high as 100 MHz. Duringminimum sunspot activity, the maximum usablefrequency can drop to as low as 10 MHz.

ATMOSPHERIC PROPAGATION

Within the atmosphere, radio waves can berefracted, reflected, and diffracted. In the followingparagraphs, we will discuss these propagationcharacteristics.

REFRACTION

A radio wave transmitted into ionized layers isalways refracted, or bent. This bending of radiowaves is called refraction. Notice the radio waveshown in figure 1-3, traveling through the earth’satmosphere at a constant speed. As the wave entersthe denser layer of charged ions, its upper portionmoves faster than its lower portion. The abrupt speedincrease of the upper part of the wave causes it tobend back toward the earth. This bending is alwaystoward the propagation medium where the radiowave’s velocity is the least.

Figure 1-3.—Radio-wave refraction.

The amount of refraction a radio wave undergoesdepends on three main factors.

1. The ionization density of the layer

2. The frequency of the radio wave

3. The angle at which the radio wave enters thelayer

1-4

Figure 1-4.—Effects of ionospheric density on radio waves.

Layer Density

Figure 1-4 shows the relationship betweenradio waves and ionization density. Each ionizedlayer has a middle region of relatively denseionization with less intensity above and below. Asa radio wave enters a region of increasingionization, a velocity increase causes it to bendback toward the earth. In the highly densemiddle region, refraction occurs more slowlybecause the ionization density is uniform. As thewave enters the upper less dense region, thevelocity of the upper part of the wave decreasesand the wave is bent away from the earth.

Frequency

The lower the frequency of a radio wave, themore rapidly the wave is refracted by a givendegree of ionization. Figure 1-5 shows threeseparate waves of differing frequencies enteringthe ionosphere at the same angle. You can see thatthe 5-MHz wave is refracted quite sharply, whilethe 20-MHz wave is refracted less sharply andreturns to earth at a greater distance than the 5-MHz wave. Notice that the 100-MHz wave is lost

into space. For any given ionized layer, there is afrequency, called the escape point, at which energytransmitted directly upward will escape intospace. The maximum frequency just below theescape point is called the critical frequency. Inthis example, the 100-MHz wave’s frequency isgreater than the critical frequency for that ionizedlayer.

Figure 1-5.—Frequency versus refractionand distance.

The critical frequency of a layer depends uponthe layer’s density. If a wave passes through a

1-5

particular layer, it may still be refracted by ahigher layer if its frequency is lower than thehigher layer’s critical frequency.

Angle of Incidence and Critical Angle

When a radio wave encounters a layer of theionosphere, that wave is returned to earth at thesame angle (roughly) as its angle of incidence.Figure 1-6 shows three radio waves of the samefrequency entering a layer at different incidenceangles. The angle at which wave A strikes thelayer is too nearly vertical for the wave to berefracted to earth, However, wave B is refractedback to earth. The angle between wave B and theearth is called the critical angle. Any wave, at agiven frequency, that leaves the antenna at anincidence angle greater than the critical angle willbe lost into space. This is why wave A was notrefracted. Wave C leaves the antenna at thesmallest angle that will allow it to be refracted andstill return to earth. The critical angle for radiowaves depends on the layer density and thewavelength of the signal.

Figure 1-6.—Incidence angles of radio waves.

As the frequency of a radio wave is increased,the critical angle must be reduced for refraction tooccur. Notice in figure 1-7 that the 2-MHz wavestrikes the ionosphere at the critical angle for thatfrequency and is refracted. Although the 5-MHzline (broken line) strikes the ionosphere at a lesscritical angle, it still penetrates the layer and islost As the angle is lowered, a critical angle isfinally reached for the 5-MHz wave and it isrefracted back to earth.

Figure 1-7.—Effect of frequency on the critical angle.

1-6

SKIP DISTANCE AND ZONE

Recall from your previous study that atransmitted radio wave separates into two parts,the sky wave and the ground wave. With thosetwo components in mind, we will now brieflydiscuss skip distance and skip zone.

Skip Distance

Look at the relationship between the sky waveskip distance, skip zone, and ground wavecoverage shown in figure 1-8. The skip distance isthe distance from the transmitter to the pointwhere the sky wave first returns to the earth. Theskip distance depends on the wave’s frequency andangle of incidence, and the degree of ionization.

Figure 1-8.—Relationship between skipzone, skip distance, and ground wave.

Skip Zone

The skip zone is a zone of silence between thepoint where the ground wave is too weak forreception and the point where the sky wave is firstreturned to earth. The outer limit of the skip zonevaries considerably, depending on the operatingfrequency, the time of day, the season of the year,sunspot activity, and the direction of transmission.

At very-low, low, and medium frequencies, askip zone is never present. However, in the high-frequency spectrum, a skip zone is often present.As the operating frequency is increased, the skipzone widens to a point where the outer limit of theskip zone might be several thousand miles away.At frequencies above a certain maximum, theouter limit of the skip zone disappears completely,and no F-layer propagation is possible.

Occasionally, the first sky wave will return toearth within the range of the ground wave. In thiscase, severe fading can result from the phasedifference between the two waves (the sky wavehas a longer path to follow).

REFLECTION

Reflection occurs when radio waves are“bounced” from a flat surface. There are basicallytwo types of reflection that occur in theatmosphere: earth reflection and ionosphericreflection. Figure 1-9 shows two

Figure 1-9.—Phase shift of reflected radio waves.

1-7

waves reflected from the earth’s surface. Waves Aand B bounce off the earth’s surface like light off ofa mirror. Notice that the positive and negativealternations of radio waves A and B are in phase beforethey strike the earth’s surface. However, afterreflection the radio waves are approximately 180degrees out of phase. A phase shift has occurred.

The amount of phase shift that occurs is notconstant. It varies, depending on the wave polarizationand the angle at which the wave strikes the surface.Because reflection is not constant, fading occurs.Normally, radio waves reflected in phase producestronger signals, while those reflected out of phaseproduce a weak or fading signal.

Ionospheric reflection occurs when certain radiowaves strike a thin, highly ionized layer in theionosphere. Although the radio waves are actuallyrefracted, some may be bent back so rapidly that theyappear to be reflected. For ionospheric reflection tooccur, the highly ionized layer can be approximatelyno thicker than one wavelength of the wave. Sincethe ionized layers are often several miles thick,ionospheric reflection mostly occurs at long wave-lengths (low frequencies).

DIFFRACTION

Diffraction is the ability of radio waves to turnsharp corners and bend around obstacles. Shown infigure 1-10, diffraction results in a change of directionof part of the radio-wave energy around the edges ofan obstacle. Radio waves with long wavelengthscompared to the diameter of an obstruction are easilypropagated around the obstruction. However, as thewavelength decreases, the obstruction causes moreand more attenuation, until at very-high frequenciesa definite shadow zone develops. The shadow zoneis basically a blank area on the opposite side of anobstruction in line-of-sight from the transmitter to thereceiver.

Diffraction can extend the radio range beyond thehorizon. By using high power and low-frequencies,radio waves can be made to encircle the earth bydiffraction.

Figure 1-10.—Diffraction around an object.

ATMOSPHERIC EFFECTSON PROPAGATION

As we stated earlier, changes in the ionospherecan produce dramatic changes in the ability tocommunicate. In some cases, communicationsdistances are greatly extended. In other cases,communications distances are greatly reduced oreliminated. The paragraphs below explain the majorproblem of reduced communications because of thephenomena of fading and selective fading.

Fading

The most troublesome and frustrating problem inreceiving radio signals is variations in signal strength,most commonly known as FADING. Severalconditions can produce fading. When a radio waveis refracted by the ionosphere or reflected from theearth’s surface, random changes in the polarizationof the wave may occur. Vertically and horizontallymounted receiving antennas are designed to receivevertically and horizontally polarized waves, respec-tively. Therefore, changes in polarization causechanges in the received signal level because of theinability of the antenna to receive polarization changes.

Fading also results from absorption of the rf energyin the ionosphere. Most ionospheric absorption occursin the lower regions of the ionosphere where ionization

1-8

density is the greatest. As a radio wave passes intothe ionosphere, it loses some of its energy to the freeelectrons and ions present there. Since the amount ofabsorption of the radio-wave energy varies with thedensity of the ionospheric layers, there is no fixedrelationship between distance and signal strength forionospheric propagation. Absorption fading occurs fora longer period than other types of fading, sinceabsorption takes place slowly. Under certainconditions, the absorption of energy is so great thatcommunication over any distance beyond the line ofsight becomes difficult.

Although fading because of absorption is themost serious type of fading, fading on the ionosphericcircuits is mainly a result of multipath propagation.

Multipath Fading

MULTIPATH is simply a term used to describethe multiple paths a radio wave may follow betweentransmitter and receiver. Such propagation pathsinclude the ground wave, ionospheric refraction,reradiation by the ionospheric layers, reflection fromthe earth’s surface or from more than one ionosphericlayer, and so on. Figure 1-11 shows a few of the pathsthat a signal can travel between two sites in a typicalcircuit. One path, XYZ, is the basic ground wave.Another path, XFZ, refracts the wave at the F layerand passes it on to the receiver at point Z. At point Z,the received signal is a combination of the groundwave and the sky wave. These two signals, havingtraveled different paths, arrive at point Z at differenttimes. Thus, the arriving waves may or may not be inphase with each other. A similar situation may resultat point A. Another path, XFZFA, results from agreater angle of incidence and two refractions fromthe F layer. A wave traveling that path and onetraveling the XEA path may or may not arrive atpoint A in phase. Radio waves that are received inphase reinforce each other and produce a strongersignal at the receiving site, while those that arereceived out of phase produce a weak or fadingsignal. Small alterations in the transmission pathmay change the phase relationship of the two signals,causing periodic fading.

Figure 1-11.—Multipath transmission.

Multipath fading may be minimized by practicescalled SPACE DIVERSITY and FREQUENCYDIVERSITY In space diversity, two or more receivingantennas are spaced some distance apart. Fadingdoes not occur simultaneously at both antennas.Therefore, enough output is almost always availablefrom one of the antennas to provide a useful signal.

In frequency diversity, two transmitters and tworeceivers are used, each pair tuned to a differentfrequency, with the same information beingtransmitted simultaneously over both frequencies.One of the two receivers will almost always produce auseful signal.

Selective Fading

Fading resulting from multipath propagationvaries with frequency since each frequency arrives atthereceiving point via a different radio path. When awide band of frequencies is transmittedsimultaneously,each frequency will vary in the amount of fading.This variation is called SELECTIVE FADING. Whenselective fading occurs, all frequencies of thetransmitted signal do not retain their original phasesand relative amplitudes. This fading causes severedistortion of the signal and limits the total signaltransmitted.

Frequency shifts and distance changes becauseof daily variations of the different ionospheric layersare summarized in table 1-1.

1-9

Table 1-1.–Daily Ionospheric Communications

D LAYER: reflects vlf waves for long-rangecommunications; refracts lf and mf forshort-range communications; has littleeffect on vhf and above; gone at night.

E LAYER: depends on the angle of the sun:refracts hf waves during the day up to 20MHz to distances of 1200 miles: greatlyreduced at night.

F LAYER: structure and density depend onthe time of day and the angle of the sun:consists of one layer at night and splitsinto two layers during daylight hours.

F1 LAYER: density depends on the angle ofthe sun; its main effect is to absorb hfwaves passing through to the F2 layer.

F2 LAYER: provides long-range hf communica-tions; very variable; height and densitychange with time of day, season, and sun-spot activity.

Figure 1-12.—Ionosphericlayers.

OTHER PHENOMENA THAT AFFECT of these layers is greatest during the summer. TheCOMMUNICATIONS F2 layer is just the opposite. Its ionization is greatest

during the winter, Therefore, operating frequenciesAlthough daily changes in the ionosphere have for F2 layer propagation are higher in the winter than

the greatest effect on communications, other phenom-ena also affect communications, both positively andnegatively. Those phenomena are discussed brieflyin the following paragraphs.

SEASONAL VARIATIONS IN THEIONOSPHERE

Seasonal variations are the result of the earth’srevolving around the sun, because the relative positionof the sun moves from one hemisphere to the otherwith the changes in seasons. Seasonal variations ofthe D, E, and F1 layers are directly related to thehighest angle of the sun, meaning the ionization density

in the summer.

SUNSPOTS

One of the most notable occurrences on the surfaceof the sun is the appearance and disappearance of dark,irregularly shaped areas known as SUNSPOTS.Sunspots are believed to be caused by violent eruptionson the sun and are characterized by strong magneticfields. These sunspots cause variations in theionization level of the ionosphere.

Sunspots tend to appear in two cycles, every 27days and every 11 years.

1-10

Twenty-Seven Day Cycle

The number of sunspots present at any one timeis constantly changing as some disappear and new onesemerge. As the sun rotates on its own axis, thesesunspots are visible at 27-day intervals, which is theapproximate period for the sun to make one completerevolution. During this time period, the fluctuationsin ionization are greatest in the F2 layer. For thisreason, calculating critical frequencies for long-distancecommunications for the F2 layer is not possible andallowances for fluctuations must be made.

Eleven-Year Cycle

Sunspots can occur unexpectedly, and the life spanof individual sunspots is variable. TheELEVEN-YEAR SUN SPOT CYCLE is a regularcycle of sunspot activity that has a minimum andmaximum level of activity that occurs every 11 years.During periods of maximum activity, the ionizationdensity of all the layers increases. Because of this,the absorption in the D layer increases and the criticalfrequencies for the E, F1, and F2 layers are higher.During these times, higher operating frequencies mustbe used for long-range communications.

IRREGULAR VARIATIONS

Irregular variations are just that, unpredictablechanges in the ionosphere that can drastically affectour ability to communicate. The more commonvariations are sporadic E, ionospheric disturbances,and ionospheric storms.

Sporadic E

Irregular cloud-like patches of unusually highionization, called the sporadic E, often format heightsnear the normal E layer. Their exact cause is notknown and their occurrence cannot be predicted.However, sporadic E is known to vary significantlywith latitude. In the northern latitudes, it appears tobe closely related to the aurora borealis or northernlights.

The sporadic E layer can be so thin that radiowaves penetrate it easily and are returned to earth bythe upper layers, or it can be heavily ionized and

extend up to several hundred miles into the ionosphere.This condition may be either harmful or helpful toradio-wave propagation.

On the harmful side, sporadic E may blank outthe use of higher more favorable layers or causeadditional absorption of radio waves at some frequen-cies. It can also cause additional multipath problemsand delay the arrival times of the rays of RF energy.

On the helpful side, the critical frequency of thesporadic E can be greater than double the criticalfrequency of the normal ionospheric layers. This maypermit long-distance communications with unusuallyhigh frequencies. It may also permit short-distancecommunications to locations that would normally bein the skip zone.

Sporadic E can appear and disappear in a shorttime during the day or night and usually does not occurat same time for all transmitting or receiving stations.

Sudden Ionospheric Disturbances

Commonly known as SID, these disturbances mayoccur without warning and may last for a few minutesto several hours. When SID occurs, long-range hfcommunications are almost totally blanked out. Theradio operator listening during this time will believehis or her receiver has gone dead.

The occurrence of SID is caused by a bright solareruption producing an unusually intense burst ofultraviolet light that is not absorbed by the F1, F2,or E layers. Instead, it causes the D-layer ionizationdensity to greatly increase. As a result, frequenciesabove 1 or 2 megahertz are unable to penetrate theD layer and are completely absorbed.

Ionospheric Storms

Ionospheric storms are caused by disturbances inthe earth’s magnetic field. They are associated withboth solar eruptions and the 27-day cycle, meaningthey are related to the rotation of the sun. The effectsof ionospheric storms are a turbulent ionosphere andvery erratic sky-wave propagation. The storms affectmostly the F2 layer, reducing its ion density andcausing the critical frequencies to be lower than

1-11

normal. What this means for communication purposesis that the range of frequencies on a given circuit issmaller than normal and that communications arepossible only at lower working frequencies.

Weather

Wind, air temperature, and water content of theatmosphere can combine either to extend radiocommunications or to greatly attenuate wave propaga-tion. making normal communications extremelydifficult. Precipitation in the atmosphere has itsgreatest effect on the higher frequency ranges.Frequencies in the hf range and below show little effectfrom this condition.

RAIN.— Attenuation because of raindrops is greaterthan attenuation for any other form of precipitation.Raindrop attenuation may be caused either byabsorption, where the raindrop acts as a poor dielectric,absorbs power from the radio wave and dissipates thepower by heat loss; or by scattering (fig. 1-13).Raindrops cause greater attenuation by scattering thanby absorption at frequencies above 100 megahertz.At frequencies above 6 gigahertz, attenuation byraindrop scatter is even greater.

Figure 1-13.–Rf energy losses fromscattering.

FOG.— Since fog remains suspended in theatmosphere, the attenuation is determined by thequantity of water per unit volume (density of the fog)and by the size of the droplets. Attenuation becauseof fog has little effect on frequencies lower than 2gigahertz, but can cause serious attenuation byabsorption at frequencies above 2 gigahertz.

SNOW.— Since snow has about 1/8 the densityof rain, and because of the irregular shape of the

snowflake, the scattering and absorption losses aredifficult to compute, but will be less than those causedby raindrops.

HAIL.— Attenuation by hail is determined by thesize of the stones and their density. Attenuation ofradio waves by scattering because of hailstones isconsiderably less than by rain.

TEMPERATURE INVERSION

When layers of warm air form above layers ofcold air, the condition known as temperature inversiondevelops. This phenomenon causes ducts or channelsto be formed, by sandwiching cool air either betweenthe surface of the earth and a layer of warm air, orbetween two layers of warm air. If a transmittingantenna extends into such a duct, or if the radio waveenters the duct at a very low angle of incidence, vhfand uhf transmissions may be propagated far beyondnormal line-of-sight distances. These long distancesare possible because of the different densities andrefractive qualities of warm and cool air. The suddenchange in densities when a radio wave enters the warmair above the duct causes the wave to be refracted backtoward earth. When the wave strikes the earth or awarm layer below the duct, it is again reflected orrefracted upward and proceeds on through the ductwith a multiple-hop type of action. An example ofradio-wave propagation by ducting is shown in figure1-14.

Figure 1-14.—Duct effect caused by temperatureinversion.

TRANSMISSION LOSSES

All radio waves propagated over the ionosphereundergo energy losses before arriving at the receivingsite. As we discussed earlier, absorption and lower

1-12

atmospheric levels in the ionosphere account for alarge part of these energy losses. There are two othertypes of losses that also significantly affectpropagation. These losses are known as groundreflection losses and freespace loss. The combinedeffect of absorption ground reflection loss, andfreespace loss account for most of the losses of radiotransmissions propagated in the ionosphere.

GROUND REFLECTION LOSS

When propagation is accomplished via multihoprefraction, rf energy is lost each time the radio waveis reflected from the earth’s surface. The amount ofenergy lost depends on the frequency of the wave, theangle of incidence, ground irregularities, and theelectrical conductivity of the point of reflection.

FREESPACE LOSS

Normally, the major loss of energy is because ofthe spreading out of the wavefront as it travels fromthe transmitter. As distance increases, the area of thewavefront spreads out, much like the beam of aflashlight. This means the amount of energycontained within any unit of area on the wavefrontdecreases as distance increases. By the time theenergy arrives at the receiving antenna, thewavefront is so spread out that the receiving antennaextends into only a small portion of the wavefront.This is illustrated in figure 1-15.

FREQUENCY SELECTION

You must have a thorough knowledge of radio-wave propagation to exercise good judgment whenselecting transmitting and receiving antennas andoperating frequencies. Selecting a usable operatingfrequency within your given allocations andavailability is of prime importance to maintainingreliable communications.

For successful communication between any twospecified locations at any given time of the day, thereis a maximum frequency, a lowest frequency and anoptimum frequency that can be used.

Figure 1-15.—Freespace loss principle.

MAXIMUM USABLE FREQUENCY

The higher the frequency of a radio wave, thelower the rate of refraction by the ionosphere.Therefore, for a given angle of incidence and time ofday, there is a maximum frequency that can be usedfor communications between two given locations. Thisfrequency is known as the MAXIMUM USABLEFREQUENCY (muf).

Waves at frequencies above the muf arenormally refracted so slowly that they return to earthbeyond the desired location or pass on through theionosphere and are lost. Variations in the ionospherethat can raise or lower a predetermined muf mayoccur at anytime. his is especially true for the highlyvariable F2 layer.

LOWEST USABLE FREQUENCY

Just as there is a muf that can be used forcommunications between two points, there is also aminimum operating frequency that can be usedknown as the LOWEST USABLE FREQUENCY (luf).As the frequency of a radio wave is lowered, the rateof refraction increases. So a wave whose frequency isbelow the established luf is refracted back to earth ata shorter distance than desired, as shown in figure 1-16.

1-13

Figure 1-16.—Refraction of frequencies belowthe lowest usable frequency (luf).

As a frequency is lowered, absorption of the radiowave increases. A wave whose frequency is too low isabsorbed to such an extent that it is too weak forreception. Atmospheric noise is also greater at lowerfrequencies. A combination of higher absorption andatmospheric noise could result in an unacceptablesignal-to-noise ratio.

For a given angle ionospheric conditions, ofincidence and set of the luf depends on the refraction

properties of the ionosphere, absorptionconsiderations, and the amount of noise present.

OPTIMUM WORKING FREQUENCY

The most practical operating frequency is onethat you can rely onto have the least number ofproblems. It should be high enough to avoid theproblems of multipath fading, absorption, and noiseencountered at the lower frequencies; but not so highas to be affected by the adverse effects of rapidchanges in the ionosphere.

A frequency that meets the above criteria isknown as the OPTIMUM WORKING FREQUENCYIt is abbreviated “fot” from the initial letters of theFrench words for optimum working frequency,“frequence optimum de travail.” The fot is roughlyabout 85% of the muf, but the actual percentagevaries and may be considerably more or less than 85percent.

In this chapter, we discussed the basics of radio-wave propagation and how atmospheric conditionsdetermine the operating parameters needed to ensuresuccessful communications. In chapter 2, we willdiscuss basic antenna operation and design tocomplete your understanding of radio-wavepropagation.

1-14

CHAPTER 2

ANTENNAS

As an Electronics Technician, you are responsiblefor maintaining systems that both radiate and receiveelectromagnetic energy. Each of these systems requiressome type of antenna to make use of this electromag-netic energy. In this chapter we will discuss antennacharacteristics, different antenna types, antenna tuning,and antenna safety.

ANTENNA CHARACTERISTICS

An antenna may be defined as a conductor or groupof conductors used either for radiating electromagneticenergy into space or for collecting it from space.Electrical energy from the transmitter is convertedinto electromagnetic energy by the antenna and radiatedinto space. On the receiving end, electromagneticenergy is converted into electrical energy by theantenna and fed into the receiver.

The electromagnetic radiation from an antennais made up of two components, the E field and theH field. The total energy in the radiated wave remainsconstant in space except for some absorption of energyby the earth. However, as the wave advances, theenergy spreads out over a greater area. This causesthe amount of energy in a given area to decrease asdistance from the source increases.

The design of the antenna system is very importantin a transmitting station. The antenna must be ableto radiate efficiently so the power supplied by thetransmitter is not wasted. An efficient transmittingantenna must have exact dimensions, determined bythe frequency being transmitted. The dimensions ofthe receiving antenna are not critical for relatively lowfrequencies, but their importance increases drasticallyas the transmitted frequency increases.

Most practical transmitting antennas are dividedinto two basic classifications, HERTZ ANTENNAS(half-wave) and MARCONI (quarter-wave) ANTEN-NAS. Hertz antennas are generally installed somedistance above the ground and are positioned to radiate

either vertically or horizontally. Marconi antennasoperate with one end grounded and are mountedperpendicular to the earth or a surface acting as aground. The Hertz antenna, also referred to as adipole, is the basis for some of the more complexantenna systems used today. Hertz antennas aregenerally used for operating frequencies of 2 MHzand above, while Marconi antennas are used foroperating frequencies below 2 MHz.

All antennas, regardless of their shape or size, havefour basic characteristics: reciprocity, directivity, gain,and polarization.

RECIPROCITY

RECIPROCITY is the ability to use the sameantenna for both transmitting and receiving. Theelectrical characteristics of an antenna apply equally,regardless of whether you use the antenna fortransmitting or receiving. The more efficient anantenna is for transmitting a certain frequency, themore efficient it will be as a receiving antenna forthe same frequency. This is illustrated by figure 2-1,view A. When the antenna is used for transmitting,maximum radiation occurs at right angles to its axis.When the same antenna is used for receiving (viewB), its best reception is along the same path; that is,at right angles to the axis of the antenna.

DIRECTIVITY

The DIRECTIVITY of an antenna or array is ameasure of the antenna’s ability to focus the energyin one or more specific directions. You can determinean antenna’s directivity by looking at its radiationpattern. In an array propagating a given amount ofenergy, more radiation takes place in certain directionsthan in others. The elements in the array can bearranged so they change the pattern and distribute theenergy more evenly in all directions. The oppositeis also possible. The elements can be arranged so theradiated energy is focused in one direction. The

2-1

Figure 2-1.—Reciprocity of antennas.

elements can be consideredfed from a common source.

GAIN

as a group of antennas

As we mentioned earlier, some antennas are highlydirectional. That is, they propagate more energy incertain directions than in others. The ratio betweenthe amount of energy propagated in these directionsand the energy that would be propagated if the antennawere not directional is known as antenna GAIN. Thegain of an antenna is constant. whether the antennais used for transmitting or receiving.

POLARIZATION

Energy from an antenna is radiated in the formof an expanding sphere. A small section of this sphereis called a wavefront. positioned perpendicular to thedirection of the radiation field (fig. 2-2). Within thiswavefront. all energy is in phase. Usually, all pointson the wavefront are an equal distance from theantenna. The farther from the antenna the wave is,the less curved it appears. At a considerable distance,the wavefront can be considered as a plane surfaceat right angles to the direction of propagation.

Figure 2-2.—Horizontal and vertical polarization.

The radiation field is made up of magnetic andelectric lines of force that are always at right anglesto each other. Most electromagnetic fields in spaceare said to be linearly polarized. The direction ofpolarization is the direction of the electric vector. Thatis, if the electric lines of force (E lines) are horizontal,the wave is said to be horizontally polarized (fig. 2-2),and if the E lines are vertical, the wave is said to bevertically polarized. Since the electric field is parallelto the axis of the dipole, the antenna is in the planeof polarization.

A horizontally placed antenna produces a horizon-tally polarized wave, and a vertically placed antennaproduces a vertically polarized wave.

In general, the polarization of a wave does notchange over short distances. Therefore, transmittingand receiving antennas are oriented alike, especiallyif they are separated by short distances.

Over long distances, polarization changes. Thechange is usually small at low frequencies, but quitedrastic at high frequencies. (For radar transmissions,a received signal is actually a wave reflected froman object. Since signal polarization varies with thetype of object, no set position of the receiving antennais correct for all returning signals). Where separateantennas are used for transmitting and receiving, thereceiving antenna is generally polarized in the samedirection as the transmitting antenna.

2-2

When the transmitting antenna is close to theground, it should be polarized vertically, becausevertically polarized waves produce a greater signalstrength along the earth’s surface. On the other hand,when the transmitting antenna is high above theground, it should be horizontally polarized to get thegreatest signal strength possible to the earth’s surface.

RADIATION OF ELECTROMAGNETICENERGY

Various factors in the antenna circuit affect theradiation of electromagnetic energy. In figure 2-3,for example, if an alternating current is applied to theA end of wire antenna AB, the wave will travel alongthe wire until it reaches the B end. Since the B endis free, an open circuit exists and the wave cannottravel further. This is a point of high impedance.The wave bounces back (reflects) from this point ofhigh impedance and travels toward the starting point,where it is again reflected. Theoretically, the energyof the wave should be gradually dissipated by theresistance of the wire during this back-and-forth motion(oscillation). However, each time the wave reachesthe starting point, it is reinforced by an impulse ofenergy sufficient to replace the energy lost during itstravel along the wire. This results in continuousoscillations of energy along the wire and a high voltageat the A end of the wire. These oscillations movealong the antenna at a rate equal to the frequency ofthe rf voltage and are sustained by properly timedimpulses at point A.

Figure 2-3.—Antenna and rf source.

The rate at which the wave travels along the wireis constant at approximately 300,000,000 meters persecond. The length of the antenna must be such thata wave will travel from one end to the other and backagain during the period of 1 cycle of the rf voltage.

The distance the wave travels during the period of1 cycle is known as the wavelength. It is found bydividing the rate of travel by the frequency.

Look at the current and voltage distribution onthe antenna in figure 2-4. A maximum movementof electrons is in the center of the antenna at all times;therefore, the center of the antenna is at a lowimpedance.

Figure 2-4.—Standing waves of current and voltage onan antenna.

This condition is called a STANDING WAVE ofcurrent. The points of high current and high voltageare known as current and voltage LOOPS. The pointsof minimum current and minimum voltage are knownas current and voltage NODES. View A shows acurrent loop and two current nodes. View B showstwo voltage loops and a voltage node. View C shows

2-3

the resultant voltage and current loops and nodes.The presence of standing waves describes the conditionof resonance in an antenna. At resonance, the wavestravel back and forth in the antenna, reinforcing eachother, and are transmitted into space at maximumradiation. When the antenna is not at resonance, thewaves tend to cancel each other and energy is lostin the form of heat.

RADIATION TYPES AND PATTERNS

A logical assumption is that energy leaving anantenna radiates equally over 360 degrees. This isnot the case for every antenna.

The energy radiated from an antenna forms a fieldhaving a definite RADIATION PATTERN. Theradiation pattern for any given antenna is determinedby measuring the radiated energy at various anglesat constant distances from the antenna and then plottingthe energy values on a graph. The shape of this patterndepends on the type of antenna being used.

Some antennas radiate energy equally in alldirections. Radiation of this type is known asISOTROPIC RADIATION. The sun is a goodexample of an isotropic radiator. If you were tomeasure the amount of radiated energy around thesun’s circumference, the readings would all be fairlyequal (fig. 2-5).

Most radiators emit (radiate) energy more stronglyin one direction than in another. These radiators arereferred to as ANISOTROPIC radiators. A flashlightis a good example of an anisotropic radiator (fig. 2-6).The beam of the flashlight lights only a portion ofthe space surrounding it. The area behind the flashlightremains unlit, while the area in front and to either sideis illuminated.

MAJOR AND MINOR LOBES

The pattern shown in figure 2-7, view B, hasradiation concentrated in two lobes. The radiationintensity in one lobe is considerably stronger than inthe other. The lobe toward point X is called a MAJORLOBE; the other is a MINOR LOBE. Since thecomplex radiation patterns associated with antennasfrequently contain several lobes of varying intensity,

Figure 2-5.—Isotropic radiation graphs.

you should learn to use the appropriate terminology,In general, major lobes are those in which the greatestamount of radiation occurs. Minor lobes are thosein which the least amount of radiation occurs.

ANTENNA LOADING

There will be times when you may want to useone antenna system to transmit on several differentfrequencies. Since the antenna must always be inresonance with the applied frequency, you must eitherlengthen it or shorten it to produce the required

2-4

Figure 2-6.—Anisotropic radiator.

resonance. Changing the antenna dimensionsphysically is impractical, but changing them electricallyis relatively simple. To change the electrical lengthof an antenna, you can insert either an inductor or acapacitor in series with the antenna. This is shownin figure 2-8, views A and B. Changing the electricallength by this method is known asLUMPED-IMPEDANCE TUNING or LOADING.If the antenna is too short for the wavelength beingused, it will be resonant at a higher frequency.Therefore, it offers a capacitive reactance at theexcitation frequency. This capacitive reactance canbe compensated for by introducing a lumped inductivereactance, as shown in view A. Similarly, if the

Figure 2-7.—Major and minor lobes.

antenna is too long for the transmitting frequency, itwill be resonant at a lower frequency and offers aninductive reactance. Inductive reactance can becompensated for by introducing a lumped capacitivereactance, as shown in view B. An antenna withnormal loading is represented in view C.

Figure 2-8.—Electrical antenna loading.

GROUND EFFECTS

As we discussed earlier, ground losses affectradiation patterns and cause high signal losses for somefrequencies. Such losses can be greatly reduced ifa good conducting ground is provided in the vicinityof the antenna. This is the purpose of the GROUNDSCREEN (fig. 2-9, view A) and COUNTERPOISE(fig. 2-9, view B).

2-5

COMMUNICATIONS ANTENNAS

Figure 2-9.—Ground screen andcounterpoise.

The ground screen in view A is composed of aseries of conductors arranged in a radial pattern andburied 1 or 2 feet below the surface of the earth.These conductors, each usually 1/2 wavelength long,reduce ground absorption losses in the vicinity of theantenna.

A counterpoise (view B) is used when easy accessto the base of the antenna is necessary. It is also usedwhen the area below the antenna is not a goodconducting surface, such as solid rock or ground thatis sandy. The counterpoise serves the same purposeas the ground screen but is usually elevated above theearth. No specific dimensions are necessary for acounterpoise, nor is the number of wires particularlycritical. The primary requirement is that the counter-poise be insulated from ground and form a grid ofreflector elements for the antenna system.

Some antennas can be used in both shore-basedand ship-based applications. Others, however, aredesigned to be used primarily in one application orthe other. The following paragraphs discuss, byfrequency range, antennas used for shore-basedcommunications.

VERY LOW FREQUENCY (VLF)

The main difficulty in vlf and lf antenna designis the physical disparity between the maximumpractical size of the antenna and the wavelength ofthe frequency it must propagate. These antennas mustbe large to compensate for wavelength and powerhandling requirements (0.25 to 2 MW), Transmittingantennas for vlf have multiple towers 600 to 1500feet high, an extensive flat top for capacitive load-ing, and a copper ground system for reducing groundlosses. Capacitive top-loading increases the bandwidthcharacteristics, while the ground plane improvesradiation efficiency.

Representative antenna configurations are shownin figures 2-10 through 2-12. Variations of these basicantennas are used at the majority of the Navy vlf sites.

Figure 2-10.—Triatic-type antenna.

2-6

Figure 2-12.—Trideco-type antenna.

Figure 2-11.—Goliath-type antenna.HIGH FREQUENCY (HF)

LOW FREQUENCY (LF)

Antennas for lf are not quite as large as antennasfor vlf, but they still occupy a large surface area. Twoexamples of If antenna design are shown in figures2-13 and 2-14. The Pan polar antenna (fig. 2-1 3) isan umbrella top-loaded monopole. It has three loadingloops spaced 120 degrees apart, interconnected betweenthe tower guy cables. Two of the loops terminate atground, while the other is used as a feed. The NORDantenna (fig. 2-14), based on the the folded-unipoleprinciple, is a vertical tower radiator grounded at thebase and fed by one or more wires connected to thetop of the tower. The three top loading wires extendfrom the top of the antenna at 120-degree intervalsto three terminating towers. Each loading wire hasa length approximately equal to the height of the maintower plus 100 feet. The top loading wires areinsulated from ground and their tower supports areone-third the height of the transmitting antenna.

High-frequency (hf) radio antenna systems are usedto support many different types of circuits, includingship-to-shore, point-to-point, and ground-to-airbroadcast. These diverse applications require the useof various numbers and types of antennas that we willreview on the following pages.

Yagi

The Yagi antenna is an end-fired parasitic array.It is constructed of parallel and coplaner dipoleelements arranged along a line perpendicular to theaxis of the dipoles, as illustrated in figure 2-15. Themost limiting characteristic of the Yagi antenna is itsextremely narrow bandwidth. Three percent of thecenter frequency is considered to be an acceptablebandwidth ratio for a Yagi antenna. The width ofthe array is determined by the lengths of the elements.The length of each element is approximately one-half

2-7

Figure 2-13.—Pan polar antenna.

wavelength, depending on its intended use (driver,reflector, or director). The required length of the arraydepends on the desired gain and directivity. Typically,the length will vary from 0.3 wavelength forthree-element arrays, to 3 wavelengths for arrays withnumerous elements. For hf applications, the maximumpractical array length is 2 wavelengths. The array’sheight above ground will determine its verticalradiation angle. Normally, array heights vary from0.25 to 2.5 wavelengths. The dipole elements areusually constructed from tubing, which provides forbetter gain and bandwidth characteristics and providessufficient mechanical rigidity for self-support. Yagiarrays of four elements or less are not structurallycomplicated. Longer arrays and arrays for lowerfrequencies, where the width of the array exceeds 40feet, require elaborate booms and supporting structures.Yagi arrays may be either fixed-position or rotatable.

LOG-PERIODIC ANTENNAS (LPAs)

An antenna arranged so the electrical length andspacing between successive elements causes the input

impedance and pattern characteristics to be repeatedperiodically with the logarithm of the driving frequencyis called a LOG-PERIODIC ANTENNA (LPA). TheLPA, in general, is a medium-power, high-gain,moderately-directive antenna of extremely broadbandwidth. Bandwidths of up to 15:1 are possible,with up to 15 dB power gain. LPAs are rathercomplex antenna systems and are relatively expensive.The installation of LPAs is normally more difficultthan for other hf antennas because of the tower heightsinvolved and the complexity of suspending theradiating elements and feedlines from the towers.

Vertical Monopole LPA

The log-periodic vertical monopole antenna (fig.2-16) has the plane containing the radiating elementsin a vertical field. The longest element is approxi-mately one-quarter wavelength at the lower cutofffrequency. The ground system for the monopolearrangement provides the image equivalent of the otherquarter wavelength for the half-dipole radiatingelements. A typical vertical monopole designed to

2-8

Figure 2-14.—NORD antenna.

Figure 2-15.—Yagi antenna.Figure 2-16.—Log-periodic vertical monopoleantenna.

2-9

cover a frequency range of 2 to 30 MHz requires onetower approximately 140 feet high and an antennalength of around 500 feet, with a ground system thatcovers approximately 3 acres of land in the immediatevicinity of the antenna.

Sector Log-Periodic Array

This version of a vertically polarized fixed-azimuthLPA consists of four separate curtains supported bya common central tower, as shown in figure 2-17.Each of the four curtains operates independently,providing antennas for a minimum of four transmitor receive systems. and a choice of sector coverage.The four curtains are also capable of radiating a rosettepattern of overlapping sectors for full coverage, asshown by the radiation pattern in figure 2-17. Thecentral supporting tower is constructed of steel andmay range to approximately 250 feet in height, withthe length of each curtain reaching 250 feet, dependingon its designed operating frequencies. A sector antennathat uses a ground plane designed to cover the entirehf spectrum takes up 4 to 6 acres of land area.

Figure 2-17.—Sector LPA and its horizontal radiationpattern.

Figure 2-18.—Rotatable log-periodic antenna.

Rotatable LPA (RLPA)

RLPAs (fig. 2-18) are commonly used inship-to-shore-to-ship and in point-to-point ecm-u-nunica-tions. Their distinct advantage is their ability to rotate360 degrees. RLPAs are usually constructed witheither tubular or wire antenna elements. The RLPAin figure 2-18 has wire elements strung on threealuminum booms of equal length, spaced equally andarranged radially about a central rotator on top of asteel tower approximately 100 feet high. Thefrequency range of this antema is 6 to 32 MHz. Thegain is 12 dB with respect to isotropic antennas.Power handling capability is 20 kw average, and vswris 2:1 over the frequency range.

INVERTED CONE ANTENNA

Inverted cone antennas are vertically polarized,omnidirectional, and have an extremely broadbandwidth. They are widely used for ship-to-shoreand ground-to-air communications. Inverted coneantennas are installed over a radial ground planesystem and are supported by poles, as shown in figure2-19. The equally-spaced vertical radiator wiresterminate in a feed ring assembly located at the bottomcenter, where a 50-ohm coaxial transmission line feedsthe antenna. Inverted cones usually have gains of 1to 5 dB above isotropic antennas, with a vswr not

2-10

Figure 2-19.—Inverted cone antenna.

greater than 2:1. They are considered medium- tohigh-power radiators, with power handling capabilitiesof 40 kW average power.

CONICAL MONOPOLE ANTENNA

Conical monopoles are used extensively in hfcommunications. A conical monopole is an efficientbroadband, vertically polarized, omnidirectional antennain a compact size. Conical monopoles are shaped liketwo truncated cones connected base-to-base. The basicconical monopole configuration, shown in figure 2-20,is composed of equally-spaced wire radiating elementsarranged in a circle around an aluminum center tower.Usually, the radiating elements are connected to thetop and bottom discs, but on some versions, there isa center waist disc where the top and bottom radiatorsare connected. The conical monopole can handle upto 40 kW of average power. Typical gain is -2 to +2dB, with a vswr of up to 2.5:1.

RHOMBIC ANTENNA

Rhombic antennas can be characterized ashigh-power, low-angle, high-gain, horizontally-polarized, highly-directive, broadband antennas ofsimple, inexpensive construction. The rhombic antenna(fig. 2-21) is a system of long-wire radiators thatdepends on radiated wave interaction for its gain anddirectivity. A properly designed rhombic antennapresents to the transmission line an input impedanceinsensitive to frequency variations up to 5:1. Itmaintains a power gain above 9 dB anywhere withina 2:1 frequency variation. At the design-centerfrequency, a gain of 17 dB is typical. The radiationpattern produced by the four radiating legs of arhombic antenna is modified by reflections from theearth under, and immediately in front of, the antenna.Because of the importance of these ground

Figure 2-20.—Conical monopole antenna.

reflections in the proper formation of the main lobe,the rhombic should be installed over reasonably smoothand level ground. The main disadvantage of therhombic antenna is the requirement for a large landarea, usually 5 to 15 acres.

QUADRANT ANTENNA

The hf quadrant antenna (fig. 2-22) is aspecial-purpose receiving antenna used inground-to-air-to-ground communications. It is uniqueamong horizontally-polarized antennas because its

2-11

Figure 2-21.—Three-wire rhombic antenna.

element arrangement makes possible a radiation pat-tern resembling that of a vertically-polarized,omnidirectional antenna. Construction and installationof this antenna is complex because of the physical

relationships between the individual elements and therequirement for a separate transmission line for eachdipole. Approximately 2.2 acres of land are requiredto accommodate the quadrant antenna.

2-12

Figure 2-22.—Quadrant antenna.

WHIP ANTENNAS

Hf whip antennas (fig. 2-23) are vertically-polarizedomnidirectional monopoles that are used forshort-range, ship-to-shore and transportable communi-cations systems. Whip antennas are made of tubularmetal or fiberglass, and vary in length from 12 feetto 35 feet, with the latter being the most prevalent.Although whips are not considered as highly efficientantennas, their ease of installation and low cost providea compromise for receiving and low-to-medium powertransmitting installations.

The self-supporting feature of the whip makes itparticularly useful where space is limited. Whips canbe tilted, a design feature that makes them suited foruse along the edges of aircraft carrier flight decks.Aboard submarines, they can be retracted into the sailstructure.

Most whip antennas require some sort of tuningsystem and a ground plane to improve their radiationefficiency throughout the hf spectrum. Without anantenna tuning system, whips generally have a narrowbandwidth and are limited in their power handling

2-13

Figure 2-23.—Whip antennas.

capabilities. Power ratings for most whips range from1 to 5 kW PEP.

WIRE-ROPE FAN ANTENNAS

Figure 2-24 shows a five-wire vertical fan antenna.This is a broadband antenna composed of five wires,

Figure 2-24.—Vertical fan antenna.

each cut for one-quarter wavelength at the lowestfrequency to be used. The wires are fanned 30 degreesbetween adjacent wires. The fan antenna providessatisfactory performance and is designed for use asa random shipboard antenna in the hf range (2-30MHz).

DISCAGE ANTENNA

The discage antenna (fig. 2-25) is a broadbandomnidirectional antenna. The diseage structure consistsof two truncated wire rope cones attached base-to-baseand supported by a central mast. The lower portionof the structure operates as a cage monopole for the4- to 12-MHz frequency range. The upper portionoperates as a discone radiator in the 10- to 30-MHzfrequency range. Matching networks limit the vswrto not greater than 3:1 at each feed point.Vinyl-covered phosphor bronze wire rope is usedfor the wire portions. The support mast and otherportions are aluminum.

VHF/UHF

At vhf and uhf frequencies, the shorter wavelengthmakes the physical size of the antenna relatively small.Aboard ship these antennas are installed as high as

2-14

.

Figure 2-25.—AS-2802/SCR discage antenna.

possible and away from any obstructions. The reasonfor the high installation is that vertical conductors,such as masts, rigging, and cables in the vicinity, causeunwanted directivity in the radiation pattern.

For best results in the vhf and uhf ranges, bothtransmitting and receiving antennas must have the samepolarization. Vertically polarized antennas (primarilydipoles) are used for all ship-to-ship, ship-to-shore,and air-to-ground vhf and uhf communications.

The following paragraphs describe the mostcommon uhf/vhf dipole antennas. All the examplesare vertically-polarized, omnidirectional, broadbandantennas.

Biconical Dipole

The biconical dipole antenna (fig. 2-26) is designedfor use at a normal rf power rating of around 250watts, with a vswr not greater than 2:1. All majorcomponents of the radiating and support structuresare aluminum. The central feed section is protectedand waterproofed by a laminated fiberglass cover.

Figure 2-26.—AS-2811/SCR biconical dipoleantenna.

2-15

Center-Fed Dipole

The center-fed dipole (fig. 2-27) is designed foruse at an average power rating of 100 watts. All majorcomponents of the radiating and support structuresare aluminum. The central feed section and radiatingelements are protected by a laminated fiberglass cover.Center-fed dipole antennas range from 29 to 47 inchesin height and have a radiator diameter of up to 3inches.

Coaxial Dipole

Figure 2-28 shows two types of coaxial dipoles.The coaxial dipole antenna is designed for use in theuhf range, with an rf power rating of 200 watts. The

Figure 2-27.—AS-2809/RC center-fed dipole antenna.

AT-150/SRC (fig. 2-28, view A) has vertical radiatingelements and a balun arrangement that electricallybalances the antenna to ground.

Figure 2-28, view B, shows an AS-390/SRCantenna assembly. This antenna is an unbalancedbroadband coaxial stub antenna. It consists of aradiator and a ground plane. The ground plane (orcounterpoise) consists of eight elements bent downward37 degrees from horizontal. The lower ends of theelements form points of a circle 23 inches in diameter.The lower section of the radiator assembly containsa stub for adjusting the input impedance of the antenna.The antenna is vertically polarized, with an rf powerrating of 200 watts, and a vswr not greater than 2:1.

SATELLITE SYSTEMS

The Navy Satellite Communication(SATCOM) provides communications

Systemlinks,

via satellites, between designated mobile units andshore sites. These links supply worldwide communica-tions coverage. The following paragraphs describesome of the more common SATCOM antenna systemsto which you will be exposed.

AS-2815/SRR-1

The AS-2815/SSR-1 fleet broadcast receivingantenna (fig. 2-29) has a fixed 360-degree horizontalpattern with a maximum gain of 4 dB at 90 degreesfrom the antenna’s horizontal plane. The maximumloss in the antenna’s vertical pattern sector is 2 dB.The vswr is less than 1.5:1, referenced to 50 ohms.This antenna should be positioned to protect it frominterference and possible front end burnout from radarand uhf transmitters.

ANTENNA GROUPS OE-82B/WSC-1(V)AND OE-82C/WSC-1(V)

Designed primarily for shipboard installations, theseantenna groups interface with the AN/WSC-3transceiver. The complete installation consists of anantenna, bandpass amplifier-filter, switching unit, andantenna control (figs. 2-30 and 2-31), Depending onrequirements, one or two antennas may be installedto provide a view of the satellite at all times. Theantenna assembly is attached to a pedestal that permits

2-16

Figure 2-28.—Coaxial dipole.

Figure 2-29.—AS-2815/SSR-1 fleet broadcastsatellite receiving antenna.

it to rotate 360 degrees and to elevate from nearhorizontal to approximately 20 degrees beyond zenith(elevation angles from +2 to +110 degrees). Theantenna tracks automatically in azimuth and manuallyin elevation. Frequency bands are 248-272 MHz forreceive and 292-312 MHz for transmit. Polarizationis right-hand circular for both transmit and receive.Antenna gain characteristics are nominally 12 dB intransmit and 11 dB in receive.

AN/WSC-5(V) SHORE STATIONANTENNA

The AN/WSC-5(V) shore station antenna (fig. 2-32)consists of four OE-82A/WSC-1(V) backplaneassemblies installed on a pedestal. This antenna isintended for use with the AN/WSC-5(V) transceiverat major shore stations. The antenna is oriented

manually and can be locked in position to receivemaximum signal strength upon capture of the satellitesignal. Hemispherical coverage is 0 to 110 degreesabove the horizon. Polarization is right-hand circularin both transmit and receive. The antenna’s operatingfrequency range is 240 to 318 MHz. With its mount,

2-17

Figure 2-30.—OE-82/WSC-1(V) antenna group.

Figure 2-31.—OE-82C/WSC-1(V) antenna group.

2-18

Figure 2-32.—OE-82A/WSC-1(V)/AN/WSC-5(V) shorestation antenna.

the antenna weighs 2500 pounds and is 15 feet high,10 feet wide, and 10 feet deep. The gain characteris-tics of this antenna are nominally 15 dB in transmitand 18 dB in receive.

ANDREW 58622 SHORE ANTENNA

The Andrew 58622 antenna (fig. 2-33) is a bifilar,16-turn helical antenna right-hand circularly polarized,with gain varying between 11.2 and 13.2 dB in the240-315 MKz frequency band. It has a 39-inch groundplate and is about 9 feet, 7 inches long. It can beadjusted manually in azimuth and elevation. Thisantenna is used at various shore installations, otherthan NCTAMS, for transmit and receive operations.

AN/WSC-6(V) SHF SATCOMANTENNA

The antennas used on current shf SATCOMshipboard terminals are parabolic reflectors withcasseegrain feeds. These antennas provide for LPI (lowprobability of intercept), with beamwidths less than2.5 degrees (fig. 2-34). The reflectors are mountedon three-axis pedestals and provide autotracking ofa beacon or communication signal by conical scanning

Figure 2-33.—Andrew 58622 shore antenna.

Figure 2-34.—AN/WSC-6(V) attenuationscale.

techniques. The antennas are radome enclosed andinclude various electronic components. Both a 7-footmodel (fig. 2-35) and a 4-foot model (fig. 2-36) areoperational in the fleet.

2-19

Figure 2-35.—Seven-foot shf SATCOM antenna.

Figure 2-36.—Four-foot shf SATCOM antenna.

MATCHING NETWORKS

An antenna matching network consists of one ormore parts (such as coils, capacitors, and lengths oftransmission line) connected in series or parallel withthe transmission line to reduce the standing wave ratioon the line. Matching networks are usually adjustedwhen they are installed and require no furtheradjustment for proper operation. Figure 2-37 showsa matching network outside of the antenna feedbox,with a sample matching network schematic.

Matching networks can also be built with variablecomponents so they can be used for impedancematching over a range of frequencies. These networksare called antenna tuners.

Antenna tuners are usually adjusted automaticallyor manually each time the operating frequency ischanged. Standard tuners are made with integralenclosures. Installation consists simply of mounting

2-20

Figure 2-37.—Matching network.

the tuner, assembling the connections with the antennaand transmission line, and pressurizing the tuner,if necessary. Access must be provided to the pressuregauge and pressurizing and purging connections.

ANTENNA TUNING

For every frequency in the frequency spectrum,there is an antenna that is perfect for radiating at thatfrequency. By that we mean that all of the powerbeing transmitted from the transmitter to the antennawill be radiated into space. Unfortunately, this is theideal and not the rule. Normally, some power is lostbetween the transmitter and the antenna. This powerloss is the result of the antenna not having the perfectdimensions and size to radiate perfectly all of thepower delivered to it from the transmitter. Naturally,it would be unrealistic to carry a separate antenna forevery frequency that a communications center iscapable of radiating; a ship would have to havemillions of antennas on board, and that would beimpossible.

To overcome this problem, we use ANTENNATUNING to lengthen and shorten antennas electricallyto better match the frequency on which we want totransmit. The rf tuner is connected electrically to theantenna and is used to adjust the apparent physicallength of the antenna by electrical means. This simply

means that the antenna does not physically changelength; instead, it is adapted electrically to the outputfrequency of the transmitter and “appears” to changeits physical length. Antenna tuning is done by usingantenna couplers, tuners, and multicouplers.

Antenna couplers and tuners are used to matcha single transmitter or receiver to one antenna whereasantenna multicouplers are used to match more thanone transmitter or receiver to o n e antenna forsimultaneous operation. Some of the many antennacouplers that are in present use are addressed in thefollowing paragraphs. For specific information ona particular coupler, refer to the appropriate equipmenttechnical manual.

Antenna Coupler Group AN/URA-38

Antenna Coupler Group AN/URA-38 is anautomatic antenna tuning system intended primarilyfor use with the AN/URT-23(V) operating in thehigh-frequency range. The equipment also includesprovisions for manual and semiautomatic tuning,making the system readily adaptable for use with otherradio transmitters. The manual tuning feature is usefulwhen a failure occurs in the automatic tuning circuitry.Tuning can also be done without the use of rf power(silent tuning). This method is useful in installationswhere radio silence must be maintained except forbrief transmission periods.

The antenna coupler matches the impedance ofa 15-, 25-, 28-, or 35-foot whip antenna to a 50-ohmtransmission line, at any frequency in the 2-to 30-MHzrange. When the coupler is used with theAN/URT-23(V), control signals from the associatedantenna coupler control unit automatically tune thecoupler’s matching network in less than 5 seconds.During manual and silent operation, the operator usesthe controls mounted on the antenna coupler controlunit to tune the antenna. A low-power (less than 250watts) cw signal is required for tuning. Once tuned,the CU 938A/URA-38 is capable of handling 1000watts PEP.

Antenna Coupler GroupsAN/SRA-56, -57, and -58

Antenna coupler groups AN/SRA-56, -57, and-58 are designed primarily for shipboard use. Each

2-21

coupler group permits several transmitters to operatesimultaneously into a single, associated, broadbandantenna, thus reducing the total number of antennasrequired in the limited space aboard ship.

These antenna coupler groups provide a couplingpath of prescribed efficiency between each transmitterand the associated antenna. They also provide isolationbetween transmitters, tunable bandpass filters tosuppress harmonic and spurious transmitter outputs,and matching networks to reduce antenna impedances.

The three antenna coupler groups (AN/SRA-56,-57, -58) are similar in appearance and function, butthey differ in frequency ranges. Antenna coupler groupAN/SRA-56 operates in the 2- to 6-MHz frequencyrange. The AN/SRA-57 operates from 4 to 12 MHz,and the AN/SRA-58 operates from 10 to 30 MHz.When more than one coupler is used in the samefrequency range, a 15 percent frequency separationmust be maintained to avoid any interference.

Antenna Coupler Group AN/SRA-33

Antenna coupler group AN/SRA-33 operates inthe uhf (225-400 Mhz) frequency range. It providesisolation between as many as four transmitter andreceiver combinations operating simultaneously intoa common uhf antenna without degrading operation.The AN/SRA-33 is designed for operation withshipboard radio set AN/WSC-3. The AN/SRA-33consists of four antenna couplers (CU-1131/SRA-33through CU-1134/SRA-33), a control power supplyC-4586/SRA-33, an electronic equipment cabinetCY-3852/SRA-33, and a set of special-purpose cables.

OA-9123/SRC

The OA-9123/SRC multicoupler enables up to fouruhf transceivers, transmitters, or receivers to operateon a common antenna. The multicoupler provideslow insertion loss and highly selective filtering in eachof the four ports. The unit is interface compatiblewith the channel select control signals from radio setsAN/WSC-3(V) (except (V)1). The unit is self-contained and is configured to fit into a standard19-inch open equipment rack.

The OA-9123/SRC consists of a cabinet assembly,control power supply assembly, and four identical filterassemblies. This multicoupler is a state-of-the-artreplacement for the AN/SRA-33 and only requiresabout half of the space.

RECEIVING ANTENNADISTRIBUTION SYSTEMS

Receiving antenna distribution systems operateat low power levels and are designed to preventmultiple signals from being received. The basicdistribution system has several antenna transmissionlines and several receivers, as shown in figure 2-38.The system includes two basic patch panels, one thatterminates the antenna transmission lines, and the otherthat terminates the lines leading to the receivers. Thus,any antenna can be patched to any receiver via patchcords.

Figure 2-38.—Receive signal distribution system.

2-22

Some distribution systems will be more complex.That is, four antennas can be patched to four receivers,or one antenna can be patched to more than onereceiver via the multicouplers.

RECEIVING MULTICOUPLERAN/SRA-12

The AN/SRA-12 filter assembly multicouplerprovides seven radio frequency channels in the 14-kHzto 32-MHz frequency range. Any of these channelsmay be used independently of the other channels, orthey may operate simultaneously. Connections to thereceiver are made by coaxial patch cords, which areshort lengths of cable with a plug attached to eachend.

ANTENNA COUPLER GROUPSAN/SRA-38, AN/SRA-39, AN/SRA-40,AN/SRA-49, AN/SRA-49A, and AN/SRA-50

These groups are designed to connect up to 20mf and hf receivers to a single antenna, with a highlyselective degree of frequency isolation. Each of thesix coupler groups consists of 14 to 20 individualantenna couplers and a single-power supply module,all are slide-mounted in a special electronic equipmentrack. An antenna input distribution line termination(dummy load) is also supplied. In addition, there areprovisions for patching the outputs from the variousantenna couplers to external receivers.

RADAR ANTENNAS

Radar antennas are usually directional antennasthat radiate energy in one lobe or beam. The two mostimportant characteristics of directional antennas aredirectivity and power gain. Most radar systems useparabolic antennas. These antennas use parabolicreflectors in different variations to focus the radiatedenergy into a desired beam pattern.

While most radar antennas are parabolic, othertypes such as the corner reflector, the broadside array,and horn radiators may also be used.

PARABOLIC REFLECTORS

To understand why parabolic reflectors are usedfor most radar antennas, you need to understand how

radio waves behave. A point source, such as anomnidirectional antenna produces a spherical radiationpattern, or spherical wavefront. As the sphere expands,the energy contained in a given surface area decreasesrapidly. At a relatively short distance from theantenna, the energy level is so small that its reflectionfrom a target would be useless in a radar system.

A solution to this problem is to form the energyinto a PLANE wavefront, In a plane wavefront, allof the energy travels in the same direction, thusproviding more energy to reflect off of a target. Toconcentrate the energy even further, a parabolicreflector is used to shape the plane wavefront’s energyinto a beam of energy. This concentration of energyprovides a maximum amount of energy to be reflectedoff of a target, making detection of the target muchmore probable.

How does the parabolic reflector focus the radiowaves? Radio waves behave much as light waves do.Microwaves travel in straight lines as do light rays.They may be focused or reflected, just as light raysmay be. In figure 2-39, a point-radiation source isplaced at the focal point F. The field leaves thisantema with a spherical wavefront. As each part ofthe wavefront moving toward the reflector reachesthe reflecting surface, it is shifted 180 degrees in phaseand sent outward at angles that cause all parts of thefield to travel in parallel paths. Because of the shapeof a parabolic surface, all paths from F to the reflectorand back to line XY are the same length. Therefore,all parts of the field arrive at line XY at the same timeafter reflection.

Figure 2-39.—Parabolic reflector radiation.

Energy that is not directed toward the paraboloid(dotted lines in fig. 2-39) has a wide-beam characteris-

2-23

tic that would destroy the narrow pattern from theparabolic reflector. This destruction is prevented bythe use of a hemispherical shield (not shown) thatdirects most of what would otherwise be sphericalradiation toward the parabolic surface. Without theshield, some of the radiated field would leave theradiator directly, would not be reflected, and wouldserve no useful purpose. The shield makes thebeamsharper, and concentrates the majority of thepower in the beam. The same results can be obtainedby using either a parasitic array to direct the radiatedfield back to the reflector, or a feed horn pointed atthe paraboloid.

The radiation pattern of the paraboloid containsa major lobe, which is directed along the axis of theparaboloid, and several minor lobes, as shown in figure2-40. Very narrow beams are possible with this typeof reflector. View A of figure 2-41 illustrates theparabolic reflector.

Truncated Paraboloid

While the complete parabolic reflector producesa pencil-shaped beam, partial parabolic reflectors

Figure 2-40.—Parabolic radiation pattern.

produce differently shaped beams. View B of figure2-41 shows a horizontally truncated, or verticallyshortened, paraboloid. This type of reflector isdesigned to produce a beam that is narrow horizontallybut wide vertically. Since the beam is wide vertically,it will detect aircraft at different altitudes withoutchanging the tilt of the antenna. It also works wellfor surface search radars to overcome the pitch androll of the ship.

Figure 2-41.—Reflector shapes.

2-24

The truncated paraboloid reflector may be usedin height-finding systems if the reflector is rotated90 degrees, as shown in view C of figure 2-41. Thistype of reflector produces a beam that is widehorizontally but narrow vertically. The beam patternis spread like a horizontal fan. Such a fan-shapedbeam can be used to determine elevation veryaccurately.

Orange-Peel Paraboloid

A section of a complete circular paraboloid, oftencalled an ORANGE-PEEL REFLECTOR because ofits shape, is shown in view D of figure 2-41. Sincethe reflector is narrow in the horizontal plane and widein the vertical, it produces a beam that is wide in thehorizontal plane and narrow in the vertical. In shape,the beam resembles a huge beaver tail. This type ofantenna system is generally used in height-findingequipment.

Cylindrical Paraboloid

When a beam of radiated energy noticeably widerin one cross-sectional dimension than in the other isdesired, a cylindrical paraboloid section approximatinga rectangle can be used. View E of figure 2-41illustrates this antenna. A parabolic cross section isin one dimension only; therefore, the reflector isdirective in one plane only. The cylindrical paraboloidreflector is either fed by a linear array of dipoles, aslit in the side of a waveguide, or by a thin waveguideradiator. Rather than a single focal point, this typeof reflector has a series of focal points forming astraight line. Placing the radiator, or radiators, alongthis focal line produces a directed beam of energy.As the width of the parabolic section is changed,different beam shapes are produced. This type ofantenna system is used in search systems and in groundcontrol approach (gca) systems.

CORNER REFLECTOR

The corner-reflector antenna consists of two flatconducting sheets that meet at an angle to form acorner, as shown in view F of figure 2-41. Thisreflector is normally driven by a half-wave radiatorlocated on a line that bisects the angle formed by thesheet reflectors.

BROADSIDE ARRAY

Desired beam widths are provided for some vhfradars by a broadside array, such as the one shownin figure 2-42. The broadside array consists of twoor more half-wave dipole elements and a flat reflector.The elements are placed one-half wavelength apartand parallel to each other. Because they are excitedin phase, most of the radiation is perpendicular orbroadside to the plane of the elements. The flatreflector is located approximately one-eighth wave-length behind the dipole elements and makes possiblethe unidirectional characteristics of the antenna system.

HORN RADIATORS

Horn radiators, like parabolic reflectors, may beused to obtain directive radiation at microwavefrequencies. Because they do not involve resonantelements, horns have the advantage of being usableover a wide frequency band.

The operation of a horn as an electromagneticdirecting device is analogous to that of acoustic horns.However, the throat of an acoustic horn usually hasdimensions much smaller than the sound wavelengthsfor which it is used, while the throat of the electromag-netic horn has dimensions that are comparable to thewavelength being used.

Horn radiators are readily adaptable for use withwaveguides because they serve both as an impedance-

Figure 2-42.—Broadside array.

2-25

matching device and as a directional radiator. Hornradiators may be fed by coaxial or other types of lines.

Horns are constructed in a variety of shapes asillustrated in figure 2-43. The shape of the horn andthe dimensions of the length and mouth largelydetermine the field-pattern shape. The ratio of thehorn length to mouth opening size determines the beamangle and, thus, the directivity. In general, the largerthe opening of the horn, the more directive is theresulting field pattern.

Figure 2-43.—Horn radiators.

FEEDHORNS

A waveguide horn, called a FEEDHORN, maybe used to feed energy into a parabolic dish. Thedirectivity of this feedhorn is added to that of theparabolic dish. The resulting pattern is a very narrowand concentrated beam. In most radars, the feedhornis covered with a window of polystyrene fiberglassto prevent moisture and dirt from entering the openend of the waveguide.

One problem associated with feedhorns is theSHADOW introduced by the feedhorn if it is in thepath of the beam. (The shadow is a dead spot directlyin front of the feedhorn.) To solve this problem thefeedhorn can be offset from center. This locationchange takes the feedhorn out of the path of the rfbeam and eliminates the shadow. An offset feedhornis shown in figure 2-44.

RADAR SYSTEMS

Now that you have a basic understanding of howradar antennas operate, we will introduce you to a fewof the radar systems currently in use.

Figure 2-44.—Offset feedhorn.

AN/GPN-27(ASR-8) AIRSURVEILLANCE RADAR

The AN/GPN-27(ASR-8) (fig. 2-45) antennaradiates a beam 1.5 degrees in azimuth and shapedin elevation to produce coverage of up to approxi-mately 32 degrees above the horizon. This providesa maplike presentation of aircraft within 55 nauticalmiles of an airport terminal. The antenna azimuth

Figure 2-45.—AN/GPN-27(ASR-8) airsurveillance radar.

2-26

pulse generator (APG), located in the rotary joint,transmits to the radar indicator azimuth informationcorresponding to beam direction. Polarization of theradiated energy can be remotely switched to eitherlinear or circular polarization. The reflector has amodified parabolic shape designed to produce anapproximately cosecant squared beam in the elevationplane. The reflector surface, covered with expandedaluminum screen, is 16.1 feet wide and 9 feet high.The antenna feedhorn, which mounts on the polarizer,provides impedance matching between the waveguidesystem and free space, and produces the desired feedpattern to illuminate the reflector. A radome overthe horn aperture excludes moisture and foreign matter,and provides a pressure seal.

pedestal assembly, the feedhorn and feedhorn supportboom, and the reflector assembly.

The base assembly provides a surface for mountingthe antenna to the ship. It also contains the azimuthdrive gearbox. The gearbox is driven by the azimuthdrive motor, which drives the pedestal in azimuththrough a pinion gear mated to a ring gear locatedat the bottom of the cone-shaped pedestal assembly,The azimuth drive circuits rotate the antenna through360 degrees at speeds of 6 rpm and 12 rpm.

The reflector and the feedhorn support boom aremounted on a trunnion, allowing the elevation angleof the rf beam to be controlled by a jackscrew locatedbehind the reflector. The jackscrew is rotated by theelevation drive gearbox, which is connected to twodc motors. The rf energy to the feedhorn is routedthrough elevation and azimuth rotary joints locatedwithin the pedestal.

AS-3263/SPS-49(V)

The AS-3263/SPS-49(V) antenna (fig. 2-46)consists of three major sections: the antenna base and

Figure 2-46.—AS-3263/SPS-49(V) antenna.

2-27

The reflector is 24 feet wide and has adouble-curved surface composed of a series ofhorizontal members that form a reflecting surface forthe horizontally polarized C-band energy. The antennahas a 28-dB gain, with a beamwidth of 9 degreesminimum vertically and approximately 3.3 degreeshorizontally. Antenna roll and pitch stabilization limitsare plus or minus 25 degrees, Stabilization accuracyis plus or minus 1 degree with the horizontal plane.

The antenna is equipped with a safety switchlocated near the antenna pedestal area. The safetyswitch disables the azimuth and elevation functionsin the antenna and the radiate function in the transmit-ter to provide protection for personnel working onthe antenna.

OE-172/SPS-55

The OE-172/SPS-55 antenna group consists of theantenna and the antenna pedestal. The antenna groupis mast-mounted by means of four bolt holes on thebase of the pedestal.

The antenna consists of two waveguide slottedarrays mounted back-to-back. One array provideslinear polarization, while the other provides circularpolarization. The array used is selected by means ofa remotely controlled waveguide switch located onthe pedestal. Linear polarization is used for mostconditions. Circular polarization is used to reducereturn echoes from precipitation. Each antenna formsa fan beam that is narrow in the azimuth plane andmoderately broad in the elevation plane.

Figure 2-47 shows a cross-section of the SPS-55antenna. During transmission, the rf signal enters theantenna through a feed waveguide and then enters afeed manifold region of 80 periodic narrow-wall slots.The slots are skewed in angle and alternated indirection of skew. They are separated by approxi-mately one-half wavelength, resulting in broadsideradiation into the sectoral horn region of the antenna.The horizontally polarized radiation from the manifoldtravels in the horn region toward the aperture, whereit encounters an array of vertical sheet metal slats.

Figure 2-47.—SPS-55 antenna cross section.

2-28

This array is a polarizing filter, which ensures thatonly horizontally polarized energy travels from thehorn region. The antenna scans at a rate of 16 rpmand produces an absolute gain of 31 dB at midband.

AN/SPN-35A AIRCRAFT CONTROLAPPROACH RADAR

The AN/SPN-35A (fig 2-48) is acarrier-controlled-approach (CCA) radar set used forprecision landing approaches during adverse weatherconditions. The radar displays both azimuth andelevation data, which enables the radar operator todirect aircraft along a predetermined glide path and

azimuth course line to a transition point approximately2 miles from the ramp of the flight deck.

The azimuth antenna, AS-1292/TPN-8, functionsin the azimuth rf line for radiation and reception ofX-band rf pulses. The azimuth antenna comprisesa truncated paraboloid-type reflector with an offsetfeedhorn and a polarizer assembly that providesremote-controlled selection of either horizontal orcircular polarization. The antenna is located abovethe azimuth drive assembly on the stabilized yoke.The azimuth drive can rotate the antenna in either 360degrees or in limited-sector modes of operation in thehorizontal plane.

Figure 2-48.—AN/SPN-3SA aircraft control approach radar.

2-29

The elevation antenna, AS-1669/SPN-35, is atruncated paraboloid-type reflector with a dual-channelfeedhorn and a polarizer assembly providingmonopulse-type radiation and reception of X-bandrf pulses. The horizontal shape of the laminatedfiberglass reflector is cosecanted. The dual-channelfeedhorn and polarizer are fixed in circular polarizationby an external grid device. The elevation antenna isstabilized-yoke mounted on the elevation driveassembly adjacent to the azimuth antenna. Theelevation drive provides the required motion for theelevation antenna and locks electrically with the searchdrive when the radar set operates in the precisionmode.

The radar operates in three modes, final, surveil-lance, and simultaneous, with each antenna actingindependently. In the final (precision) mode, theazimuth antenna scans a 30-degree sector (60-degreesector optional) while the elevation antenna scans a10-degree sector (35-degree sector optional). In thesurveillance mode the azimuth antenna rotates throughthe full 360-degree search pattern at 16 rpm whilethe elevation antenna scans a 10-degree sector. Inthe simultaneous mode, the azimuth antenna rotatesthrough the full 360-degrees search pattern in60-degree increments while the elevation antenna scansa 10-degree sector. The data rate in this mode isapproximately 16 azimuth sweeps and 24 elevationsweeps every 60 seconds.

The antenna pedestal control stabilizes the azimuthand elevation antennas for plus or minus 3 degreesof pitch and plus or minus 10 degrees of roll.

RF SAFETY PRECAUTIONS

Although radio frequency emissions are usuallyharmless, there are still certain safety precautions youshould follow whenever you are near high-power rfsources. Normally, electromagnetic radiation fromtransmission lines and antennas isn’t strong enoughto electrocute personnel. However, it may lead to otheraccidents and can compound injuries. Voltages maybe induced into metal objects both above and belowground, such as wire guys, wire cable, hand rails, andladders. If you come into contact with these objects,you may receive a shock or an rf burn. The shockcan cause you to jump involuntarily, to fall into nearby

equipment, or, when working aloft, to fall from theelevated work area. Take care to ensure that alltransmission lines or antennas are de-energized beforeworking on or near them.

When working aloft aboard ship, be sure to usea working aloft chit. This will ensure that all radiators,not only those on your own ship but also those nearbyare secured while you are aloft.

ALWAYS obey rf radiation warning signs andkeep a safe distance from radiating antennas. Thesix types of warning signs for rf radiation hazards areshown in figure 2-49.

The two primary safety concerns associated withrf fields are rf burns and injuries caused by dielectricheating.

RF BURNS

Close or direct contact with rf transmission linesor antennas may result in rf burns caused by inducedvoltages. These burns are usually deep, penetrating,third-degree burns. To heal properly, rf burns mustheal from the inside toward the skin’s surface. DoNOT take rf burns lightly. To prevent infection, youmust give proper attention to ALL rf burns, includingthe small pinhole burns. ALWAYS seek treatmentfor any rf burn or shock and report the incident toyour supervisor so appropriate action can be takento correct the hazard.

DIELECTRIC HEATING

While the severity of rf burns may vary from minorto major, burns or other damage done by DIELEC-TRIC HEATING may result in long-term injury, oreven death. Dielectric heating is the heating of aninsulating material caused by placing it in ahigh-frequency electric field. The heat results fromthe rapid reversal of molecular polarization dielectricmaterial.

When a human is in an rf field, the body acts asthe dielectric. If the power in the rf field exceeds 10milliwatts per centimeter, the individual will have anoticeable rise in body temperature. Basically, thebody is “cooking” in the rf field. The vital organs

2-30

Figure 2-49.—Rf radiation warning signs

2-31

of the body are highly susceptible to dielectric heating.The eyes are also highly susceptible to dielectricheating. Do NOT look directly into devices radiatingrf energy. Remember, rf radiation can be dangerous.For your own safety, you must NOT stand directlyin the path of rf radiating devices.

PRECAUTIONS WHEN WORKINGALOFT

As we mentioned earlier, it is extremely importantto follow all safety precautions when working aloft.Before you work on an antenna, ensure that it is taggedout properly at the switchboard to prevent it frombecoming operational. Always be sure to secure themotor safety switches for rotating antennas. Have

the switches tagged and locked open before you beginworking on or near the antenna.

When working near a stack, draw and wear therecommended oxygen breathing apparatus. Amongother toxic substances, stack gas contains carbonmonoxide. Carbon monoxide is too unstable to buildup to a high concentration in the open, but prolongedexposure to even small quantities is dangerous.

For more detailed information concerning thedangers and hazards of rf radiation, refer to theNAVELEX technical manual, ElectromagneticRadiation Hazards. NAVELEX 0967-LP-624-6010.

This completes chapter 2. In chapter 3, we willdiscuss transmission lines and waveguides.

2-32

CHAPTER 3

INTRODUCTION TOTRANSMISSION LINES AND WAVEGUIDES

A TRANSMISSION LINE is a device designedto guide electrical energy from one point to another.It is used, for example, to transfer the output rf energyof a transmitter to an antenna. This energy will nottravel through normal electrical wire without greatlosses. Although the antenna can be connecteddirectly to the transmitter, the antenna is usuallylocated some distance away from the transmitter. Onboard ship, the transmitter is located inside a radioroom, and its associated antenna is mounted on a mast.A transmission line is used to connect the transmitterand the antenna.

The transmission line has a single purpose for boththe transmitter and the antenna. This purpose is totransfer the energy output of the transmitter to theantenna with the least possible power loss. How wellthis is done depends on the special physical andelectrical characteristics (impedance and resistance)of the transmission line.

TRANSMISSION LINE THEORY

The electrical characteristics of a two-wiretransmission line depend primarily on the constructionof the line. The two-wire line acts like a longcapacitor. The change of its capacitive reactance isnoticeable as the frequency applied to it is changed.Since the long conductors have a magnetic field aboutthem when electrical energy is being passed throughthem, they also exhibit the properties of inductance.The values of inductance and capacitance presenteddepend on the various physical factors that wediscussed earlier. For example, the type of line used,the dielectric in the line, and the length of the linemust be considered. The effects of the inductive andcapacitive reactance of the line depend on thefrequency applied. Since no dielectric is perfect,electrons manage to move from one conductor to theother through the dielectric. Each type of two-wiretransmission line also has a conductance value. Thisconductance value represents the value of the current

flow that may be expected through the insulation,If the line is uniform (all values equal at each unitlength), then one small section of the line mayrepresent several feet. This illustration of a two-wiretransmission line will be used throughout the discussionof transmission lines; but, keep in mind that theprinciples presented apply to all transmission lines.We will explain the theories using LUMPED CON-STANTS and DISTRIBUTED CONSTANTS to furthersimplify these principles.

LUMPED CONSTANTS

A transmission line has the properties of induc-tance, capacitance, and resistance just as the moreconventional circuits have. Usually, however, theconstants in conventional circuits are lumped into asingle device or component. For example, a coil ofwire has the property of inductance. When a certainamount of inductance is needed in a circuit, a coil ofthe proper dimensions is inserted. The inductanceof the circuit is lumped into the one component. Twometal plates separated by a small space, can be usedto supply the required capacitance for a circuit. Insuch a case, most of the capacitance of the circuit islumped into this one component. Similarly, a fixedresistor can be used to supply a certain value of circuitresistance as a lumped sum. Ideally, a transmissionline would also have its constants of inductance,capacitance, and resistance lumped together, as shownin figure 3-1. Unfortunately, this is not the case.Transmission line constants are as described in thefollowing paragraphs.

DISTRIBUTED CONSTANTS

Transmission line constants, called distributedconstants, are spread along the entire length of thetransmission line and cannot be distinguished sepa-rately. The amount of inductance, capacitance, andresistance depends on the length of the line, the sizeof the conducting wires, the spacing between the

3-1

Figure 3-1.—Two-wire transmission line.

wires, and the dielectric (air or insulating medium)between the wires. The following paragraphs willbe useful to you as you study distributed constantson a transmission line.

Inductance of a Transmission Line

When current flows through a wire, magnetic linesof force are set up around the wire. As the currentincreases and decreases in amplitude, the field aroundthe wire expands and collapses accordingly. Theenergy produced by the magnetic lines of forcecollapsing back into the wire tends to keep the currentflowing in the same direction. This represents a certainamount of inductance, which is expressed inmicrohenrys per unit length. Figure 3-2 illustratesthe inductance and magnetic fields of a transmissionline.

Capacitance of a Transmission Line

Capacitance also exists between the transmissionline wires, as illustrated in figure 3-3. Notice thatthe two parallel wires act as plates of a capacitor andthat the air between them acts as a dielectric. Thecapacitance between the wires is usually expressedin picofarads per unit length. This electric fieldbetween the wires is similar to the field that existsbetween the two plates of a capacitor.

Figure 3-2.—Distributed inductance.

Figure 3-3.—Distributed capacitance.

Resistance of a Transmission Line

The transmission line shown in figure 3-4 haselectrical resistance along its length. This resistanceis usually expressed in ohms per unit length and isshown as existing continuously from one end of theline to the other.

Figure 3-4.—Distributed resistance.

Leakage Current

Since any dielectric, even air, is not a perfectinsulator, a small current known as LEAKAGECURRENT flows between the two wires. In effect,the insulator acts as a resistor, permitting current topass between the two wires. Figure 3-5 shows thisleakage path as resistors in parallel connected betweenthe two lines. This property is called CONDUC-TANCE (G) and is the opposite of resistance.Conductance in transmission lines is expressed as thereciprocal of resistance and is usually given inmicromhos per unit length.

Figure 3-5.—Leakage in a transmission line.

3-2

ELECTROMAGNETIC FIELDS CHARACTERISTIC IMPEDANCE

The distributed constants of resistance, inductance,and capacitance are basic properties common to alltransmission lines and exist whether or not any currentflow exists. As soon as current flow and voltage existin a transmission line, another property becomes quiteevident. This is the presence of an electromagneticfield, or lines of force, about the wires of thetransmission line. The lines of force themselves arenot visible; however, understanding the force that anelectron experiences while in the field of these linesis very important to your understanding of energytransmission.

There are two kinds of fields; one is associatedwith voltage and the other with current. The fieldassociated with voltage is called the ELECTRIC (E)FIELD. It exerts a force on any electric charge placedin it. The field associated with current is called aMAGNETIC (H) FIELD, because it tends to exerta force on any magnetic pole placed in it. Figure 3-6illustrates the way in which the E fields and H fieldstend to orient themselves between conductors of atypical two-wire transmission line. The illustrationshows a cross section of the transmission lines. TheE field is represented by solid lines and the H fieldby dotted lines. The arrows indicate the direction ofthe lines of force. Both fields normally exist togetherand are spoken of collectively as the electromagneticfield.

Figure 3-6.—Fields between conductors.

You can describe a transmission line in terms ofits impedance. The ratio of voltage to current (Ein/Iin)at the input end is known as the INPUT IMPEDANCE(Zin). This is the impedance presented to the transmit-ter by the transmission line and its load, the antenna.The ratio of voltage to current at the output (EOUT/IOUT)end is known as the OUTPUT IMPEDANCE (ZOUT).This is the impedance presented to the load by thetransmission line and its source. If an infinitely longtransmission line could be used, the ratio of voltageto current at any point on that transmission line wouldbe some particular value of impedance. This imped-ance is known as the CHARACTERISTIC IMPED-ANCE.

The maximum (and most efficient) transfer ofelectrical energy takes place when the source imped-ance is matched to the load impedance. This fact isvery important in the study of transmission lines andantennas. If the characteristic impedance of thetransmission line and the load impedance are equal,energy from the transmitter will travel down thetransmission line to the antenna with no power losscaused by reflection.

LINE LOSSES

The discussion of transmission lines so far has notdirectly addressed LINE LOSSES; actually some lossesoccur in all lines. Line losses may be any of threetypes—COPPER, DIELECTRIC, and RADIATIONor INDUCTION LOSSES.

NOTE: Transmission lines are sometimes referredto as rf lines. In this text the terms are used inter-changeably.

Copper Losses

One type of copper loss is I2R LOSS. In rf linesthe resistance of the conductors is never equal to zero.Whenever current flows through one of these conduc-tors, some energy is dissipated in the form of heat.This heat loss is a POWER LOSS. With copper braid,which has a resistance higher than solid tubing, thispower loss is higher.

3-3

Another type of copper loss is due to SKINEFFECT. When dc flows through a conductor, themovement of electrons through the conductor’s crosssection is uniform, The situation is somewhat differentwhen ac is applied. The expanding and collapsingfields about each electron encircle other electrons.This phenomenon, called SELF INDUCTION, retardsthe movement of the encircled electrons. The fluxdensity at the center is so great that electron movementat this point is reduced. As frequency is increased,the opposition to the flow of current in the center ofthe wire increases. Current in the center of the wirebecomes smaller and most of the electron flow is onthe wire surface. When the frequency applied is 100megahertz or higher, the electron movement in thecenter is so small that the center of the wire couldbe removed without any noticeable effect on current.You should be able to see that the effective cross-sectional area decreases as the frequency increases.Since resistance is inversely proportional to thecross-sectional area, the resistance will increase as thefrequency is increased. Also, since power lossincreases as resistance increases, power losses increasewith an increase in frequency because of skin effect.

Copper losses can be minimized and conductivityincreased in an rf line by plating the line with silver.Since silver is a better conductor than copper, mostof the current will flow through the silver layer. Thetubing then serves primarily as a mechanical support.

Dielectric Losses

DIELECTRIC LOSSES result from the heatingeffect on the dielectric material between the conductors.Power from the source is used in heating the dielectric.The heat produced is dissipated into the surroundingmedium. When there is no potential differencebetween two conductors, the atoms in the dielectricmaterial between them are normal and the orbits ofthe electrons are circular. When there is a potentialdifference between two conductors, the orbits of theelectrons change. The excessive negative charge onone conductor repels electrons on the dielectric towardthe positive conductor and thus distorts the orbits ofthe electrons. A change in the path of electronsrequires more energy, introducing a power loss.

The atomic structure of rubber is more difficultto distort than the structure of some other dielectricmaterials. The atoms of materials, such as polyethyl-ene, distort easily. Therefore, polyethylene is oftenused as a dielectric because less power is consumedwhen its electron orbits are distorted.

Radiation and Induction Losses

RADIAION and INDUCTION LOSSES aresimilar in that both are caused by the fields surround-ing the conductors. Induction losses occur when theelectromagnetic field about a conductor cuts throughany nearby metallic object and a current is inducedin that object. As a result, power is dissipated in theobject and is lost.

Radiation losses occur because some magnetic linesof force about a conductor do not return to theconductor when the cycle alternates. These lines offorce are projected into space as radiation, and thisresults in power losses. That is, power is suppliedby the source, but is not available to the load.

VOLTAGE CHANGE

In an electric circuit, energy is stored in electricand magnetic fields. These fields must be broughtto the load to transmit that energy. At the load, energycontained in the fields is converted to the desired formof energy.

Transmission of Energy

When the load is connected directly to the sourceof energy, or when the transmission line is short,problems concerning current and voltage can be solvedby applying Ohm’s law. When the transmission linebecomes long enough so the time difference betweena change occurring at the generator and a changeappearing at the load becomes appreciable, analysisof the transmission line becomes important.

Dc Applied to a Transmission Line

In figure 3-7, a battery is connected through arelatively long two-wire transmission line to a loadat the far end of the line. At the instant the switch

3-4

is closed, neither current nor voltage exists on the line.When the switch is closed, point A becomes a positivepotential, and point B becomes negative. These pointsof difference in potential move down the line.However, as the initial points of potential leave pointsA and B, they are followed by new points of differencein potential, which the battery adds at A and B. Thisis merely saying that the battery maintains a constantpotential difference between points A and B. A shorttime after the switch is closed, the initial points ofdifference in potential have reached points A’ and B’;the wire sections from points A to A’ and points Bto B’ are at the same potential as A and B, respec-tively. The points of charge are represented by plus(+) and minus (-) signs along the wires, The directions

of the currents in the wires are represented by thearrowheads on the line, and the direction of travel isindicated by an arrow below the line. Conventionallines of force represent the electric field that existsbetween the opposite kinds of charge on the wiresections from A to A’ and B to B’. Crosses (tails ofarrows) indicate the magnetic field created by theelectric field moving down the line. The movingelectric field and the accompanying magnetic fieldconstitute an electromagnetic wave that is moving fromthe generator (battery) toward the load. This wavetravels at approximately the speed of light in freespace. The energy reaching the load is equal to thatdeveloped at the battery (assuming there are no lossesin the transmission line). If the load absorbs all ofthe energy, the current and voltage will be evenlydistributed along the line.

Ac Applied to a Transmission Line

When the battery of figure 3-7 is replaced by anac generator (fig. 3-8), each successive instantaneousvalue of the generator voltage is propagated down the

Figure 3-7.—Dc voltage applied to a line.

Figure 3-8.—Ac voltage applied to a line.

line at the speed of light. The action is similar to thewave created by the battery, except the applied voltageis sinusoidal instead of constant. Assume that theswitch is closed at the moment the generator voltageis passing through zero and that the next half cyclemakes point A positive. At the end of one cycle ofgenerator voltage, the current and voltage distributionwill be as shown in figure 3-8.

In this illustration the conventional lines of forcerepresent the electric fields. For simplicity, themagnetic fields are not shown. Points of charge areindicated by plus (+) and minus (-) signs, the largersigns indicating points of higher amplitude of bothvoltage and current. Short arrows indicate directionof current (electron flow). The waveform drawn belowthe transmission line represents the voltage (E) andcurrent (I) waves. The line is assumed to be infinitein length so there is no reflection. Thus, travelingsinusoidal voltage and current waves continually travelin phase from the generator toward the load, or farend of the line. Waves traveling from the generatorto the load are called INCIDENT WAVES. Wavestraveling from the load back to the generator are calledREFLECTED WAVES and will be explained in laterparagraphs.

STANDING-WAVE RATIO

The measurement of standing waves on a transmis-sion line yields information about equipment operating

3-5

conditions. Maximum power is absorbed by the loadwhen ZL = Z0. If a line has no standing waves, thetermination for that line is correct and maximum powertransfer takes place.

You have probably noticed that the variation ofstanding waves shows how near the rf line is to beingterminated in Z0. A wide variation in voltage alongthe length means a termination far from Z0. A smallvariation means termination near Z0. Therefore, theratio of the maximum to the minimum is a measureof the perfection of the termination of a line. Thisratio is called the STANDING-WAVE RATIO (swr)and is always expressed in whole numbers. Forexample, a ratio of 1:1 describes a line terminated inits characteristic impedance (Z0).

Voltage Standing-Wave Ratio

The ratio of maximum voltage to minimum voltageon a line is called the VOLTAGE STANDING-WAVERATIO (vswr). Therefore:

The vertical lines in the formula indicate that theenclosed quantities are absolute and that the two valuesare taken without regard to polarity, Depending onthe nature of the standing waves, the numerical valueof vswr ranges from a value of 1 (ZL = Z0, no standingwaves) to an infinite value for theoretically completereflection. Since there is always a small loss on aline, the minimum voltage is never zero and the vswris always some finite value. However, if the vswris to be a useful quantity. the power losses along theline must be small in comparison to the transmittedpower.

voltage. Since power is proportional to the squareof the voltage, the ratio of the square of the maximumand minimum voltages is called the power stand-ing-wave ratio. In a sense, the name is misleadingbecause the power along a transmission line does notvary.

Current Standing-Wave Ratio

The ratio of maximum to minimum current alonga transmission line is called CURRENT STAND-ING- WAVE RATIO (iswr). Therefore:

This ratio is the same as that for voltages. It can beused where measurements are made with loops thatsample the magnetic field along a line. It gives thesame results as vswr measurements.

TRANSMISSION MEDIUMS

The Navy uses many different types of TRANS-MISSION MEDIUMS in its electronic applications.Each medium (line or waveguide) has a certaincharacteristic impedance value, current-carryingcapacity, and physical shape and is designed to meeta particular requirement.

The five types of transmission mediums that wewill discuss in this topic include PARALLEL-LINE,TWISTED PAIR, SHIELDED PAIR, COAXIALLINE, and WAVEGUIDES. The use of a particularline depends, among other things, on the appliedfrequency, the power-handling capabilities, and thetype of installation.

Power Standing-Wave Ratio Parallel Line

The square of the vswr is called the POWER One type of parallel line is the TWO-WIRE OPEN

STANDING-WAVE RATIO (pswr). Therefore: LINE, illustrated in figure 3-9. This line consists oftwo wires that are generally spaced from 2 to 6 inchesapart by insulating spacers. This type of line is mostoften used for power lines, rural telephone lines, andtelegraph lines. It is sometimes used as a transmission

This ratio is useful because the instruments used to line between a transmitter and an antenna or between

detect standing waves react to the square of the an antenna and a receiver. An advantage of this type

3-6

Figure 3-9.—Two-wire open

of line is its simple construction.

line.

The principaldisadvantages of this type of line are the high radiationlosses and electrical noise pickup because of the lackof shielding. Radiation losses are produced by thechanging fields created by the changing current in eachconductor.

Another type of parallel line is the TWO-WIRERIBBON (TWIN LEAD) LINE, illustrated in figure3-10. This type of transmission line is commonly usedto connect a television receiving antenna to a hometelevision set. This line is essentially the same as thetwo-wire open line except that uniform spacing isassured by embedding the two wires in a low-lossdielectric, usually polyethylene. Since the wires areembedded in the thin ribbon of polyethylene, thedielectric space is partly air and partly polyethylene.

Twisted Pair

The TWISTED PAIR transmission line is illustratedin figure 3-11. As the name implies, the line consistsof two insulated wires twisted together to form aflexible line without the use of spacers. It is not usedfor transmitting high frequency because of the highdielectric losses that occur in the rubber insulation.When the line is wet, the losses increase greatly.

Figure 3-10.—Two-wire ribbon line.

Figure 3-11.—Twisted pair.

Shielded Pair

The SHIELDED PAIR, shown in figure 3-12,consists of parallel conductors separated from eachother and surrounded by a solid dielectric. Theconductors are contained within a braided coppertubing that acts as an electrical shield. The assemblyis covered with a rubber or flexible compositioncoating that protects the line from moisture andmechanical damage. Outwardly, it looks much likethe power cord of a washing machine or refrigerator.

Figure 3-12.—Shielded pair.

The principal advantage of the shielded pair is thatthe conductors are balanced to ground; that is, thecapacitance between the wires is uniform throughoutthe length of the line. This balance is due to theuniform spacing of the grounded shield that surroundsthe wires along their entire length. The braided coppershield isolates the conductors from stray magneticfields.

Coaxial Lines

There are two types of COAXIAL LINES, RIGID(AIR) COAXIAL LINE and FLEXIBLE (SOLID)COAXIAL LINE. The physical construction of bothtypes is basically the same; that is, each contains twoconcentric conductors.

3-7

The rigid coaxial line consists of a central, insulatedwire (inner conductor) mounted inside a tubular outerconductor. This line is shown in figure 3-13. In someapplications, the inner conductor is also tubular. Theinner conductor is insulated from the outer conductorby insulating spacers or beads at regular intervals.The spacers are made of Pyrex, polystyrene, or someother material that has good insulating characteristicsand low dielectric losses at high frequencies.

Figure 3-13.—Air coaxial line.

The chief advantage of the rigid line is its abilityto minimize radiation losses. The electric and magneticfields in a two-wire parallel line extend into space forrelatively great distances and radiation losses occur.However, in a coaxial line no electric or magneticfields extend outside of the outer conductor. The fieldsare confined to the space between the two conductors,resulting in a perfectly shielded coaxial line. Anotheradvantage is that interference from other lines isreduced.

The rigid line has the following disadvantages:(1) it is expensive to construct; (2) it must be keptdry to prevent excessive leakage between the twoconductors; and (3) although high-frequency lossesare somewhat less than in previously mentioned lines,they are still excessive enough to limit the practicallength of the line.

Leakage caused by the condensation of moistureis prevented in some rigid line applications by the useof an inert gas, such as nitrogen, helium, or argon.It is pumped into the dielectric space of the line ata pressure that can vary from 3 to 35 pounds persquare inch. The inert gas is used to dry the line whenit is first installed and pressure is maintained to ensurethat no moisture enters the line.

Flexible coaxial lines (fig. 3-14) are made withan inner conductor that consists of flexible wireinsulated from the outer conductor by a solid,continuous insulating material. The outer conductoris made of metal braid, which gives the line flexibility.Early attempts at gaining flexibility involved usingrubber insulators between the two conductors.However, the rubber insulators caused excessive lossesat high frequencies.

Figure 3-14.—Flexible coaxial line.

Because of the high-frequency losses associatedwith rubber insulators, polyethylene plastic wasdeveloped to replace rubber and eliminate these losses.Polyethylene plastic is a solid substance that remainsflexible over a wide range of temperatures. It isunaffected by seawater, gasoline, oil, and most otherliquids that may be found aboard ship. The use ofpolyethylene as an insulator results in greaterhigh-frequency losses than the use of air as aninsulator. However, these losses are still lower thanthe losses associated with most other solid dielectricmaterials.

This concludes our study of transmission lines.The rest of this chapter will be an introduction intothe study of waveguides.

WAVEGUIDE THEORY

The two-wire transmission line used in conventionalcircuits is inefficient for transferring electromagneticenergy at microwave frequencies. At these frequencies,energy escapes by radiation because the fields are notconfined in all directions, as illustrated in figure 3-15.Coaxial lines are more efficient than two-wire linesfor transferring electromagnetic energy because thefields are completely confined by the conductors, asillustrated in figure 3-16. Waveguides are the most

3-8

efficient way to transfer electromagnetic energy.WAVEGUIDES are essentially coaxial lines withoutcenter conductors. They are constructed fromconductive material and may be rectangular, circular,or elliptical in shape, as shown in figure 3-17.

Figure 3-15.—Fields confined in two directions only.

Figure 3-16.—Fields confined in all directions.

WAVEGUIDE ADVANTAGES

Waveguides have several advantages over two-wireand coaxial transmission lines. For example, the largesurface area of waveguides greatly reduces COPPER(12R) LOSSES. Two-wire transmission lines have largecopper losses because they have a relatively smallsurface area. The surface area of the outer conductor

Figure 3-17.—Waveguide shapes.

of a coaxial cable is large, but the surface area of theinner conductor is relatively small. At microwavefrequencies, the current-carrying area of the inner con-ductor is restricted to a very small layer at thesurface of the conductor by an action called SKINEFFECT.

Skin effect tends to increase the effective resistanceof the conductor. Although energy transfer in coaxialcable is caused by electromagnetic field motion, themagnitude of the field is limited by the size of thecurrent-carrying area of the inner conductor. The smallsize of the center conductor is even further reducedby skin effect, and energy transmission by coaxialcable becomes less efficient than by waveguides.DIELECTRIC LOSSES are also lower in waveguidesthan in two-wire and coaxial transmission lines.Dielectric losses in two-wire and coaxial lines arecaused by the heating of the insulation between theconductors. The insulation behaves as the dielectricof a capacitor formed by the two wires of thetransmission line. A voltage potential across the twowires causes heating of the dielectric and results ina power loss. In practical applications, the actualbreakdown of the insulation between the conductorsof a transmission line is more frequently a problemthan is the dielectric loss.

This breakdown is usually caused by stationaryvoltage spikes or “nodes,” which are caused bystanding waves. Standing waves are stationary andoccur when part of the energy traveling down the line

3-9

is reflected by an impedance mismatch with the load.The voltage potential of the standing waves at thepoints of greatest magnitude can become large enoughto break down the insulation between transmissionline conductors.

The dielectric in waveguides is air, which has amuch lower dielectric loss than conventional insulatingmaterials. However, waveguides are also subject todielectric breakdown caused by standing waves.Standing waves in waveguides cause arcing, whichdecreases the efficiency of energy transfer and canseverely damage the waveguide. Also since theelectromagnetic fields are completely contained withinthe waveguide, radiation losses are kept very low.

Power-handling capability is another advantageof waveguides. Waveguides can handle more powerthan coaxial lines of the same size becausepower-handling capability is directly related to thedistance between conductors. Figure 3-18 illustratesthe greater distance between conductors in awaveguide.

Figure 3-18.—Comparisonand a circular waveguide.

of spacing in coaxial cable

In view of the advantages of waveguides, youwould think that waveguides should be the only typeof transmission lines used. However, waveguides havecertain disadvantages that make them practical for useonly at microwave frequencies.

WAVEGUIDE DISADVANTAGES

Physical size is the primary lower-frequencylimitation of waveguides. The width of a waveguidemust be approximately a half wavelength at thefrequency of the wave to be transported. For example,a waveguide for use at 1 megahertz would be about700 feet wide. This makes the use of waveguides atfrequencies below 1000 megahertz increasinglyimpractical. The lower frequency range of any systemusing waveguides is limited by the physical dimensionsof the waveguides.

Waveguides are difficult to install because of theirrigid, hollow-pipe shape. Special couplings at thejoints are required to assure proper operation. Also,the inside surfaces of waveguides are often plated withsilver or gold to reduce skin effect losses. Theserequirements increase the costs and decrease thepracticality of waveguide systems at any other thanmicrowave frequencies.

DEVELOPING THE WAVEGUIDEFROM PARALLEL LINES

You may better understand the transition fromordinary transmission line concepts to waveguidetheories by considering the development of awaveguide from a two-wire transmission line. Figure3-19 shows a section of a two-wire transmission linesupported on two insulators. At the junction with theline, the insulators must present a very high impedanceto ground for proper operation of the line. A lowimpedance insulator would obviously short-circuit theline to ground, and this is what happens at very highfrequencies. Ordinary insulators display the character-istics of the dielectric of a capacitor formed by thewire and ground. As the frequency increases, the

overall impedance decreases. A better high-frequencyinsulator is a quarter-wave section of transmissionline shorted at one end. Such an insulator is shownin figure 3-20. The impedance of a shorted quar-ter-wave section is very high at the open-end junctionwith the two-wire transmission line. This type ofinsulator is known as a METALLIC INSULATORand may be placed anywhere along a two-wire line.

3-10

Figure 3-19.—Two-wire transmission line.

Figure 3-20.—Quarter-wave section of transmissionline shorted at one end.

Note that quarter-wave sections are insulators at onlyone frequency. This severely limits the bandwidth,efficiency, and application of this type of two-wireline.

Figure 3-21 shows several metallic insulators oneach side of a two-wire transmission line. As moreinsulators are added, each section makes contact withthe next, and a rectangular waveguide is formed. Thelines become part of the walls of the waveguide, asillustrated in figure 3-22. The energy is thenconducted within the hollow waveguide instead ofalong the two-wire transmission line.

The comparison of the way electromagnetic fieldswork on a transmission line and in a waveguide isnot exact. During the change from a two-wire lineto a waveguide, the electromagnetic field configurationsalso undergochanges, the

many changes. As a result of thesewaveguide does not actually operate

Figure 3-21.—Metallic insulator on each sidetwo-wire line.

Figure 3-22.—Forming a waveguide by addingquarter-wave sections.

like a two-wire line that is completely shuntedquarter-wave sections. If it did, the use of a wave-guide would be limited to a single-frequency wavelength that was four times the length of the quarter-wave sections. In fact, waves of this length cannotpass efficiently through waveguides. Only a smallrange of frequencies of somewhat shorter wavelength(higher frequency) can pass efficiently.

of a

by

3-11

As shown in figure 3-23, the widest dimensionof a waveguide is called the “a” dimension anddetermines the range of operating frequencies. Thenarrowest dimension determines the power-handlingcapability of the waveguide and is called the “b”dimension.

Figure 3-23.—Labeling waveguide dimensions,

NOTE: This method of labeling waveguides isnot standard in all texts, Different methods may beused in other texts on microwave principles, but thismethod is in accordance with Navy Military Standards(MIL-STDS).

In theory, a waveguide could function at an infinitenumber of frequencies higher than the designedfrequency; however, in practice, an upper frequencylimit is caused by modes of operation, which will bediscussed later.

If the frequency of a signal is decreased so muchthat two quarter-wavelengths are longer than the widedimension of a waveguide, energy will no longer passthrough the waveguide. This is the lower frequencylimit, or CUTOFF FREQUENCY of a givenwaveguide. In practical applications, the widedimension of a waveguide is usually 0.7 wavelengthat the operating frequency. This allows the waveguideto handle a small range of frequencies both above andbelow the operating frequency. The “b” dimensionis governed by the breakdown potential of thedielectric, which is usually air. Dimensions rangingfrom 0.2 to 0.5 wavelength are common for the “b”sides of a waveguide.

ENERGY PROPAGATION INWAVEGUIDES

Since energy is transferred through waveguidesby electromagnetic fields, you need a basic understand-ing of field theory. Both electric (E FIELD) andmagnetic fields (H FIELD) are present in waveguides,and the interaction of these fields causes energy totravel through the waveguide. This action is bestunderstood by first looking at the properties of thetwo individual fields.

E Field

An electric field exists when a difference ofpotential causes a stress in the dielectric between twopoints. The simplest electric field is one that formsbetween the plates of a capacitor when one plate ismade positive compared to the other, as shown in viewA of figure 3-24. The stress created in the dielectricis an electric field.

Electric fields are represented by arrows that pointfrom the positive toward the negative potential. Thenumber of arrows shows the relative strength of thefield. In view B, for example, evenly spaced arrowsindicate the field is evenly distributed. For ease ofexplanation, the electric field is abbreviated E field,and the lines of stress are called E lines.

H Field

The magnetic field in a waveguide is made up ofmagnetic lines of force that are caused by current flowthrough the conductive material of the waveguide.Magnetic lines of force, called H lines, are continuousclosed loops, as shown in figure 3-25. All of the Hlines associated with current are collectively calleda magnetic field or H field. The strength of the Hfield, indicated by the number of H lines in a givenarea, varies directly with the amount of current.

Although H lines encircle a single, straight wire,they behave differently when the wire is formed intoa coil, as shown in figure 3-26. In a coil the individualH lines tend to form around each turn of wire. Since

3-12

Figure 3-24.—Simple electric fields.

Figure 3-25.—Magnetic field on a single wire.

the H lines take opposite directions between adjacentturns, the field between the turns is canceled. Insideand outside the coil, where the direction of each Hfield is the same, the fields join and form continuousH lines around the entire coil. A similar action takesplace in a waveguide.

Figure 3-26.—Magnetic field on a coil.

BOUNDARY CONDITIONS INA WAVEGUIDE

The travel of energy down a waveguide is similar,but not identical, to the travel of electromagnetic wavesin free space. The difference is that the energy in a

waveguide is confined to the physical limits of theguide. Two conditions, known as BOUNDARYCONDITIONS, must be satisfied for energy to travelthrough a waveguide.

The first boundary condition (illustrated in fig.3-27, view A can be stated as follows:

For an electric field to exist at the surfaceof a conductor, it must be perpendicularto the conductor.

Thein view

Figure 3-27.—E field boundary condition.

opposite of this boundary condition, shownB, is also true. An electric field CANNOT

exist parallel to a perfect conductor.

The second boundary condition, which is illustratedin figure 3-28, can be stated as follows:

For a varying magnetic field to exist, it mustform closed loops in parallel with theconductors and be perpendicular to theelectric field.

3-13

Figure 3-28.—H field boundary condition.

Since an E field causes a current flow that in turnproduces an H field, both fields always exist at thesame time in a waveguide. If a system satisfies oneof these boundary conditions, it must also satisfy theother since neither field can exist alone.

WAVEFRONTS WITHIN AWAVEGUIDE

Electromagnetic energy transmitted into spaceconsists of electric and magnetic fields that are at rightangles (90 degrees) to each other and at right anglesto the direction of propagation. A simple analogy toestablish this relationship is by use of the right-handrule for electromagnetic energy, based on thePOYNTING VECTOR. It indicates that a screw(right-hand thread) with its axis perpendicular to theelectric and magnetic fields will advance in thedirection of propagation if the E field is rotated tothe right (toward the H field). This rule is illustratedin figure 3-29.

Figure 3-29.—The Poynting vector.

The combined electric and magnetic fields forma wavefront that can be represented by alternatenegative and positive peaks at half-wavelength

intervals, as illustrated in figure 3-30. Angle isthe direction of travel of the wave with respect to somereference axis.

Figure 3-30.—Wavefronts in space.

The reflection of a single wavefront off the “b”wall of a waveguide is shown in figure 3-31. Thewavefront is shown in view A as small particles, Inviews B and C particle 1 strikes the wall and isbounced back from the wall without losing velocity.If the wall is perfectly flat, the angle at which it thewall, known as the angle of incidence is the sameas the angle of reflection An instant after particle1 strikes the wall, particle 2 strikes the wall, as shown

Figure 3-31.—Reflection of a single wavefront.

3-14

in view C, and reflects in the same manner. Becauseall the particles are traveling at the same velocity,particles 1 and 2 do not change their relative positionwith respect to each other. Therefore, the reflectedwave has the same shape as the original. Theremaining particles as shown in views D, E, and Freflect in the same manner. This process results ina reflected wavefront identical in shape, but oppositein polarity, to the incident wave.

Figure 3-32, views A and B, each illustrate thedirection of propagation of two different electromag-netic wavefronts of different frequencies being radiatedinto a waveguide by a probe. Note that only thedirection of propagation is indicated by the lines andarrowheads. The wavefronts are at right angles tothe direction of propagation. The angle of incidence

and the angle of reflection of the wavefrontsvary in size with the frequency of the input energy,but the angles of reflection are equal to each otherin a waveguide. The CUTOFF FREQUENCY in awaveguide is a frequency that would cause angles ofincidence and reflection to be perpendicular to thewalls of the guide. At any frequency below the cutofffrequency, the wavefronts will be reflected back andforth across the guide (setting up standing waves) andno energy will be conducted down the waveguide.

Figure 3-32.—Different frequencies in a waveguide.

The velocity of propagation of a wave along awaveguide is less than its velocity through free space(speed of light). This lower velocity is caused by thezigzag path taken by the wavefront. Theforward-progress velocity of the wavefront in awaveguide is called GROUP VELOCITY and issomewhat slower than the speed of light.

The group velocity of energy in a waveguide isdetermined by the reflection angle of the wavefrontsoff the “b” walls. The reflection angle is determinedby the frequency of the input energy. This basicprinciple is illustrated in figure 3-33. As frequencyis decreased. the reflection angle increases, causingthe group velocity to decrease. The opposite is alsotrue; increasing frequency increases the group velocity.

Figure 3-33.—Reflection angle at various frequencies.

WAVEGUIDE MODES OFOPERATION

The waveguide analyzed in the previous paragraphsyields an electric field configuration known as thehalf-sine electric distribution. This configuration,called a MODE OF OPERATION, is shown in figure3-34. Recall that the strength of the field is indicatedby the spacing of the lines; that is, the closer the lines,the stronger the field. The regions of maximumvoltage in this field move continuously down thewaveguide in a sine-wave pattern. To meet boundaryconditions. the field must always be zero at the “b”walls.

3-15

Figure 3-34.—Half-sine E field distribution.

The half-sine field is only one of many fieldconfigurations, or modes, that can exist in a rectangularwaveguide. A full-sine field can also exist in arectangular waveguide because, as shown in figure3-35, the field is zero at the “b” walls.

Figure 3-35.—Full-sine E field distribution.

The magnetic field in a rectangular waveguide isin the form of closed loops parallel to the surface ofthe conductors. The strength of the magnetic fieldis proportional to the electric field. Figure 3-36illustrates the magnetic field pattern associated witha half-sine electric field distribution. The magnitudeof the magnetic field varies in a sine-wave patterndown the center of the waveguide in “time phase” withthe electric field. TIME PHASE means that the peakH lines and peak E lines occur at the same instant intime, although not necessarily at the same point alongthe length of the waveguide.

The dominant mode is the most efficient mode.Waveguides are normally designed so that only thedominant mode will be used. To operate in thedominant mode, a waveguide must have an “a” (wide)dimension of at least one half-wavelength of thefrequency to be propagated. The “a” dimension ofthe waveguide must be kept near the minimumallowable value to ensure that only the dominant modewill exist. In practice, this dimension is usually 0.7wavelength.

Figure 3-36.—Magnetic field caused by a half-sineE field.

Of the possible modes of operation available fora given waveguide, the dominant mode has the lowestcutoff frequency. The high-frequency limit of arectangular waveguide is a frequency at which its “a”dimension becomes large enough to allow operationin a mode higher than that for which the waveguidehas been designed.

Circular waveguides are used in specific areas ofradar and communications systems, such as rotatingjoints used at the mechanical point where the antennasrotate. Figure 3-37 illustrates the dominant mode ofa circular waveguide. The cutoff wavelength of acircular guide is 1.71 times the diameter of thewaveguide. Since the “a” dimension of a rectangularwaveguide is approximately one half-wavelength atthe cutoff frequency, the diameter of an equivalentcircular waveguide must be 2/1.71, or approximately

Figure 3-37.—Dominant mode in a circularwaveguide.

3-16

1.17 times the “a” dimension of a rectangularwaveguide.

MODE NUMBERING SYSTEMS

So far, only the most basic types of E and H fieldarrangements have been shown. More complicatedarrangements are often necessary to make possiblecoupling, isolation, or other types of operation. Thefield arrangements of the various modes of operationare divided into two categories: TRANSVERSEELECTRIC (TE) and TRANSVERSE MAGNETIC(TM).

In the transverse electric (TE) mode, the entireelectric field is in the transverse plane, which isperpendicular to the waveguide, (direction of energytravel). Part of the magnetic field is parallel tothe length axis.

In the transverse magnetic (TM) mode, theentire magnetic field is in the transverse plane andhas no portion parallel to the length axis.

Since there are several TE and TM modes,subscripts are used to complete the description of thefield pattern. In rectangular waveguides, the first

subscript indicates the number of half-wave patternsin the “a” dimension, and the second subscript indicatesthe number of half-wave patterns in the “b” dimension.

The dominant mode for rectangular waveguidesis shown in figure 3-38. It is designated as the TEmode because the E fields are perpendicular to the“a” walls. The first subscript is 1, since there is onlyone half-wave pattern across the “a” dimension. There

Figure 3-38.—Dominant mode in a rectangular

are no E-field patterns across the “b” dimension, sothe second subscript is 0. The complete modedescription of the dominant mode in rectangularwaveguides is TE1,0. Subsequent description ofwaveguide operation in this text will assume thedominant (TE1,0) mode unless otherwise noted.

A similar system is used to identify the modes ofcircular waveguides. The general classification of TEand TM is true for both circular and rectangularwaveguides. In circular waveguides the subscriptshave a different meaning. The first subscript indicatesthe number of fill-wave patterns around the circumfer-ence of the waveguide. The second subscript indicatesthe number of half-wave patterns across the diameter.

In the circular waveguide in figure 3-39, the Efield is perpendicular to the length of the waveguidewith no E lines parallel to the direction of propagation.Thus, it must be classified as operating in the TEmode. If you follow the E line pattern in a counter-clockwise direction starting at the top, the E linesgo from zero, through maximum positive (tail ofarrows), back to zero, through maximum negative(head of arrows), and then back to zero again. Thisis one full wave, so the first subscript is 1. Alongthe diameter, the E lines go from zero throughmaximum and back to zero, making a half-wavevariation. The second subscript, therefore, is also 1.TE1,1 is the complete mode description of the dominantmode in circular waveguides. Several modes arepossible in both circular and rectangular waveguides.Figure 3-40 illustrates several different modes thatcan be used to verify the mode numbering system.

Figure 3-39.—Counting wavelengths in a circularwaveguide.

waveguide.

3-17

Figure 3-40.—Various modes of operation for rectangular and circular waveguides.

WAVEGUIDE INPUT/OUTPUTMETHODS

A waveguide, as explained earlier in this topic,operates differently from an ordinary transmission line.Therefore, special devices must be used to put energyinto a waveguide at one end and remove it from theother end.

The three devices used to injector remove energyfrom waveguides are PROBES, LOOPS, and SLOTS.Slots may also be called APERTURES or WINDOWS.

When a small probe is inserted into a waveguideand supplied with microwave energy, it acts as aquarter-wave antenna. Current flows in the probe andsets up an E field such as the one shown in figure3-41, view A. The E lines detach themselves fromthe probe. When the probe is located at the point ofhighest efficiency, the E lines set up an E field ofconsiderable intensity.

The most efficient place to locate the probe is inthe center of the “a” wall, parallel to the “b” wall, andone quarter-wavelength from the shorted end of thewaveguide, as shown in figure 3-41, views B and

C. This is the point at which the E field is maximumin the dominant mode. Therefore, energy transfer(coupling) is maximum at this point. Note that thequarter-wavelength spacing is at the frequency requiredto propagate the dominant mode.

In many applications a lesser degree of energytransfer, called loose coupling, is desirable. Theamount of energy transfer can be reduced by decreasingthe length of the probe, by moving it out of the centerof the E field, or by shielding it. Where the degreeof coupling must be varied frequently, the probe ismade retractable so the length can be easily changed.

The size and shape of the probe determines itsfrequency, bandwidth, and power-handling capability.As the diameter of a probe increases, the bandwidthincreases. A probe similar in shape to a door knobis capable of handling much higher power and a largerbandwidth than a conventional probe. The greaterpower-handling capability is directly related to theincreased surface area. Two examples o fbroad-bandwidth probes are illustrated in figure 3-41,view D. Removal of energy from a waveguide is

simply a reversal of the injection process using thesame type of probe.

Another way of injecting energy into a waveguideis by setting up an H field in the waveguide. Thiscan be accomplished by inserting a small loop thatcarries a high current into the waveguide, as shownin figure 3-42, view A. A magnetic field builds uparound the loop and expands to fit the waveguide, asshown in view B. If the frequency of the current inthe loop is within the bandwidth of the waveguide,energy will be transferred to the waveguide.

3-18

Figure 3-41.—Probe coupling in a rectangular waveguide.

Figure 3-42.—Loop coupling in a rectangularwaveguide.

For the most efficient coupling to the waveguide,the loop is inserted at one of several points where themagnetic field will be of greatest strength. Four ofthose points are shown in figure 3-42, view C.

When less efficient coupling is desired, you canrotate or move the loop until it encircles a smallernumber of H lines. When the diameter of the loopis increased, its power-handling capability alsoincreases. The bandwidth can be increased byincreasing the size of the wire used to make the loop.

When a loop is introduced into a waveguide inwhich an H field is present, a current is induced inthe loop. When this condition exists, energy isremoved from the waveguide.

Slots or apertures are sometimes used when veryloose (inefficient) coupling is desired, as shown infigure 3-43. In this method energy enters througha small slot in the waveguide and the E field expandsinto the waveguide. The E lines expand first acrossthe slot and then across the interior of the waveguide.

3-19

Figure 3-43.—Slot coupling in a waveguide.

Minimum reflections occur when energy is injectedor removed if the size of the slot is properly propor-tioned to the frequency of the energy.

After learning how energy is coupled into and outof a waveguide with slots, you might think that leavingthe end open is the most simple way of injecting orremoving energy in a waveguide. This is not the case,however, because when energy leaves a waveguide,fields form around the end of the waveguide. Thesefields cause an impedance mismatch which, in turn,causes the development of standing waves and a drasticloss in efficiency. Various methods of impedancematching and terminating waveguides will be coveredin the next section.

WAVEGUIDE IMPEDANCEMATCHING

Waveguide transmission systems are not alwaysperfectly impedance matched to their load devices.The standing waves that result from a mismatch causea power loss, a reduction in power-handling capability,and an increase in frequency sensitivity. Imped-ance-changing devices are therefore placed in thewaveguide to match the waveguide to the load. Thesedevices are placed near the source of the standingwaves.

Figure 3-44 illustrates three devices, called irises,that are used to introduce inductance or capacitanceinto a waveguide. An iris is nothing more than a metalplate that contains an opening through which the wavesmay pass. The iris is located in the transverse planeof either the magnetic or electric field.

An inductive iris and its equivalent circuit areillustrated in figure 3-44, view A. The iris places ashunt inductive reactance across the waveguide thatis directly proportional to the size of the opening.Notice that the inductive iris is in the magnetic plane.The shunt capacitive reactance, illustrated in viewB, basically acts the same way. Again, the reactanceis directly proportional to the size of the opening, butthe iris is placed in the electric plane. The iris,illustrated in view C, has portions in both the magnetic

Figure 3-44.—Waveguide irises.

3-20

and electric transverse planes and forms an equivalentparallel-LC circuit across the waveguide. At theresonant frequency, the iris acts as a high shuntresistance. Above or below resonance, the iris actsas a capacitive or inductive reactance.

POSTS and SCREWS made from conductivematerial can be used for impedance-changing devicesin waveguides. Views A and B of figure 3-45,illustrate two basic methods of using posts and screws.A post or screw that only partially penetrates into thewaveguide acts as a shunt capacitive reactance. Whenthe post or screw extends completely through thewaveguide, making contact with the top and bottomwalls, it acts as an inductive reactance. Note that whenscrews are used, the amount of reactance can be varied.

Figure 3-45.—Conducting posts and screws.

WAVEGUIDE TERMINATIONS

Electromagnetic energy is often passed througha waveguide to transfer the energy from a source intospace. As previously mentioned, the impedance ofa waveguide does not match the impedance of space,and without proper impedance matching standing wavescause a large decrease in the efficiency of thewaveguide.

Any abrupt change in impedance causes standingwaves, but when the change in impedance at the endof a waveguide is gradual, almost no standing wavesare formed. Gradual changes in impedance can beobtained by terminating the waveguide with afunnel-shaped HORN, such as the three types illustratedin figure 3-46. The type of horn used depends uponthe frequency and the desired radiation pattern.

Figure 3-46.—Waveguide horns.

As you may have noticed, horns are really simpleantennas. They have several advantages over otherimpedance-matching devices, such as their largebandwidth and simple construction.

A waveguide may also be terminated in a resistiveload that is matched to the characteristic impedanceof the waveguide. The resistive load is most oftencalled a DUMMY LOAD, because its only purposeis to absorb all the energy in a waveguide withoutcausing standing waves.

There is no place on a waveguide to connect afixed termination resistor; therefore, several specialarrangements are used to terminate waveguides. Onemethod is to fill the end of the waveguide with agraphite and sand mixture, as illustrated in figure 3-47,view A. When the fields enter the mixture, theyinduce a current flow in the mixture that dissipatesthe energy as heat. Another method (view B) is touse a high-resistance rod placed at the center of theE field. The E field causes current to flow in the rod,and the high resistance of the rod dissipates the energyas a power loss, again in the form of heat.

Still another method for terminating a waveguideis the use of a wedge of highly resistive material, asshown in view C of figure 3-47. The plane of thewedge is placed perpendicular to the magnetic lines

3-21

Figure 3-47.—Terminating waveguides.

of force. When the H lines cut through the wedge,current flows in the wedge and causes a power loss.As with the other methods, this loss is in the formof heat. Since very little energy reaches the end ofthe waveguide, reflections are minimum.

All of the terminations discussed so far aredesigned to radiate or absorb the energy withoutreflections. In many instances, however, all of theenergy must be reflected from the end of thewaveguide. The best way to accomplish this is topermanently weld a metal plate at the end of thewaveguide, as shown in view D of figure 3-47.

WAVEGUIDE PLUMBING

Since waveguides are really only hollow metalpipes, the installation and the physical handling ofwaveguides have many similarities to ordinaryplumbing. In light of this fact, the bending, twisting,joining, and installation of waveguides is commonlycalled waveguide plumbing. Naturally, waveguidesare different in design from pipes that are designed

to carry liquids or other substances. The design ofa waveguide is determined by the frequency and powerlevel of the electromagnetic energy it will carry. Thefollowing paragraphs explain the physical factorsinvolved in the design of waveguides.

Waveguide Bends

The size, shape, and dielectric material of awaveguide must be constant throughout its length forenergy to move from one end to the other withoutreflections. Any abrupt change in its size or shapecan cause reflections and a loss in overall efficiency.When such a change is necessary, the bends, twists,and joints of the waveguides must meet certainconditions to prevent reflections.

Waveguides maybe bent in several ways that donot cause reflections. One way is the gradual bendshown in figure 3-48. This gradual bend is knownas an E bend because it distorts the E fields. The Ebend must have a radius greater than two wavelengthsto prevent reflections.

Figure 3-48.—Gradual E bend.

Another common bend is the gradual H bend (fig.3-49). It is called an H bend because the H fieldsare distorted when a waveguide is bent in this manner.Again, the radius of the bend must be greater thantwo wavelengths to prevent reflections. Neither theE bend in the “a” dimension nor the H bend in the“b” dimension changes the normal mode of operation.

Figure 3-49.—Gradual H bend.

3-22

A sharp bend in either dimension may be usedif it meets certain requirements. Notice the two45-degree bends in figure 3-50; the bends are 1/4λapart. The reflections that occur at the 45-degree bendscancel each other, leaving the fields as though noreflections have occurred.

Figure 3-50.—Sharp bends.

Sometimes the electromagnetic fields must berotated so that they are in the proper phase to matchthe phase of the load. This may be accomplished bytwisting the waveguide as shown in figure 3-51. Thetwist must be gradual and greater than

Figure 3-51.—Waveguide twist.

The flexible waveguide (fig. 3-52) allows specialbends, which some equipment applications mightrequire. It consists of a specially wound ribbon ofconductive material, the most commonly used is brass,with the inner surface plated with chromium. Powerlosses are greater in the flexible waveguide becausethe inner surfaces are not perfectly smooth. Therefore,it is only used in short sections where no otherreasonable solution is available.

Waveguide Joints

beSince an entire waveguide system cannot possibly

molded into one piece, the waveguide must be

Figure 3-52.—Flexible waveguide.

constructed in sections and the sections connected withjoints. The three basic types of waveguide joints arethe PERMANENT, the SEMIPERMANENT, and theROTATING JOINTS. Since the permanent joint isa factory-welded joint that requires no maintenance,only the semipermanent and rotating joints will bediscussed.

Sections of waveguide must be taken apart formaintenance and repair. A semipermanent joint, calleda CHOKE JOINT, is most commonly used for thispurpose. The choke joint provides good electromag-netic continuity between the sections of the waveguidewith very little power loss.

A cross-sectional view of a choke joint is shownin figure 3-53. The pressure gasket shown betweenthe two metal surfaces forms an airtight seal. Noticein view B that the slot is exactly from the “a”wall of the waveguide. The slot is also deep,as shown in view A, and because it is shorted at point1, a high impedance results at point 2. Point 3 is from point 2. The high impedance at point 2 resultsin a low impedance, or short, at point 3. This effectcreates a good electrical connection between the twosections that permits energy to pass with very littlereflection or loss.

Whenever a stationary rectangular waveguide isto be connected to a rotating antenna, a rotating jointmust be used. A circular waveguide is normally usedin a rotating joint. Rotating a rectangular waveguidewould cause field pattern distortion. The rotatingsection of the joint, illustrated in figure 3-54, uses achoke joint to complete the electrical connection withthe stationary section. The circular waveguide isdesigned so that it will operate in the TM0,1 mode.

3-23

The rectangular sections are attached as shown in theillustration to prevent the circular waveguide fromoperating in the wrong mode. Distance “O” is so that a high impedance will be presented to anyunwanted modes. This is the most common designused for rotating joints, but other types may be usedin specific applications.

WAVEGUIDE MAINTENANCE

The installation of a waveguide system presentsproblems that are not normally encountered whendealing with other types of transmission lines. Theseproblems often fall within the technician’s area ofresponsibility. A brief discussion of waveguidehandling, installation, and maintenance will helpprepare you for this maintenance responsibility,Detailed information concerning waveguide mainte-nance in a particular system may be found in thetechnical manuals for the system.

Figure 3-53.—Choke joint.

Figure 3-54.—Rotating joint.

Since a waveguide naturally has a low loss ratio,most losses in a waveguide system are caused by otherfactors. Improperly connected joints or damaged innersurfaces can decrease the efficiency of a system tothe point that it will not work at all. Therefore, youmust take great care when working with waveguidesto prevent physical damage. Since waveguides aremade from a soft, conductive material, such as copperor aluminum, they are very easy to dent or deform.Even the slightest damage to the inner surface of awaveguide will cause standing waves and, often,internal arcing. Internal arcing causes further damageto the waveguide in an action that is oftenself-sustaining until the waveguide is damaged beyonduse. Part of your job as a technician will be to inspectthe waveguide system for physical damage. Thepreviously mentioned dents are only one type ofphysical damage that can decrease the efficiency ofthe system. Another problem occurs becausewaveguides are made from a conductive material suchas copper while the structures of most ships are madefrom steel. When two dissimilar metals, such ascopper and steel, are in direct contact, an electricalaction called ELECTROLYSIS takes place that causesvery rapid corrosion of the metals. Waveguides canbe completely destroyed by electrolytic corrosion ina relatively short period of time if they are not isolatedfrom direct contact with other metals. Any inspection

3-24

of a waveguide system should include a detailedinspection of all support points to ensure that electro-lytic corrosion is not taking place. Any waveguidethat is exposed to the weather should be painted andall joints sealed. Proper painting prevents naturalcorrosion, and sealing the joints prevents moisture fromentering the waveguide.

Moisture can be one of the worst enemies of awaveguide system. As previously discussed, thedielectric in waveguides is air, which is an excellentdielectric as long as it is free of moisture. Wet air,however, is a very poor dielectric and can cause seriousinternal arcing in a waveguide system. For this reason,care is taken to ensure that waveguide systems arepressurized with air that is dry. Checking the pressureand moisture content of the waveguide air may be oneof your daily system maintenance duties.

More detailed waveguide installation and mainte-nance information can be found in the technicalmanuals that apply to your particular system. Anothergood source is the Electronics Installation andMaintenance Handbooks (EIMB) published by NavalSea Systems Command. Installation Standards (EIMB)Handbook, NAVSEA 0967-LP-000-0110, is the volumethat deals with waveguide installation and maintenance.

WAVEGUIDE DEVICES

The discussion of waveguides, up to this point,has been concerned only with the transfer of energyfrom one point to another. Many waveguide deviceshave been developed, however, that modify the energyin some fashion during the transmission. Some devicesdo nothing more than change the direction of theenergy. Others have been designed to change the basiccharacteristics or power level of the electromagneticenergy.

This section will explain the basic operatingprinciples of some of the more common waveguidedevices, such as DIRECTIONAL COUPLERS,CAVITY RESONATORS, and HYBRID JUNCTIONS.

Directional Couplers

The directional coupler is a device that providesa method of sampling energy from within a waveguide

for measurement or use in another circuit. Mostcouplers sample energy traveling in one direction only.However, directional couplers can be constructed thatsample energy in both directions. These are calledBIDIRECTIONAL couplers and are widely used inradar and communications systems.

Directional couplers may be constructed in manyways. The coupler illustrated in figure 3-55 isconstructed from an enclosed waveguide section ofthe same dimensions as the waveguide in which theenergy is to be sampled. The “b” wall of this enclosedsection is mounted to the “b” wall of the waveguidefrom which the sample will be taken. There are twoholes in the “b” wall between the sections of thecoupler. These two holes are apart. The uppersection of the directional coupler has a wedge-o fenergy-absorbing material at one end and a pickupprobe connected to an output jack at the other end.The absorbent material absorbs the energy not directedat the probe and a portion of the overall energy thatenters the section.

Figure 3-55.—Directional coupler.

Figure 3-56 illustrates two portions of the incidentwavefront in a waveguide. The waves travel downthe waveguide in the direction indicated and enter thecoupler section through both holes. Since both portionsof the wave travel the same distance, they are in phasewhen they arrive at the pickup probe. Because thewaves are in phase, they add together and provide asample of the energy traveling down the waveguide.The sample taken is only a small portion of the energythat is traveling down the waveguide. The magnitudeof the sample, however, is proportional to themagnitude of the energy in the waveguide. Theabsorbent material is designed to ensure that the ratio

3-25

between the sample energy and the energy in thewaveguide is constant. Otherwise, the sample wouldcontain no useful information. The ratio is usuallystamped on the coupler in the form of an attenuationfactor.

and the probe are in opposite positions from thedirectional coupler designed to sample the incidentenergy. This positioning causes the two portions ofthe reflected energy to arrive at the probe in phase,providing a sample of the reflected energy. Thetransmitted energy is absorbed by the absorbentmaterial.

Figure 3-56.—Incident wave in a directional couplerdesigned to sample incident waves.

The effect of a directional coupler on any reflectedenergy is illustrated in figure 3-57. Note that thesetwo waves do not travel the same distance to thepickup probe. The wave represented by the dottedline travels further and arrives at the probe 180degrees out of phase with the wave, represented bythe solid line. Because the waves are 180 degreesout of phase at the probe, they cancel each other andno energy is induced into the pickup probe. Whenthe reflected energy arrives at the absorbent material,it adds and is absorbed by the material.

Figure 3-57.—Reflected wave in a directionalcoupler.

A directional coupler designed to sample reflectedenergy is shown in figure 3-58. The absorbent material

Figure 3-58.—Directional coupler designed to sampleretlected energy.

A simple bidirectional coupler for sampling bothtransmitted and reflected energy can be constructedby mounting two directional couplers on opposite sidesof a waveguide, as shown in figure 3-59.

Figure 3-59.—Bidirectional coupler.

Resonators

definition, a resonant cavity is any space

Cavity

Bycompletely enclosed by conducting- walls that cancontain oscillating electromagnetic fields and possessresonant properties. The cavity has many advantages

3-26

and uses at microwave frequencies. Resonant cavitieshave a very high Q and can be built to handlerelatively large amounts of power. Cavities with aQ value in excess of 30,000 are not uncommon. Thehigh Q gives these devices a narrow bandpass andallows very accurate tuning. Simple, rugged construc-tion is an additional advantage.

Although cavity resonators, built for differentfrequency ranges and applications, have a variety ofshapes, the basic principles of operation are the samefor all.

One example of a cavity resonator is the rectangularbox shown in figure 3-60, view A. It may be thoughtof as a section of rectangular waveguide closed at bothends by conducting plates. The frequency at whichthe resonant mode occurs is of the distancebetween the end plates. The magnetic field patternsin the rectangular cavity are shown in view B.

There are two variables that determine the primaryfrequency of any resonant cavity. The first variableis PHYSICAL SIZE. In general, the smaller thecavity, the higher its resonant frequency. The secondcontrolling factor is the SHAPE of the cavity. Figure3-61 illustrates several cavity shapes that are commonlyused. Remember from the previously stated definitionof a resonant cavity that any completely enclosedconductive surface, regardless of its shape, can actas a cavity resonator.

Energy can be inserted or removed from a cavityby the same methods that are used to couple energyinto and out of waveguides. The operating principlesof probes, loops, and slots are the same whether usedin a cavity or a waveguide. Therefore, any of the threemethods can be used with cavities to inject or removeenergy.

The resonant frequency of a cavity can be variedby changing any of the three parameters: cavityvolume, cavity capacitance, or cavity inductance.Changing the frequencies of a cavity is known asTUNING. The mechanical methods of tuning a cavitymay vary with the application, but all methods usethe same electrical principles.

Figure 3-60.—Rectangular waveguide cavityresonator.

Waveguide Junctions

You may have assumed that when energy travelingdown a waveguide reaches a junction it simply dividesand follows the junction. This is not strictly true.

3-27

Figure 3-61.—Types of cavities.

Different types of junctions affect the energy individed into two basic types, the E TYPE and the H

different ways. Since waveguide junctions are usedTYPE. HYBRID JUNCTIONS are more complicated

extensively in most systems, you need to understanddevelopments of the basic T junctions. The MAGIC-T

the basic operating principles of those most commonlyand the HYBRID RING are the two most commonly

used.used hybrid junctions.

The T JUNCTION is the most simple of theE-TYPE T JUNCTION.— An E-type T junction

commonly used waveguide junctions. T junctions areis illustrated in figure 3-62, view A.

Figure 3-62.—E fields in an E-type T junction.

3-28

It is called an E-type T junction because the junctionarm extends from the main waveguide in the samedirection as the E field in the waveguide.

Figure 3-62, view B, illustrates cross-sectionalviews of the E-type T junction with inputs fed intothe various arms. For simplicity, the magnetic linesthat are always present with an electric field have beenomitted. In view K, the input is fed into arm b andthe outputs are taken from the a and c arms. Whenthe E field arrives between points 1 and 2, point 1becomes positive and point 2 becomes negative. Thepositive charge at point 1 then induces a negativecharge on the wall at point 3. The negative chargeat point 2 induces a positive charge at point 4. Thesecharges cause the fields to form 180 degrees out ofphase in the main waveguide; therefore, the outputswill be 180 degrees out of phase with each other.In view L, two in-phase inputs of equal amplitude arefed into the a and c arms. The signals at points 1 and

2 have the same phase and amplitude. No differenceof potential exists across the entrance to the b arm,and no energy will be coupled out. However, whenthe two signals fed into the a and c arms are 180degrees out of phase, as shown in view M, points1 and 2 have a difference of potential. This differenceof potential induces an E field from point 1 to point2 in the b arm, and energy is coupled out of this arm.Views N and P illustrate two methods of obtainingtwo outputs with only one input.

H-TYPE T JUNCTION.— An H-type T junctionis illustrated in figure 3-63, view A. It is called anH-type T junction because the long axis of the “b”arm is parallel to the plane of the magnetic lines offorce in the waveguide. Again, for simplicity, onlythe E lines are shown in this figure. Each X indicatesan E line moving away from the observer. Each dotindicates an E line moving toward the observer.

Figure 3-63.—E field in an H-type T junction.

3-29

In view 1 of figure 3-63, view B, the signal is fedinto arm b and in-phase outputs are obtained fromthe a and c arms. In view 2, in-phase signals are fedinto arms a and c and the output signal is obtainedfrom the b arm because the fields add at the junctionand induce E lines into the b arm. If180-degree-out-of-phase signals are fed into arms aand c, as shown in view 3, no output is obtained fromthe b arm because the opposing fields cancel at thejunction. If a signal is fed into the a arm, as shownin view 4 , outputs will be obtained from the b andc arms. The reverse is also true. If a signal is fedinto the c arm, outputs will be obtained from the aand b arms.

MAGIC-T HYBRID JUNCTION.— A simpli-fied version of the magic-T hybrid junction is shownin figure 3-64. The magic-T is a combination of theH-type and E-type T junctions. The most commonapplication of this type of junction is as the mixersection for microwave radar receivers.

Figure 3-64.—Magic-T hybrid junction.

If a signal is fed into the b arm of the magic-T,it will divide into two out-of-phase components. Asshown in figure 3-65, view A, these two componentswill move into the a and c arms. The signal enteringthe b arm will not enter the d arm because of the zeropotential existing at the entrance of the d arm. Thepotential must be zero at this point to satisfy theboundary conditions of the b arm. This absence ofpotential is illustrated in views B and C where themagnitude of the E field in the b arm is indicated bythe length of the arrows. Since the E lines are atmaximum in the center of the b arm and minimumat the edge where the d arm entrance is located, nopotential difference exists across the mouth of the darm.

Figure 3-65.—Magic-T with input to arm b.

In summary, when an input is applied to arm bof the magic-T hybrid junction, the output signals fromarms a and c are 180 degrees out of phase with eachother, and no output occurs at the d arm.

The action that occurs when a signal is fed intothe d arm of the magic-T is illustrated in figure 3-66.As with the H-type T junction, the signal entering thed arm divides and moves down the a and c arms asoutputs that are in phase with each other and with theinput. The shape of the E fields in motion is shownby the numbered curved slices. As the E field movesdown the d arm, points 2 and 3 are at an equalpotential. The energy divides equally into arms a andc, and the E fields in both arms become identical inshape. Since the potentials on both sides of the b armare equal, no potential difference exists at the entranceto the b arm, resulting in no output.

3-30

Figure 3-66.—Magic-T with input to arm d.

When an input signal is fed into the a arm asshown in figure 3-67, a portion of the energy iscoupled into the b arm as it would be in an E-typeT junction. An equal portion of the signal iscoupled through the d arm because of the action ofthe H-type junction. The c arm has two fieldsacross it that are out of phase with each other.Therefore, the fields cancel, resulting in no outputat the c arm. The reverse of this action takes placeif a signal is fed into the c arm, resulting inoutputs at the b and d arms and no output at the aarm.

Figure 3-67.—Magic-T with input to arm a.

Unfortunately, when a signal is applied to anyarm of a magic-T, the flow of energy in the outputarms is affected by reflections. Reflections arecaused by impedance mismatching at thejunctions. These reflections are the cause of thetwo major disadvantages of the magic-T. First, thereflections represent a power loss since all theenergy fed into the junction does not reach theload that the arms feed. Second, the reflectionsproduce standing waves that can result in internalarcing. Thus, the maximum power a magic-T canhandle is greatly reduced.

Reflections can be reduced by using somemeans of impedance matching that does not

destroy the shape of the junctions. One method isshown in figure 3-68. A post is used to match theH plane, and an iris is used to match the E plane.Even though this method reduces reflections, itlowers the power-handling capability even further.

Figure 3-68.—Magic-T impedance matching.

HYBRID RING.— A type of hybrid junctionthat overcomes the power limitation of the magic-T is the hybrid ring, also called a RAT RACE. Thehybrid ring, illustrated in figure 3-69, view A, isactually a modification of the magic-T. It isconstructed of rectangular waveguides moldedinto a circular pattern. The arms are joined to thecircular waveguide to form E-type T junctions.View B shows, in wavelengths, the dimensionsrequired for a hybrid ring to operate properly.

The hybrid ring is used primarily in high-powered radar and communications systems toperform two functions. During the transmitperiod, the hybrid ring couples microwave energyfrom the transmitter to the antenna and allows noenergy to reach the receiver. During the receivecycle, the hybrid ring couples energy from theantenna to the receiver and allows no energy toreach the transmitter. Any device that performsboth of these functions is called a DUPLEXER. Aduplexer permits a system to use the sameantenna for both transmitting and receiving.

SUMMARY

This concludes our discussion on transmissionlines and waveguides. In this volume you havebeen given a basic introduction on wavepropagation from the time it leaves thetransmitter to the point of reception. In volume 8you will be introduced to a variety of electronicsupport systems.

3-31

Figure 3-69.—Hybrid ring with wavelengthmeasurements.

3-32

APPENDIX I

GLOSSARY

ABSORPTION—(1) Absorbing light waves. Doesnot allow any reflection or refraction; (2)Atmospheric absorption of rf energy with noreflection or refraction (adversely affects long-distance communications).

ACOUSTICS—The science of sound.

AMPLITUDE—The portion of a cycle measured froma reference line to a maximum value above (orto a maximum value below) the line.

ANGLE OF INCIDENCE—The angle between theincident wave and the normal.

ANGLE OF REFLECTION—The angle betweenthe reflected wave and the normal.

ANGLE OF REFRACTION—The angle betweenthe normal and the path of a wave through thesecond medium.

ANGSTROM UNIT—The unit used to define thewavelength of light waves.

ANISOTROPIC—The property of a radiator to emitstrong radiation in one direction.

ANTENNA—A conductor or set of conductors usedeither to radiate rf energy into space or to collectrf energy from space.

APERTURE—See SLOT.

ARRAY OF ARRAYS—See COMBINATIONARRAY.

BAY—Part of an antenna array.

BEARING—An angular measurement that indicatesthe direction of an object in degrees from truenorth. Also called azimuth.

BEVERAGE ANTENNA—A horizontal, longwireantenna designed for reception and transmissionof low-frequency, vertically polarized groundwaves. Also known as WAVE ANTENNA.

BIDIRECTIONAL ARRAY—An array that radiatesin opposite directions along the line of maximumradiation.

BROADSIDE ARRAY—An array in which thedirection of maximum radiation is perpendicularto the plane containing the elements.

BOUNDARY CONDITIONS—The two conditionsthat the E-field and H-field within a waveguidemust meet before energy will travel down thewaveguide. The E-field must be perpendicularto the walls and the H-field must be in closedloops, parallel to the walls, and perpendicular tothe E-field.

CAVITY RESONATOR—A space totally enclosedby a metallic conductor and supplied with energyin such a way that it becomes a source ofelectromagnetic oscillations. The size and shapeof the enclosure determine the resonant frequency.

CENTER-FEED METHOD—Connecting the centerof an antenna to a transmission line, which is thenconnected to the final (output) stage of thetransmitter. Also known as CURRENT-FEEDMETHOD.

CHARACTERISTIC IMPEDANCE—The ratio ofvoltage to current at any given point on atransmission line. Represented by a value of

impedance.

CHOKE JOINT—A joint between two sections ofwaveguide that provides a good electricalconnection without power losses or reflections.

AI-1

COAXIAL LINE—A type of transmission line that

contains two concentric conductors.

COLLINEAR ARRAY—An array with all the

elements in a straight line. Maximum radiation

is perpendicular to the axis of the elements.

COMBINATION ARRAY—An array system that

uses the characteristics of more than one array.

Also known as ARRAY OF ARRAYS.

COMPLEX WAAE—A wave produced by combiningtwo or more pure tones at the same time.

CONDUCTANCE—The opposite of resistance in

transmission lines. The minute amount of

resistance that is present in the insulator of a

transmission line.

CONNECTED ARRAY—see DRIVEN ARRAY

COPPER LOSS—Power loss in copper conductors

caused by the internal resistance of the conductors

to current flow. Also know as 12R LOSS.

CORNER-REFLECTOR ANTENNA—A half-wave

antenna with a reflector consisting of two flat

metal surfaces meeting at an angle behind the

radiator.

COUNTERPOISE—A network of wire that is

connected to a quarter-wave antenna at one end

and provides the equivalent of an additional ¼

wavelength.

COUPLING DEVICE—A coupling coil that con-

nects the transmitter to the feeder.

CREST (TOP)—The peak of the positive alternation

(maximum value above the line) of a wave.

CRITICAL ANGLE—The maximum angle at which

radio waves can be transmitted and still be

refracted back to earth.

CRITICAL FREQUENCY—The maximum fre-

quency at which a radio wave can be transmitted

vertically and still be refracted back to earth.

CURRENT-FEED METHOD—See CENTER-FEEDMETHOD.

C U R R E N T S T A N D I N G - W A V E R A T I O(ISWR)—The ratio of maximum to minimumcurrent along a transmission line.

CUTOFF FREQUENCY—The frequency at whichthe attenuation of a waveguide increases sharplyand below which a traveling wave in a givenmode cannot be maintained. A frequency witha half wavelength that is greater than the widedimension of a waveguide.

CYCLE—One complete alternation of a sine wavethat has a maximum value above and a maximumvalue below the reference line.

DAMPING—Reduction of energy by absorption.

DENSITY—(1) The compactness of a substance;(2) Mass per unit volume.

DETECTOR—The device that responds to a waveor disturbance.

DIELECTRIC HEATING—The heating of aninsulating material by placing it in a highfrequency electric field.

DIELECTRIC LOSSES—The losses resulting fromthe heating effect on the dielectric materialbetween conductors.

DIELECTRIC CONSTANT—The ratio of a givendielectric to the dielectric value of a vacuum.

DIFFRACTION—The bending of the paths of waveswhen the waves meet some form of obstruction.

DIPOLE—A common type of half-wave antennamade from a straight piece of wire cut in half.Each half operates at a quarter wavelength of theoutput.

AI-2

DIRECTIONAL.—Radiation that varies with direction.

DIRECTIONAL COUPLER—A device that samplesthe energy traveling in a waveguide for use inanother circuit.

DIRECTOR—The parasitic element of an array thatreinforces energy coming from the driver towarditself.

DIRECTIVITY—The property of an array that causesmore radiation to take place in certain directionsthan in others.

DISTRIBUTED CONSTANTS—The constants ofinductance, capacitance, and resistance in atransmission line. The constants are spread alongthe entire length of the line and cannot bedistinguished separately.

DOMINANT MODE—The easiest mode to producein a waveguide, and also, the most efficient modein terms of energy transfer.

DOPPLER EFFECT—The apparent change infrequency or pitch when a sound source moveseither toward or away from a listener.

DOUBLET—Another name for the dipole antenna.

DRIVEN ARRAY—An array in which all of theelements are driven. Also known as CON-NECTED ARRAY

DRIVEN ELEMENT—An element of an antenna(transmitting or receiving) that is connecteddirectly to the transmission line.

DUMMY LOAD—A device used at the end of atransmission line or waveguide to converttransmitted energy into heat so no energy isradiated outward or reflected back.

E-FIELD—Electric field that exists when a differencein electrical potential causes a stress in thedielectric between two points. AlSO known asELECTRIC FIELD.

E-TYPE T-JUNCTION—A waveguide junction inwhich the junction arm extends from the main

waveguide in the same direction as the E-fieldin the waveguide.

ECHO—The reflection of the original sound waveas it bounces off a distant surface.

ELECTROMAGNETIC FIELD—The combinationof an electric (E) field and a magnetic (H) field.

ELECTROMAGNETIC INTERFERENCE—Man-made or natural interference that degrades thequality of reception of radio waves.

E L E C T R O M A G N E T I C R A D I A T I O N — T h eradiation of radio waves into space.

ELECTRIC FIELD—See E-FIELD.

ELEMENT—A part of an antenna that can be eitheran active radiator or a parasitic radiator.

END-FEED METHOD—Connecting one end of anantenna through a capacitor to the final outputstage of a transmitter. Also known asVOLTAGE-FEED METHOD.

END-FIRE ARRAY—An array in which the directionof radiation is parallel to the axis of the array.

ELEVATION ANGLE—The angle between the lineof sight to an object and the horizontal plane.

FADING—Variations in signal strength by atmo-spheric conditions.

FEEDER—A transmission line that carries energyto the antenna.

FLAT LINE—A transmission line that has nostanding waves. This line requires no specialtuning device to transfer maximum power.

FLEXIBLE COAXIAL LINE— coaxial line madewith a flexible inner conductor insulated fromthe outer conductor by a solid, continuousinsulating material.

FOLDED DIPOLE—An ordinary half-wave antenna(dipole) that has one or more additional conduc-tors connected across the ends parallel to eachother.

AI-3

FOUR-ELEMENT ARRAY—An array with threeparasitic elements and one driven element.

FREE-SPACE LOSS—The loss of energy of a radiowave because of the spreading of the wavefrontas it travels from the transmitter.

FREQUENCY—The number of cycles that occur inone second. Usually expressed in Hertz.

FREQUENCY DIVERSITY—Transmitting (andreceiving) of radio waves on two differentfrequencies simultaneously.

FRONT-TO-BACK RATIO—The ratio of the energyradiated in the principal direction to the energyradiated in the opposite direction.

FUNDAMENTAL FREQUENCY—The bas icfrequency or first harmonic frequency.

GAIN—The ratio between the amount of energypropagated from an antenna that is directionalto the energy from the same antenna that wouldbe propagated if the antenna were not directional.

GENERATOR END—See INPUT END

GROUND PLANE—The portion of a groundplaneantenna that acts as ground.

GROUND-PLANE ANTENNA—A type of antennathat uses a ground plane as a simulated groundto produce low-angle radiation.

GROUND REFLECTION LOSS—The loss of rfenergy each time a radio wave is reflected fromthe earth’s surface.

GROUND SCREEN—A series of conductors buriedbelow the surface of the earth and arranged ina radial pattern. Used to reduce losses in theground.

GROUND WAVES—Radio waves that travel near thesurface of the earth.

GROUP VELOCITY—The forward progress velocityof a wave front in a waveguide.

H-FIELD—Any space or region in which a magneticforce is exerted. The magnetic field may beproduced by a current-carrying coil or conductor,by a permanent magnet, or by the earth itself.Also known as MAGNETIC FIELD.

H-TYPE T-JUNCTION—A waveguide junction inwhich the junction arm is parallel to the magneticlines of force in the main waveguide.

HALF-WAVE DIPOLE ANTENNA—An antennaconsisting of two rods (¼ wavelength h) in astraight line, that radiates electromagnetic energy.

HARMONIC—A frequency that is a whole numbermultiple of a smaller base frquency.

HERTZ ANTENNA—A half-wave antenna installedsome distance above ground and positioned eithervertically or horizontally.

HORN—A funnel-shaped section of waveguide usedas a termination device and as a radiating antenna.

HORIZONTAL AXIS—On a graph, the straight lineaxis plotted from left to right.

HORIZONTAL PATTERN—The part of a radiationpattern that is radiated in all directions along thehorizontal plane.

HORIZONTALLY POLARIZED—Waves that areradiated with their E-field component parallelto the earth’s surface.

HYBRID JUNCTION—A waveguide junction thatcombines two or more basic T-junctions.

HYBRID RING—A hybrid-waveguide junction thatcombines a series of E-type T-junctions in a ringconfiguration.

I2R LOSS—See COPPER LOSS.

AI-4

INCIDENT WAVE—(1) The wave that strikes thesurface of a medium; (2) The wave that travelsfrom the sending end to the receiving end of atransmission line.

INDUCTION FIELD—The electromagnetic fieldproduced about an antenna when current andvoltage are present on the same antenna.

INDUCTION LOSSES—The losses that occur whenthe electromagnetic field around a conductor cutsthrough a nearby metallic object and induces acurrent into that object.

INPUT END—The end of a two-wire transmissionline that is connected to a source. Also knownas a GENERATOR END or a TRANSMITTEREND.

INPUT IMPEDANCE—The impedance presentedto the transmitter by the transmission line andits load.

INTERFERENCE—Any disturbance that producesan undesirable response or degrades a wave.

IONOSPHERE—The most important region of theatmosphere extending from 31 miles to 250 milesabove the earth. Contains four cloud-like layersthat affect radio waves.

IONOSPHERIC STORMS—Disturbances in theearth’s magnetic field that make communicationspractical only at lower frequencies.

IONIZATION—The process of upsetting electricalneutrality.

IRIS—A metal plate with an opening through whichelectromagnetic waves may pass. Used as animpedance matching device in waveguides.

ISOTROPIC RADIATION—The radiation of energyequally in all directions.

LEAKAGE CURRENT—The small amount ofcurrent that flows between the conductors of atransmission line through the dielectric.

LOAD END—See OUTPUT END.

LOAD ISOLATOR—A passive attenuator in whichthe loss in one direction is much greater than thatin the opposite direction. An example is a ferriteisolator for waveguides that allow energy to travelin only one direction.

LOADING—See LUMPED-IMPEDANCE TUNING.

LOBE—An area of a radiation pattern plotted on apolar-coordinate graph that represents maximumradiation.

LONG-WIRE ANTENNA—An antenna that is awavelength or more long at its operating fre-quency.

LONGITUDINAL WAVES—Waves in which thedisturbance (back and forth motion) takes placein the direction of propagation. Sometimes called

compression waves.

LOOP—(1) The curves of a standing wave or antennathat represent amplitude of current or voltage;

(2) A curved conductor that connects the endsof a coaxial cable or other transmission line andprojects into a waveguide or resonant cavity forthe purpose of injecting or extracting energy.

LOWEST USABLE FREQUENCY—The minimumoperating frequency that can be used for commu-nications between two points.

LUMPED CONSTANTS—The propert ies o finductance, capacitance, and resistance in atransmission line.

LUMPED-IMPEDANCE TUNING—The inser-tion of an inductor or capacitor in series with anantenna to lengthen or shorten the antennaelectrically. Also known as LOADING.

LOOSE COUPLING—Inefficient coupling of energyfrom one circuit to another that is desirable insome applications. Also called weak coupling.

AI-5

MAGIC-T JUNCTION—A combination of theH-type and E-type T-junctions.

MAGNETIC FIELD—See H-FIELD.

MAJOR LOBE—The lobe in which the greatestamount of radiation occurs.

MARCONI ANTENNA—A quarter-wave antennaoriented perpendicular to the earth and operatedwith one end grounded. Also known asQUARTER-WAVE ANTENNA.

MAXIMUM USABLE FREQUENCY—Maximumfrequency that can be used for communicationsbetween two locations for a given time of dayand a given angle of incidence.

MEDIUM—The substance through which a wavetravels from one point to the next. Air, water,wood, etc., are examples of a medium.

METALLIC INSULATOR—A shorted quarter-wavesection of transmission line.

MICROWAVE REGION—The portion of theelectromagnetic spectrum from 1,000 megahertzto 100,000 megahertz.

MINOR LOBE—The lobe in which the radiationintensity is less than a major lobe.

MULTIELEMENT ARRAY—An array consistingof one or more arrays and classified as todirectivity.

MULTIELEMENT PARASITIC ARRAY—An arraythat contains two or more parasitic elements anda driven element.

MULTIPATH—The multiple paths a radio wave mayfollow between transmitter and receiver.

NEGATIVE ALTERNATION—The portion of asine wave below the reference line.

NODE—The fixed minimum points of voltage orcurrent on a standing wave or antenna.

NONDIRECTIONAL—See OMNIDIRECTIONAL,

NONRESONANT LINE—A transmission line thathas no standing waves of current or voltage.

NORMAL—The imaginary line perpendicular to thepoint at which the incident wave strikes thereflecting surface. Also called the perpendicular.

NULL—On a polar-coordinate graph, the area thatrepresents minimum or 0 radiation.

OMNIDIRECTIONAL—Transmitting in all direc-tions. Also known as NONDIRECTIONAL.

OPEN-ENDED LINE—A transmission line that hasan infinitely large terminating impedance.

OPTIMUM WORKING FREQUENCY—The mostpractical operating frequency that can be usedwith the least amount of problems; roughly 85percent of the maximum usable frequency.

ORIGIN—The point on a graph where the verticaland horizontal axes cross each other.

OUTPUT END—The end of a transmission line thatis opposite the source. Also known as RECEIV-ING END.

OUTPUT IMPEDANCE—The impedance presentedto the load by the transmission line and its source.

PARALLEL RESONANT CIRCUIT—A circuit thatacts as a high impedance at resonance.

PARALLEL-WIRE—A type of transmission lineconsisting of two parallel wires.

PARASITIC ARRAY—An array that has one ormore parasitic elements.

PARASITIC ELEMENT—The passive element ofan antenna array that is connected to neither thetransmission line nor the driven element.

PERIOD—The amount of time required for comple-tion of one full cycle.

AI-6

PHASE SHIFTER—A device used to change thephase relationship between two ac signals.

PLANE OF POLARIZATION—The plane (verticalor horizontal) with respect to the earth in whichthe E-field propagates.

POSITIVE ALTERNATION—The portion of asine wave above the reference line.

POWER GAIN—The ratio of the radiated powerof an antenna compared to the output power ofa standard antenna. A measure of antennaefficiency usually expressed in decibels. Alsoreferred to as POWER RATIO.

POWER LOSS—The heat loss incurrent flows through it.

a conductor as

POWER RATIO—See POWER GAIN.

P O W E R S T A N D I N G — W A V E R A T I O(PSWR)—The ratio of the square of the maxi-mum and minimum voltages of a transmissionline.

PROPAGATION—Waves traveling through amedium.

PROBE—A metal rod that projects into, but isinsulated from, a waveguide or resonant cavityand used to inject or extract energy.

QUARTER-WAVE ANTENNA—See MARCONIANTENNA.

RADIATION FIELD—The electromagnetic field thatdetaches itself from an antenna and travelsthrough space.

RADIATION LOSSES—The losses that occur whenmagnetic lines of force about a conductor areprojected into space as radiation and are notreturned to the conductor as the cycle alternates.

RADIATION PATTERN—A plot of the radiatedenergy from an antenna.

RADIATION RESISTANCE—The resistance, which

if inserted in place of an antenna, would consume

the same amount of power as that radiated by

the antenna.

RADIO FREQUENCIES—Electromagnetic frequen-

cies that fall between 3 kilohertz and 300

gigahertz and are used for radio communications.

RADIO HORIZON—The boundary beyond thenatural horizon in which radio waves cannot bepropagated over the earth’s surface.

RADIO WAVE—(1) A form of radiant energy that

can neither be seen nor felt; (2) An electromag-

netic wave generated by a transmitter.

RAREFIED WAVE—A longitudinal wave that hasbeen expanded or rarefied (made less dense) as

it moves away from the source.

RECEIVER—The object that responds to a wave or

disturbance. Same as detector.

RECEIVING ANTENNA—The device used to pick

up an rf signal from space.

RECEIVING END—See OUTPUT END.

RECIPROCITY—The ability of an antenna to bothtransmit and receive electromagnetic energy with

equal efficiency.

REFLECTED WAVE—(1) The wave that reflects

back from a medium; (2) Waves traveling from

the load back to the generator on a transmission

line; (3) The wave moving back to the sending

end of a transmission line after reflection has

occurred.

REFLECTION WAVES—Waves that are neither

transmitted nor absorbed, but are reflected from

the surface of the medium they encounter.

AI-7

REFLECTOR—The parasitic element of an arraythat causes maximum energy radiation in adirection toward the driven element.

REFRACTION—The changing of direction as a waveleaves one medium and enters another mediumof a different density.

REFRACTIVE INDEX—The ratio of the phasevelocity of a wave in free space to the phasevelocity of the wave in a given substance(dielectric).

RERADIATION—The reception and retransmissionof radio waves caused by turbulence in thetroposphere.

RESONANCE—The condition produced when thefrequency of vibrations are the same as the naturalfrequency (of a cavity), The vibrations reinforceeach other.

RESONANT LINE—A transmission line that hasstanding waves of current and voltage.

RHOMBIC ANTENNA—A diamond-shaped antennaused widely for long-distance, high-frequencytransmission and reception.

RIGID COAXIAL LINE—A coxial line consist-ing of a central, insulated wire (inner conductor)mounted inside a tubular outer conductor.

ROTATING JOINT—A joint that permits one sec-tion of a transmission line or waveguide to rotatecontinuously with respect to another while passingenergy through the joint. Also called a rotarycoupler.

SCATTER ANGLE—The angle at which thereceiving antenna must be aimed to capture thescattered energy of tropospheric scatter.

SELF-INDUCTION—The phenomenon caused bythe expanding and collapsing fields of an electronthat encircles other electrons and retards themovement of the encircled electrons.

SERIES RESONANT CIRCUIT—A circuit thatacts as a low impedance at resonance.

SHIELDED PAIR—A line consisting of parallelconductors separated from each other andsurrounded by a solid dielectric.

SHORT-CIRCUITED LINE—A transmission linethat has a terminating impedance equal to 0.

SKIN EFFECT—The tendency for alternating currentto concentrate in the surface layer of a conductor.The effect increases with frequency and servesto increase the effective resistance of the conduc-tor.

SKIP DISTANCE—The distance from a transmitterto the point where the sky wave is first returnedto earth.

SKIP ZONE—A zone of silence between the pointwhere the ground wave becomes too weak forreception and the point where the sky wave isfirst returned to earth.

SKY WAVES—Radio waves reflected back to earthfrom the ionosphere.

SLOT—Narrow opening in a waveguide wall usedto couple energy in or out of the waveguide. Alsocalled an APERTURE or a WINDOW.

SOURCE—(1) The object that produces waves ordisturbance; (2) The name given to the end ofa two-wire transmission line that is connectedto a source.

SPACE DIVERSITY—Reception of radio waves bytwo or more antennas spaced some distance apart,

SPACE WAVE—A radio wave that travels directlyfrom the transmitter to the receiver and remainsin the troposphere.

SPECTRUM—(1) The entire range of electromagneticwaves; (2) VISIBLE. The range of electromag-netic waves that stimulate the sense of sight;

AI-8

(3) ELECTROMAGNETIC. The entire range ofelectromagnetic waves arranged in order of theirfrequencies.

SPORADIC E LAYER—Irregular cloud-like patchesof unusually high ionization. Often forms atheights near the normal E-layer.

SPREADER—Insulator used with transmission linesand antennas to keep the parallel wires separated.

STANDING WAVE—The distribution of voltage andcurrent formed by the incident and reflectedwaves, which have minimum and maximumpoints on a resultant wave that appears to standstill.

STANDING-WAVE RATIO (SWR)—The ratio ofthe maximum to the minimum amplitudes ofcorresponding components of a field, voltage,or current along a transmission line or waveguidein the direction of propagation measured at agiven frequency. Measures the perfection of thetermination of the line.

STRATOSPHERE—Located between the troposphereand the ionosphere. Has little effect on radiowaves.

STUB—Short section of a transmission line used tomatch the impedance of a transmission line toan antenna. Can also be used to produce desiredphase relationships between connected elementsof an antenna.

SUDDEN IONOSPHERIC DISTURBANCE—Anirregular ionospheric disturbance that can totallyblank out hf radio communications.

SURFACE WAVE—A radio wave that travels alongthe contours of the earth, thereby being highlyattenuated.

TEMPERATURE INVERSION—The condition inwhich warm air is formed above a layer of coolair that is near the earth’s surface.

THREE-ELEMENT ARRAY—An array with twoparasitic elements (reflector and director) and adriven element.

TRANSMISSION LINE—A device designed to guide

electrical energy from one point to another.

TRANSMITTING ANTENNA—The device used

to send the transmitted signal energy into space.

TRANSMISSION MEDIUMS—The various types

of lines and waveguides used as transmission

lines.

TRANSMITTER END—See INPUT END.

TRANSVERSE WAVE MOTION—The up anddown motion of a wave as the wave moves

outward.

TRANSVERSE ELECTRIC MODE—The entireelectric field in a waveguide is perpendicular to

the wide dimension and the magnetic field is

parallel to the length. Also called the TE mode.

TRANSVERSE MAGNETIC MODE—The entire

magnetic field in a waveguide is perpendicular

to the wide dimension (“a” wall) and some portion

of the electric field is parallel to the length. Also

called the TM mode.

TROPOSPHERE—The portion of the atmosphere

closest to the earth’s surface, where all weather

phenomena take place.

TROPOSPHERIC SCATTER—The propagationof radio waves in the troposphere by means of

scatter.

TROUGH (BOTTOM)—The peak of the negative

alternation (maximum value below the line).

TUNED LINE—Another name for the resonant line.This line uses tuning devices to eliminate the

reactance and to transfer maximum power from

the source to the line.

TURNSTILE ANTENNA—A type of antenna used

in vhf communications that is omnidirectional

AI-9

and consists of two horizontal half-wave antennasmounted at right angles to each other in the samehorizontal plane.

TWISTED PAIR—A line consisting of two insulatedwires twisted together to form a flexible linewithout the use of spacers.

TWO-WIRE OPEN LINE—A parallel line consistingof two wires that are generally spaced from 2to 6 inches apart by insulating spacers.

TWO-WIRE RIBBON (TWIN LEAD)—A parallelline similar to a two-wire open line except thatuniform spacing is assured by embedding the twowires in a low-loss dielectric.

UNIDIRECTIONAL ARRAY—An array that radiatesin only one general direction.

UNTUNED LINE—Another name for the flat ornonresonant line.

V ANTENNA—A bidirectional antenna, shaped likea V, which is widely used for communications.

VELOCITY—The rate at which a disturbance travelsthrough a medium.

VERTICAL AXIS—On a graph, the straight line axisoriented from bottom to top.

VERTICAL PATTERN—The part of a radiationpattern that is radiated in the vertical plane.

VERTICAL PLANE—An imaginary plane that isperpendicular to the horizontal plane.

VERTICALLY POLARIZED—Waves radiated withthe E-field component perpendicular to the earth’ssurface.

VOLTAGE-FEED METHOD—See END-FEEDMETHOD.

V O L T A G E S T A N D I N G - W A V E R A T I O(VSWR)—The ratio of maximum to minimumvoltage of a transmission line.

WAVE ANTENNA—See BEVERAGE ANTENNA.

WAVE MOTION—A recurring disturbance advanc-ing through space with or without the use of aphysical medium.

WAVE TRAIN—A continuous series of waves withthe same amplitude and wavelength.

WAVEFRONT—A small section of an expandingsphere of electromagnetic radiation, perpendicularto the direction of travel of the energy.

WAVEGUIDE—A rectangular, circular, or ellipticalmetal pipe designed to transport electro-magneticwaves through its interior.

W A V E G U I D E M O D E O F O P E R A T I O N —Particular field configuration in a waveguide thatsatisfies the boundary conditions. Usually dividedinto two broad types: the transverse electric (TE)and the transverse magnetic (TM).

WAVEGUIDE POSTS—A rod of conductive materialused as impedance-changing devices inwaveguides.

WAVEGUIDE SCREW—A screw that projects intoa waveguide for the purpose of changing theimpedance.

WAVELENGTH—(1) The distance in space occupiedby 1 cycle of a radio wave at any given instant;(2) The distance a disturbance travels during oneperiod of vibration.

WINDOW—See Slot.

YAGI ANTENNA—A multielement parasitic array.Elements lie in the same plane as those of theend-fire array.

AI-10

APPENDIX II

REFERENCES USED TO DEVELOP THIS TRAMAN

Shipboard Antenna Systems, Vol 1, Communications Antenna Fundamentals, NAVSEA 0967-LP-177-3010, Naval SeaSystems Command, Washington, DC, 1972.

Shipboard Antenna Systems, Vol 2, Instatallation Details Communications Antenna Systems, NAVSEA0967-LP-177-3020, Naval Sea Systems Command, Washington, DC, 1973.

Shipboard Antenna Systems, Vol 3, Antenna Couplers Communications Antenna Systems, NAVSEA0967-LP-177-3030, Naval Sea Systems Command, Washington, DC, 1973.

Shipboard Antenna Systems, Vol 4, Testing and Maintenance Communications Antenna Systems, NAVSEA0967-LP-177-3040, Naval Sea Systems Command, Washington, DC, 1972.

Navy Electricity and Electronics Training Series, Module 10, Introduction to Wave Propagation, Transmission Lines,and Antennas, NAVEDTRA B72-10-00-93, Naval Education and Training Program Management SupportActivity, Pensacola FL, 1993.

Navy Electricity and Electronics Training Series, Module 11, Microwave Principles, NAVEDTRA 172-11-00-87,Naval Education and Training Program Management Support Activity, Pensacola FL, 1987.

Navy UHF Satellite Communication System Description, FSCS-200-83-1, Naval Ocean Systems Center, San Diego,CA, 1991.

AII-1

INDEX

A

Antennas/antenna radiationanisotropic radiation, 2-4characteristics, 2-1counterpoise, 2-5, 2-6directivity, 2-1gain, 2-2ground screen, 2-5, 2-6Hertz antennas, 2-1isotropic radiation, 2-4loading, 2-4lobe, 2-4loop, 2-3low probability of intercept (LPI), 2-19lumped-impedance tuning, 2-5major lobe, 2-4Marconi antennas, 2-1minor lobe, 2-4node, 2-3, 3-9period, 2-3polarization, 2-2reciprocity, 2-1standing wave, 2-3wavelength, 2-3

Atmosphere, 1-1ionosphere, 1-1stratosphere, 1-1temperature inversion, 1-12troposphere, 1-1weather, 1-12

log-periodic (LPA), 2-8, 2-16long-wire, 2-11low frequency (lf), 2-7NORD, 2-9pan polar, 2-8parasitic array, 2-7quadrant, 2-11, 2-13rhombic, 2-11, 2-12rotatable LPA (RLPA), 2-10sector log-periodic array, 2-10Trideco, 2-7tuning system, 2-13ultra high frequency (uhf), 2-14vertical monopole LPA, 2-8, 2-9very high frequency (vhf), 2-14very low frequency (vlf), 2-6whip, 2-13, 2-14wire rope fan, 2-14Yagi, 2-7, 2-9

Coupler groups, 2-21 to 23coupler group AN/SRA-33, 2-22coupler group AN/SRA-39, 2-23coupler group AN/SRA-40, 2-23coupler group AN/SRA-49, 2-23coupler group AN/SRA-49A, 2-23coupler group AN/SRA-50, 2-23coupler group AN/SRA-56, 2-21, 2-22coupler group AN/SRA-57, 2-21, 2-22coupler group AN/SRA-58, 2-21, 2-22coupler group AN/URA-38, 2-21, 2-23multicoupler (receive filter) AN/SR4-12, 2-23multicoupler OA-9123/SRC, 2-22

C

ICommunications antennas, 2-6

biconical dipole, 2-15boom, 2-10center-fed dipole, 2-16coaxial dipole, 2-16, 2-17conical monopole, 2-11discage, 2-14, 2-15Goliath, 2-7ground plane, 2-10, 2-13high frequency (hf), 2-7inverted cone, 2-10, 2-11

Ionosphere, 1-1D layer, 1-3E layer, 1-4F/F1/F2 layer, 1-4ionization 1-2ionized layers, 1-2, 1-3ionospheric storms, 1-11ions, 1-2regular variations, 1-1seasonal variations, 1-10

INDEX-1

solar flare, 1-4sporadic E, 1-4, 1-11sudden ionospheric disturbances (SID), 1-11sunspot activity. 1-4, 1-10sunspots cycles, 1-11

M

Matching networks, 2-20antenna couplers, 2-21antenna tuners, 2-20antenna tuning, 2-21receive distribution system, 2-22

P

Propagation, 1-4angle of incidence 1-6, 3-14angle of reflection 3-14critical angle, 1-6critical frequency, 1-5, 1-11escape point, 1-5fading, 1-7frequency diversity. 1-8layer density, 1-5multipath fading, 1-8reflection, 1-7refraction, 1-4selective fading, 1-8skip distance, 1-7skip zone, 1-7sky wave, 1-7space diversity, 1-8

R

Radar antennas, 2-23AN/GPN-27 (ASR-8), 2-26AN/SPN-35A, 2-29AS-1292/TPN-8, 2-29AS-32631SPS-49(V), 2-27azimuth pulse generator (APG), 2-27broadside array, 2-25carrier-controlled approach (CCA), 2-29corner reflector, 2-24, 2-25cylindrical paraboloid, 2-24, 2-25feedhorn, 2-26focal point, 2-23height-finding, 2-25

horn radiator, 2-25, 2-26OE-172/SPS-55, 2-28orange-peel paraboloid, 2-24, 2-25parabolic reflector, 2-23paraboloid, 2-24truncated paraboloid, 2-24AS-1669/SPN-35, 2-29, 2-30

RF safety, 2-30dielectric heating, 2-30radiation warning signs, 2-31rf burns, 2-30working aloft, 2-32

S

Satellite communications antennas, 2-16AN/WSC-6(V), 2-19, 2-20Andrew 58622, 2-19AS-2815/SRR-1, 2-16, 2-17backplane, 2-17, 2-19OE-82A/WSC-1(V), 2-17, 2-19OE-82B/WSC-1(V), 2-16, 2-180E-82C/WSC-1(V), 2-16, 2-18

T

Transmission, 1-12absorption, 1-14freespace losses, 1-13frequency selection, 1-13ground reflection losses, 1-13lowest usable frequency (luf), 1-13maximum usable frequency (muf), 1-13optimum working frequency (fot), 1-14plane wavefront, 2-23wavefront, 1-13, 2-2, 2-23, 3-14

Transmission line, 3-1ac, 3-5capacitance, 3-1, 3-2characteristic impedance (Z0), 3-3coaxial line (flexible/rigid), 3-6, 3-7conductance (G), 3-1, 3-2copper losses (I2R), 3-3, 3-9current standing-wave ratio (iswr), 3-6dc, 3-4, 3-5dielectric losses, 3-3, 3-4, 3-9distributed constants, 3-1

INDEX-2

electric (E) field, 3-3, 3-12electromagnetic fields, 3-3incident wave, 3-5inductance, 3-1, 3-2induction losses, 3-3, 3-4input impedance (Zin), 3-3leakage current, 3-2line losses, 3-3lumped constants, 3-1magnetic (H) field, 3-3, 3-12output impedance (Zout), 3-3parallel line, 3-6power loss, 3-3power standing-wave ratio (pswr), 3-6radiation losses, 3-3, 3-4reflected wave, 3-5resistance, 3-1self-induction, 3-4shielded pair, 3-6, 3-7skin effect, 3-4, 3-9standing-wave ratio (SWR), 3-5twisted pair, 3-6, 3-7two-wire, 3-1two-wire open line, 3-6two-wire ribbon line, 3-7voltage standing-wave ratio, (vswr), 3-6waveguides, 3-6

W

Waveguide input/output, 3-18apertures, 3-18, 3-19bidirectional coupler, 3-25, 3-26cavity resonators, 3-25, 3-26, 3-27directional coupler, 3-25, 3-26dummy load, 3-21duplexer, 3-31horn, 3-21hybrid junctions, 3-25, 3-28, 3-30hybrid ring, 3-28, 3-31, 3-32impedance matching, 3-19

iris, 3-20junctions, 3-27loop, 3-19magic-T, 3-28, 3-30posts, 3-20probes, 3-18resistive load, 3-21, 3-22screws, 3-21slots, 3-18, 3-19T junction (E and H type), 3-28, 3-29terminations, 3-20windows, 3-18

Waveguides, 3-6“a” dimension, 3-12“b” dimension, 3-12angle of incidence 3-14, 3-15angle of reflection 3-14, 3-15arcing, 3-10bends, 3-22boundary conditions, 3-13choke joint, 3-23, 3-24circular, 3-16cutoff frequency, 3-12, 3-15dominant mode, 3-16E bend, 3-22electrolysis, 3-24group velocity, 3-15H bend, 3-22joints, 3-23metallic insulator, 3-10, 3-11mode numbering, 3-17mode of operation, 3-12, 3-15plumbing, 3-22Poynting vector, 3-14rotating joint, 3-23, 3-24sharp bend, 3-23size, 3-10transverse electric (TE), 3-17transverse magnetic (TM), 3-17twist, 3-23

INDEX-3

Assignment Questions

Information: The text pages that you are to study areprovided at the beginning of the assignment questions.

ASSIGNMENT 1

Textbook Assignment: “Wave Propagation,” chapter 1, pages 1-1 through 1-14.

1-1.

1-2.

1-3.

1-4.

1-5.

Which of the followingcan affect atmosphericconditions?

1. Geographic height

factors

2. Geographic location3. Changes in time4. All of the above

In what portion of theatmosphere does the majorityof weather phenomena takeplace?

1. Ionosphere2. Stratosphere3. Troposphere4. Hydrosphere

Because the stratosphere is arelatively calm region withlittle or no temperaturechange, it will have almost noeffect on radio wavepropagation.

1. True2. False

Variations in the ionosphereresulting from changes in thesun’s activity are known as

1. regular variations2. irregular variations3. both 1 and 2 above4. seasons

The regular variations in theionosphere can be separatedinto how many classes?

1. One2. Two3. Three4. Four

1-6. In ionization, when an electronis knocked free from a neutralgas atom, what is the overallcharge of the atom?

1. Negative2. Positive3. Neutral4. Inverted

1-7. The frequency of ultravioletlight passing through theatmosphere has whatrelationship to the ionosphericlayer it ionizes?

1. It is inverselyproportional

2. It is directly proportional3. It is inversely

proportional during the dayand directly proportionalat night

4. It is directly proportionalduring the day andinversely proportional atnight

1-8. What term best describes theprocess that returns positiveions to their original neutralstate?

1. Refraction2. Recombination3. Ionization4. Polarization

1-9. At what approximate time of dayis the density of theionospheric layers at itslowest level?

1. Just before sunrise2. Mid-morning3. Afternoon4. Sunset

1–10. How many distinct layers makeup

1.2.3.4.

the ionosphere?

OneTwoThreeFour

1

1-11. At what frequencies does thecombination of the earth’ssurface and the D layer act asa waveguide?

1. Vlf2. Lf3. Mf4. Hf

1-12. The D layer loses itsabsorptive qualities atfrequencies above what level?

1. 30 MHz2. 20 MHz3. 10 MHz4. 3 MHz

1-13. What is the approximate rangeof the E layer above theearth’s surface?

1. 30–54 miles2. 55–90 miles3. 91–130 miles4. 131–160 miles

1-14. Frequencies above what levelpass through the E layerunaffected?

1. 50 MHz2. 100 MHz3. 150 MHz4. 200 MHz

1-15. During daylight hours, the Flayer will divide into how manyseparate layers?

1. Five2. Two3. Three4. Four

1-16. Most high–frequency, long-rangecommunications occur in whatlayer(s) of the ionosphere?

1. D2. E3. F4. H

1-17. Which of the following is NOT afactor for radio waverefraction?

1. Ionization density of thelayer

2. Frequency of the radio wave3. Angle of incidence4. Transmitter power

1-18. For any given ionized layer,the critical frequency is justbelow the escape point.

1. True2. False

1-19. The critical angle for radiowave propagation depends onwhat two factors?

1. Angle of incidence andlayer density only

2. Layer density andwavelength only

3. Angle of incidence andwavelength only

4. Wavelength and antennaheight only

1-20. What term best describes thearea located between thetransmitting antenna and thepoint where the sky wave firstreturns to the earth?

1. Ground wave2. Skip zone3. Skip distance4. Ace area

1-21. Which of the following factorswill affect the outer limits ofthe skip zone?

1. Frequency2. Sunspot activity3. Angle of transmission4. All of the above

1-22. Radio waves reflecting from theearth’s surface or theionosphere, 180 degrees out ofphase, have what effect, ifany, at the receiving station?

1. The signal will be weak orfaded

2. The signal will be stronger3. The signal will be garbled4. None

2

1-23.

1-24.

1-25.

1-26.

1-27.

1-28.

For ionospheric reflection tooccur, the ionized layer mustnot be thicker than how manywavelengths of the transmittedfrequency?

1. One2. Two3. Three4. Four

The ability of radio waves toturn sharp corners and bendaround obstacles is known as

1. reflection2. refraction3. diffraction4. waveshaping

Which of the followingdefinitions best describes ashadow zone?

1. The area of completecoverage at vlf frequencies

2. The area within thediameter of an obstruction

3. The area ranging the heightof the obstruction

4. The area on the oppositeside of the obstruction, inline-of-site from thetransmitter to the receiver

What type of fading occurs forthe longest amount of time?

1. Phase shift2. Absorption3. Multipath4. Diffraction

Which of the following areexamples of multipath radiowave transmissions?

1. Groundwaves2. Ionospheric refractions3. Reflection from the earth’s

surface4. All of the above

Fading on the majority of theionospheric circuits is aresult of what particular typeof fading?

1. Selective2. Absorption3. Multipath4. Weather

IN ANSWERING QUESTIONS 1-29 AND 1-30,SELECT FROM THE FOLLOWING LIST THEDEFINITION OF THE INDICATED TERM.

A.

B.

C.

D.

Two or more receivingantennas spaced apart toproduce a usable signal

Two or more receivingantennas of varying heightslocated together

The use of two separatetransmitters and receiverson different frequenciestransmitting the sameinformation

The use of two separatetransmitters and receiverson the same frequencytransmitting the sameinformation

1-29. Space diversity.1. A2. B3. C4. D

1-30. Frequency diversity.

1. A2. B3. C4. D

1-31. A wide band of frequencies istransmitted and selectivefading occurs. Which of thefollowing statements bestdescribes the effect of thefading on the signal?

1. It affects variousfrequencies

2. It can cause changes inphase and amplitude

3. It can cause severedistortion and limit totalsignal strength

4. All of the above

1-32. Which ionospheric layer is mostdense during the winter?

1. E2. D3. F24. F1

3

1-33. During the 27–day sunspotcycle, which ionospheric layerexperiences the greatestfluctuations in density?

1. D2. E3. F14. F2

IN ANSWERING QUESTIONS 1–34 THROUGH1-38, SELECT FROM THE FOLLOWING LISTTHE DEFINITION OF THE INDICATED TERM.

A. Depends on the angle of thesun; refracts hf wavesduring the day, up to 20MHz, to distances of 1200miles; greatly reduced atnight

B. Reflects vlf waves forlong–range communications;refracts lf and mf forshort–range communications;has little effect on vhfand above; gone at night

C. Density depends on theangle of the sun; its maineffect is absorption of hfwaves passing through tothe F2 layer

D. Provides long-range hfcommunications; veryvariable; height anddensity change with time ofday, season, and sunspotactivity

E. Structure and densitydepend on the time of dayand the angle of the sun;consists of one layer atnight and two layers duringthe day

1–34. D layer.

1. A2. B3. C4. D

1-35. E layer.

1. A2. B3. C4. E

1-36. F layer.

1. B2. C3. D4. E

1-37. F1 layer.

1. A2. B3. C4. D

1-38. F2 layer.

1. A2. C3. D4. E

1-39. During periods of maximumsunspot activity within theeleven-year cycle, criticalfrequencies for all layersincrease.

1. True2. False

1-40. Which of the following problemsis NOT a negative side effectof the sporadic E layer?

1. Causes increased multipathproblems

2. Provides additionalabsorption

3. Blanks out more favorablelayers

4. Increased static in line ofsight communications

1-41. When sudden ionosphericdisturbances (SID) occurs,which ionospheric layer isaffected the most?

1. D2. E3. F14. F2

1-42. What effect do ionosphericstorms have on (a) the range offrequencies and (b) the workingfrequency used forcommunications?

1. (a) Increase (b) increase2. (a) Decrease (b) decrease3. (a) Increase (b) decrease4. (a) Decrease (b) increase

4

1-43.

1-44.

What form of precipitation hasthe greatest absorption effecton RF energy?

1. Fog2. Snow3. Rain4. Hail

The duct effect produced bytemperature inversion allowsfor long-distancecommunications over whatfrequency band?

1. Vlf2. Lf3. Hf4. Vhf

1-45. Which of the following factorsaffect(s) the amount of groundreflection loss when a radiowave is reflected from theearth’s surface?

1. Angle of incidence2. Ground irregularities3. Electrical conductivity at

the point of reflection4. All of the above

1-46. As an RF wave increases indistance, the wavefront spreadsout, reducing the amount ofenergy available within anygiven unit of area. Thisaction produces what type ofenergy loss?

1. Absorption2. Ground reflection3. Freespace4. Spread

1-47. Radio waves above the MUF willexperience what effect whenrefracted from the ionosphere?

1. They will fall short of thedesired location

2. They will overshoot thedesired location

3. They will be absorbed bylower layers

4. They will experiencemultipath fading

1-48. Variations in the ionospheremay change a preexisting muf.This is especially true becauseof the volatility of which ofthe following layers?

1. F12. F23. D4. E

1-49. Radio waves that are propagatedbelow the LUF are affected bywhat problem(s)?

1. Increased absorption2. Higher levels of

atmospheric noise3. Higher rate of refraction4. All of the above

1-50. The frequency that will avoidthe problems of multipathfading, absorption, noise, andrapid changes in the ionosphereis known by what term?

1. LUF2. MUF3. FOT4. LOS

5

ASSIGNMENT 2

Textbook Assignment: “Antennas,“ chapter 2, pages 2-1 through 2–32.

2-1.

2-2.

2-3.

2-4.

2–5.

2-6.

Electromagnetic radiation froman antenna is made up of whattwo components?

1. E and H fields2. Ground and sky waves3. Vertical and horizontal

wavefronts4. Reflected and refracted

energy

What determines the size of atransmitting antenna?

1. Transmitter power2. Available space3. Operating frequency4. Distance to be

transmitted

Most practical transmittingantennas are divided into twoclassifications, Hertz andMarconi.

1. T2. F

Hertz antennas are designed tooperate at what wavelength inrelationship to theiroperating frequency?

1. Quarter–wave2. Half–wave3. Three quarter–wave4. Full-wave

Marconi antennas are used foroperating frequencies belowwhat level?

1. 10 MHz2. 6 MHz3. 4 MHz4. 2 MHz

All antennas regardless oftheir shape or size have howmany basic characteristics?

1. 12. 23. 34. 4

2-7. The ability to use the sameantenna for both transmittingand receiving is known by whatterm?

1.2.3.4.

2-8. The

GainReciprocityDirectivityPolarization

ability of an antenna or

2-9.

2-10.

2-11.

array to focus energy in oneor more specific directions isrepresented by a measurementof what antenna property?

1. Signal Strength2. Reciprocity3. Directivity4. Polarization

The gain of a transmittingantenna is 9 dB, what will thegain be for the same antennaused for receiving?

1. 9 dB2. 6 dB3. 4 dB4. 3 dB

Which, if any, of thefollowing components of aradiated electromagnetic fielddetermines its direction ofpolarization?

1. H lines2. E lines3. Angle of Propagation4. None of the above

Over long distances thepolarization of a radiatedwave changes, at whatfrequencies will this changebe the most dramatic?

1. VLF2. LF3. MF4. HF

6

2-12. A transmitting antenna at 2-18. An antenna that radiatesground level should be energy more strongly in onepolarized in what manner to direction than another is saidachieve best signal strength? to have what type of radiation

pattern?1. Horizontally2. Vertically 1. Isotropic3. Circularly 2. Anisotropic4. Linearly 3. Bysotropic

4. Circumstropic2-13. What term describes the

distance a wave travels duringthe

1.2.3.4.

period of one cycle?

WavelengthFrequencyTravel timeRadiation rate

IN ANSWERING QUESTIONS 2-14 AND 2-15,REFER TO FIGURE 2–4 OF THE TEXT.

2-14. The points of high current andvoltage are best described bywhich of the following terms?

1. Peaks2. Crescents3. Loops4. Highs

2-15. The points of minimum voltageand minimum current arerepresented by which of thefollowing terms?

1. Lows2. Valleys3. Descents4. Nodes

2-16. An antenna at resonance willtransmit at maximumefficiency; an antenna that isnot at resonance will losepower in which of thefollowing ways?

1. Skin effect loss2. Heat loss3. Ground absorption4. Wave scattering

2-17. An antenna that radiatesenergy in all directions issaid to have what type ofradiation pattern?

1. Isotropic2. Anisotropic3. Bysotropic4. Circumstropic

2-19. When viewing a radiationpattern graph, you can expectthe areas of maximum andminimum radiation beidentified by which of thefollowing terms?

1. High and low probes2. Maximum and minimum

points3. Major and minor lobes4. Positive and negative

lobes

2-20. If an antenna is too short forthe wavelength being used,what electrical compensationmust be introduced for theantenna to achieve resonance?

1. Lumped resistance2. Lumped capacitive

reactance3. Lumped inductive

reactance4. More power

2-21. If an antenna is too long forthe wavelength being used,what electrical compensationmust be introduced for theantenna to achieve resonance?

1. Lumped resistance2. Lumped capacitive

reactance3. Lumped inductive

reactance4. Less power

2-22. A ground screen is a series ofconductors buried 1 or 2 feetbelow the surface in a radialpattern and is usually of whatlength in comparison to thewavelength being used?

1. One-quarter wavelength2. One–half wavelength3. Three-quarter wavelength4. Full wavelength

7

2-29. The most distinct advantage of2-23. When would a counterpoise beused?

1. When easy access to theantenna base is necessary

2. When the surface below issolid rock

3. When the surface below issandy ground

4. All the above

2-24. Capacitive top-loading helpsto increase which of thefollowing antennacharacteristics?

1. Bandwidth2. Power-handling3. Directivity4. Radiation efficiency

2-25. What is the most limitingcharacteristic of the Yagiantenna?

1. Power-handling2. Narrow bandwidth3. Physical size4. Lack of directivity

2-26. In general, log-periodicantennas have which of thefollowing characteristics?

1. Medium power handlingcapabilities

2. High gain3. Extremely broad bandwidth4. All the above

2-27. A typical vertical monopolelog periodic antenna designedto cover a frequency range of2 to 30 MHz will requireapproximately how many acresof land for its ground planesystem?

1. 1 acre2. 2 acres3. 3 acres4. 4 acres

2-28. A sector log–periodic arraycan act as an antenna for aminimum of what number oftransmit or receive systems?

1. 12. 23. 34. 4

2-30.

2–31.

2-32.

2-33.

2-34.

the rotatable log–periodicantenna is its ability toperform what function?

1. Rotate 360 degrees2. Rotate from horizontal to

vertical and back3. Ability to handle high

transmitter power4. Ability to produce high

antenna gain

What is the average powerhandling capability of anInverted Cone antenna?

1. 20 kw2. 30 kw3. 40 kw4. 50 kw

What determines the gain anddirectivity of a Rhombicantenna?

1. Transmitter power2. Antenna height3. Radiated wave interaction4. Transmitted frequency

Most Whip antennas requiresome kind of a tuning systemto improve bandwidth and powerhandling capabilities.

1. T2. F

Why are UHF and VHF antennason board ship installed ashigh as possible?

1. To prevent radiationhazard to personnel

2. To prevent radiationhazard to ordinance

3. To increase powerhandling capabilities

4. To prevent unwanteddirectivity in theradiation pattern frommast structures

The central feed section forboth the biconical and center-fed dipole are protected bywhat type of covering?

1. SCOTCHCOAT2. RTV3. Laminated fiberglass4. Rubber shield

8

2-35.

2-36.

2-37.

2-38.

2-39.

2-40.

The adjustable stub on theAS-390/SRC uhf antenna is usedto adjust what antennacharacteristic?

1. The counterpoise angle2. The input impedance3. The radiation angle4. The feed point

The OE-82B/WSC-I(V) antennagroup uses what type ofpolarization?

1. Vertical2. Horizontal3. Right–hand circular4. Left-hand circular

The AN/WSC-5 (V) shore stationantenna consists of whatnumber of OE-82A/WSC-1 (V)assemblies?

1. 12. 23. 34. 4

What does the acronym LPIstand for?

1. Low power interference2. Low probability of

intercept3. Low phase intercept4. Last pass intercept

The reflectors for the AN/WSC-6 (V) are mounted on three-axis pedestals and provideauto tracking using whatscanning technique?

1. Conical2. Peripheral3. Vertical4. Horizontal

Antenna tuning is accomplishedusing what piece or pieces ofequipment?

1. Couplers2. Tuners3. Multicouplers4. All the above

2-41.

2-42.

2-43.

2-44.

2-45.

2-46.

Antenna multicouplers are usedto match more than onetransmitter or receiver towhat number of antennas?

1. 12. 23. 34. 4

The AN/URA-38 antenna coupleris an automatic tuning systemprimarily used with whichradio transmitter?

1. AN/WSC-32. AN/URT–233. AN/URC-804. AN/FRT-84

The AN/SRA-57 coupler groupoperates in which of thefollowing frequency ranges?

1. 2– 6 Mhz2. 4-12 Mhz3. 10-30 Mhz4. 40–60 Mhz

How many channels are providedwith the AN/SRA-12multicoupler?

1. 42. 53. 64. 7

What type of radar would use atruncated paraboloid reflectorthat has been rotated 90degrees?

1. A surface search2. An air search3. A navigation4. A height-finder

Of the following methods,which is NOT used to feed acylindrical paraboloidreflector?

1. A linear array of dipoles2. A slit in the side of a

waveguide3. A thin waveguide radiator4. A quarter–wave stub

9

2-47. The elements of a broadsidearray are spaced one-halfwavelength apart and arespaced how many wavelengthsaway from the reflector? 1.

1. One–eighth2. One-quarter3. One–half4. Three-quarter

2-48. What is the advantage, if any,to offsetting a feedhornradiator for a parabolic dish?

1. A broader beam angle2. The elimination of

shadows3. A narrower beam angle4. No advantage

2-49. What is the range in nauticalmiles of the AN/GPN-27 radar?

1. 552. 753. 1054. 155

2-50. What is the purpose of thejackscrew on theAS–3263/SPS–49(V) antenna?

1. To adjust the beam width2. To vary the antenna feed

horn focal distance3. To adjust the beam

elevation angle4. To lockdown the antenna

for PM

2-51. The OE-172/SPS–55 antennanormally operates in thelinearly polarized mode, forwhat reason would you use thecircular polarized mode?

2-52. Which of the following is NOTa mode of operation for theAN/SPN-35A radar set?

Final2. Dual3. Surveillance4. Simultaneous

2-53. The two primary safetyconcerns associated with rffields are rf burns andinjuries caused by dielectricheating.

1. T2. F

2-54. When a person is standing inan rf field, power in excessof what level will cause anoticeable rise in bodytemperature?

1. 5 milliwatts2. 10 milliwatts3. 15 milliwatts4. 20 milliwatts

2-55. When working aloft, whatsafety precaution(s) must befollowed?

1. Tag out the antenna atthe switchboard toprevent it from becomingoperational

2. Secure motor safetyswitches for rotatingantennas

3. Wear the proper oxygenbreathing apparatus whenworking near a stack

4. All the above

1. To compensate for theships pitch and roll

2. To prevent jamming3. To reduce return echoes

from precipitation4. To achieve over the

horizon coverage

10

ASSIGNMENT 3

Textbook Assignment: “Transmission Lines and Waveguides,” chapter 3, pages 3-1through 3-32.

3-1. A transmission line is designedto perform which of thefollowing functions?

1. Disperse energy in alldirections

2. Detune a transmitter tomatch the load

3. Guide electrical energyfrom point to point

4. Replace the antenna in acommunications system

3-2. The conductance value of atransmission line representswhich of the following values?

1.

2.

3.

4.

Expected value of currentflow through the insulationExpected value of voltagesupplied by the transmitterValue of the lump anddistributed constants ofthe line divided byimpedanceValue of the lumpedconstants of the line asseen by the source and theload

3-3. Distributed constants in atransmission line aredistributed in which of thefollowing ways?

1. Between ground and anysingle point on the line

2. Along the length of theline

3. According to the thicknessof the line

4. According to the cross-sectional area of the line

3-4. Leakage current in a two–wiretransmission line is thecurrent that flows through whatcomponent?

1. The resistor2. The inductor3. The insulator

3-5. Conductance is the reciprocalof what electrical property?

1. Inductance2. Resistance3. Capacitance4. Reciprocity

3-6. A transmission line that hascurrent flowing through it haswhich of the following fieldsabout it?

1. Electric only2. Magnetic only3. Both electric and magnetic4. Capacitive

3-7. A measurement of the voltage tocurrent ratio (Ein/ Iout) at theinput end of a transmissionline is called the

1. input–gain rate2. voltage–gain ratio3. output impedance4. input impedance

3-8. The characteristic impedance(2.) of a transmission line iscalculated by using which ofthe following ratios?

1. Rsource to Rload of the line2. Imax to Im i n at every point

along the line3. E to I at every point along

the line4. E in to Eout of the line

4. The conductor

11

3-9. Maximum transfer of energy fromthe source to the transmissionline takes place when whatimpedance relationship existsbetween the source and thetransmission line?

1. When the load impedanceequals the source impedance

2. When the load impedance istwice the source impedance

3. When the load impedance ishalf the source impedance

4. When the load impedance isone-fourth the sourceimpedance

3-10. Which of the following sets ofterms represents a type of lossin a transmission line?

1. I2R and induction only2. Induction and dielectric

only3. Dielectric and radiation

only4. I2R , induction, and

dielectric

3-11. Skin effect is classified aswhich of the following types ofloss?

1. Copper2. Voltage3. Induction4. Dielectric

3-12. What transmission-line loss iscaused by magnetic lines offorce not returning to theconductor?

1. Copper2. Radiation3. Induction4. Dielectric

3-13. When a dc voltage is applied toa transmission line and theload absorbs all the energy,what is the resultingrelationship between currentand voltage?

1. They are in phase with eachother

2. They are equal to ZO of theline

3. They are out of phase witheach other

4. They are evenly distributedalong the line

3-14. The initial waves that travelfrom the generator to the loadof a transmission line arereferred to as what type ofwaves?

1. Incident2. Refracted3. Reflected4. Diffracted

3-15. Waves that travel from theoutput end to the input end ofa transmission line arereferred to as what type ofwaves?

1. Incident2. Refracted3. Reflected4. Diffracted

3-16. The ratio of maximum voltage tominimum voltage on atransmission line is referredto as the

1. rswr2. pswr3. vswr4. iswr

3-17. Which of the following ratiossamples the magnetic fieldalong a line?

1. Vswr2. Pswr3. Iswr4. Rswr

3-18. Which of the following lines isNOT a transmission medium?

1. Load line2. Coaxial line3. Two-wire open line4. Twisted-pair line

3-19. Electrical power lines are mostoften made of which of thefollowing types of transmissionlines?

1. Twin-lead line2. Shielded-pair line3. Two–wire open line4. Two–wire ribbon line

12

3-20. Uniform capacitance throughoutthe length of the line is anadvantage of which of thefollowing transmission lines?

1. Coaxial line2. Twisted pair3. Shielded pair4. Two-wire open line

3-21. What is the primary advantageof a rigid coaxial line?

1. Low radiation losses2. Inexpensive construction3. Low high–frequency losses4. Easy maintenance

3-22. The most efficient transfer ofelectromagnetic energy can beprovided by which of thefollowing mediums?

1. Waveguides2. Twin–lead flat lines3. Single-conductor lines4. Coaxial transmission lines

3-23. Copper I2R losses are reducedby what physical property ofwaveguides?

1. Small surface area2. Large surface area3. Shape of the waveguides4. Waveguide material being

used

3-24. In a coaxial line, the current-carrying area of the innerconductor is restricted to asmall surface layer because ofwhich of the followingproperties?

1. Skin effect2. Copper loss3. Conductor density4. Waveguide material being

used

3-25. Which of the followingdielectrics is used inwaveguides?

1. Air2. Mica3. Insulating oil4. Insulating foam

3-26. Which of the followingcharacteristics of a waveguidecause its lower–frequencylimitation?

1. I2R loss2. Physical size3. Wall thickness4. Dielectric loss

3-27. At very high frequencies,ordinary insulators in a two-wire transmission line displaythe characteristics of whatelectrical component?

1. An inductor2. A resistor3. A capacitor4. A transformer

3-28. At high frequencies, which ofthe following devices worksbest as an insulator?

1. An open half-wave section2. An open quarter–wave

section3. A shorted half–wave section4. A shorted quarter-wave

section

3-29. The range of operatingfrequencies is determined bywhich of the followingwaveguide dimensions?

1. The widest (height/width)2. The narrowest (height/

width)3. The shortest (length)4. The longest (length)

3–30. The cutoff frequency for awaveguide is controlled by thephysical dimensions of thewaveguide and is defined as thefrequency at which two quarterwavelengths are

1. shorter than the “a”dimension

2. shorter than the “b”dimension

3. longer than the “a”dimension

4. longer than the “b”dimension

13

3-31. In practical applications,which of the followingdimensions describes the widedimension of the waveguide atthe operating frequency?

1. 0.1 wavelength2. 0.2 wavelength3. 0.5 wavelength4. 0.7 wavelength

3-32. Which of the following fieldsis/are present in waveguides?

1. E field only2. H field only3. E and H fields4. Stationary fields

3-33. A difference in potentialacross a dielectric causeswhich of the following fieldsto develop?

1. Electric only2. Magnetic only3. Electromagnetic

3-34. H lines have which of thefollowing distinctivecharacteristics?

1. They are continuousstraight lines

2. They are generated byvoltage

3. They form closed loops4. They form only in the

waveguide

3-35. For an electric field to existat the surface of a conductor,the field must have whatangular relationship to theconductor?

1. 0°2. 30°3. 45°4. 90°

3-36. If the wall of a waveguide isperfectly flat, the angle ofreflection is equal to which ofthe

1.2.3.4.

following angles?

CutoffIncidenceRefractionPenetration

3-37.

3–38.

3-39.

3–40.

3-41.

The cutoff frequency in awaveguide occurs at exactlywhat angle of reflection?

1. 10°2. 30°3. 45°4. 90°

How does the group velocity ofan electromagnetic field in awaveguide compare to thevelocity of a wavefront throughfree space?

1. Group velocity is somewhatfaster

2. Group velocity is somewhatslower

3. Group velocity is twicethat of free velocity

4. Free velocity is twice thatof group velocity

The group velocity of awavefront in a waveguide may beincreased by which of thefollowing actions?

1. Decreasing the frequency ofthe input energy

2. Increasing the frequency ofthe input energy

3. Increasing the power of theinput energy

4. Decreasing the power of theinput energy

The various field configura–tions that can exist in awaveguide are referred to as

1.2.3.4.

The

wavefrontsmodes of operationfields of operationfields of distribution

most efficient transfer ofenergy occurs in a waveguide inwhat mode?

1. Sine2. Dominant3. Transverse4. Time–phase

14

3-42. How is the cutoff wavelengthfor a circular waveguidecomputed?

1. 1.17 times the radius ofthe waveguide

2. 1.17 times the diameter ofthe waveguide

3. 1.71 times the diameter ofthe waveguide

4. 1.71 times the radius ofthe waveguide

3-43. The field configuration inwaveguides is divided into whattwo categories?

1. Half-sine and dominant2. Transverse electric and

transverse magnetic3. Transverse electric and

dominant4. Transverse magnetic and

half-sine

3-44. With a mode description ofTE 1,0, what maximum number ofhalf-wave patterns exist acrossthe “a” dimension of awaveguide?

1. One2. Two3. Three4. Four

3–45. With the mode description,TE 1,1, what maximum number ofhalf-wave patterns exist acrossthe diameter of a circularwaveguide?

1. One2. Two3. Three4. Four

3-46. Which of the following devicesCANNOT be used to inject orremove energy from a waveguide?

1. A slot2. A loop3. A probe4. A horn

3-47. Loose coupling is a method usedto reduce the amount of energybeing transferred from awaveguide. How is loosecoupling achieved when using aprobe?

1. By doubling the size of theprobe

2. By increasing the length ofthe probe

3. By decreasing the length ofthe probe

4. By placing the probedirectly in the center ofthe energy field

3-48. Increasing the size of the loopwire increases which of thefollowing loop capabilities?

1. Efficiency2. Bandwidth coverage3. Power–handling capability4. Each of the above

3-49. A waveguide that is notperfectly impedance matched toits load is not efficient.Which of the followingconditions in a waveguidecauses this inefficiency?

1. Sine waves2. Dominant waves3. Standing waves4. Transverse waves

3-50. A waveguide iris that coverspart of both the electric andmagnetic planes acts as whattype of equivalent circuit atthe resonant frequency?

1. As an inductive reactance2. As a shunt resistance3. As a capacitive reactance4. As a shorted 1/4 wave stub

3-51. A horn can be used as awaveguide termination devicebecause it provides which ofthe following electricalfunctions?

1. A reflective load2. An absorptive load3. An abrupt change in

impedance4. A gradual change in

impedance

15

3-52. For a waveguide to beterminated with a resistiveload, that load must be matchedto which of the followingproperties of the waveguide?

1. The bandwidth2. The frequency3. The inductance4. The characteristic

impedance

3-53. A resistive device with thesole purpose of absorbing allthe energy in a waveguidewithout causing reflections isa/an

1. iris2. horn3. antenna4. dummy load

3-54. A resistive load most oftendissipates energy in which ofthe following forms?

1. Heat2. Light3. Magnetic4. Electrical

3-55. Reflections will be caused byan abrupt change in which ofthe following waveguide’sphysical characteristics?

1. Size and shape only2. Size and dielectric

material only3. Dielectric material and

shape only4. Size, shape, and dielectric

material

3-56. A waveguide bend that in the Eand H plane must be greaterthan two wavelengths to prevent

1. cracking2. reflections3. energy gaps4. electrolysis

3-57. A flexible waveguide is used inshort sections because of thepower-loss disadvantages. Whatis the cause of this powerloss?

1. Walls are not smooth2. E and H fields are not

perpendicular3. Cannot be terminated in its

characteristic impedance4. Wall size cannot be kept

consistent

3-58. The choke joint is used forwhat purpose in a waveguide?

1. To reduce standing waves2. To restrict the volume of

electron flow3. To prevent the field from

rotating4. To provide a joint that can

be disassembled duringmaintenance

3-59. A circular waveguide isnormally used in a rotatingjoint because rotating arectangular waveguide wouldcause which of the followingunwanted conditions?

1. Oscillation2. Large power loss3. Decrease in bandwidth4. Field–pattern distortion

3-60. In your waveguide inspection,you should be alert for whichof the following problems?

1. Corrosion2. Damaged surfaces3. Improperly sealed joints4. Each of the above

3–61. What type of corrosion occurswhen dissimilar metals are incontact with each other?

1. Contact2. Metallic3. Electrical4. Electrolytic

16

3–62. Internal arcing in a waveguideis usually a symptom of whichof the following conditions?

1. Change in mode2. Electrolysis at a joint3. Moisture in the waveguide4. Gradual change in frequency

3-63. What is the primary purpose ofa directional coupler?

1. To sample the energy in awaveguide

2. To change the phase of theenergy in the waveguide

3. To change the direction ofenergy travel in thewaveguide

4. To allow energy in thewaveguide to travel in onedirection only

3-64. What is the electrical distancebetween the two holes in asimple directional coupler?

1. 1/8 wavelength2. 1/4 wavelength3. 1/2 wavelength4. 3/4 wavelength

3-65. Of the followingcharacteristics, which is NOTrequired for a device to beconsidered a resonant cavity?

1. Be enclosed by conductingwalls

2. Possess resonant properties3. Contain oscillating

electromagnetic fields4. Be round or elliptical in

shape

3-66. What property gives a resonantcavity a narrow bandpass andallows very accurate tuning?

1. Low Q2. High Q3. Inductive reactance4. Capacitive reactance

3-67. What factor(s) determine(s) theprimary frequency of a resonantcavity?

3-68. Tuning is the process ofchanging what property of aresonant cavity?

1. The Q2. The power3. The cutoff frequency4. The resonant frequency

3-69. What are the two basic types ofwaveguide T junctions?

1. H and T2. H and E3. Hybrid Ring and magic T4. Q and magic T

3-70. A waveguide junction in whichthe arm area extends from themain waveguide in the samedirection as the electric fieldis an example of what typejunction?

1. E–type T2. H-type T3. H-type T junction4. H–type junction

3-71. Low power handling capabilitiesand internal power losses arethe primary disadvantages ofwhich of the followingjunctions?

1. Magic T2. Rat race3. Duplexer4. Hybrid ring

3-72. The hybrid ring is usually usedas what type of device in radarsystems?

1. Mixer2. Detector3. Duplexer4. Impedance matcher

1. Size only2. Shape only3. Size and shape4. Q of the cavity

17