Antennas by Drdo

66

DRDO SCIENCE SCECTRUM 2009

1. INTRODUCTION

Antennas are basic components of any electric systemand are connecting links between the transmitter and freespace or free space and the receiver. Thus antennas playvery important role in finding the characteristics of thesystem in which antennas are employed. Antennas areemployed in different systems in different forms. That is,in some systems the operational characteristic of the systemare designed around the directional properties of the antennasor in some others systems, the antennas are used simplyto radiate electromagnetic energy in an omnidirectinal orfinally in some systems for point-to-point communicationpurpose in which increased gain and reduced wave interferenceare required.

1.2 Antenna Definitions

There are several definitions of antenna, and are asfollows:• The IEEE Standard Definitions of Terms (IEEE Std 145-

1983):--A means for radiating or receiving radio waves

• “An antenna is any device that converts electronicsignals to electromagnetic waves (and vice versa)”effectively with minimum loss of signals as shown inFig.1.

• An antenna is basically a transforming device that willconvert impedance of transmitter output (50/75 Ohm)into free space impedance (120pi or 377 Ohm).

• Region of transition between guided and free spacepropagation

• Concentrates incoming wave onto a sensor (receivingcase)

• Launches waves from a guiding structure into spaceor air (transmitting case)

• Often part of a signal transmitting system over somedistance.

DRDO Science Spectrum, March 2009, pp 66-78© 2009, DESIDOC

Figure 1. Wireless communication system.

Antennas and its Applications

Pramod DhandeArmament Research & Development Establishment, Dr Homi Bhabha Rd, Pashan, Pune-411 021

ABSTRACT

In the world of modern wireless communication, engineer who wants to specialize in the communication field needsto have a basic understanding of the roles of electromagnetic radiation, antennas, and related propagation phenomena.These papers discuss on the performance, characteristic, testing, measurement and application of antennas in modernwireless communication systems. Antenna is an important part of any wireless communication system as it convertsthe electronic signals (propagating in the RF Transreceiver) into Electromagnetic Waves (Propagating in the free space)efficiently with minimum loss. We use antennas when nothing else is possible, as in communication with a missile orover rugged mountain terrain where cables are expensive and take a long time to install. The performance characteristicsof the parent system are heavily influenced by the selection, position and design of the antenna suite. To understandthe concept of antenna one should know the behaviour of Electromagnetic waves in free space. So I am briefly coveringthe basics of Electromagnetic waves and its propagation modes in free space. Apart from that I am also covering Antennaclassifications (based on Frequency, aperture, polarization and radiation pattern), its performance parameters (Gain,Directivity, Beam area and beam efficiency, radiation pattern, VSWR/Return loss, polarization, Efficiency), measurementtechniques (Outdoor and Indoor Testing) and its defence applications (Naval antennas, Airborne Antennas and EarthStation Antennas). Finally I discuss about Pyramidal horn antennas, Monopole antennas.

Keywords: Antenna, wireless communication, pyramidal horn antennas, monopole antennas

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DHANDE: ANTENNAS AND ITS APPLICATIONS

1.2.1 Antenna Definitions

• The radiation pattern and radiation resistance of anantenna is the same when it transmits and when itreceives, if no non-reciprocal devices are used. So,Same antenna can be used for Transmission and Receptionof Electromagnetic Waves

• Does not apply to active antennas.NB: Antenna is a passive device, it does not amplify

the signals, it only directs the signal energy in a particulardirection in reference with isotropic antenna.

2. IMPORTANCE OF ANTENNA IN AIRBORNEAPPLICATION

As shown in Fig.3, different frequency band antennasare placed on aircraft/missile body for different communication.

is clear that if we move towards high frequency, wavelengthof the signal being smaller (from Equation 1); hence thedimensions of the antenna and RF component becomesmaller. So at higher frequency the size of the wirelesssystem becomes compact.

1f

λ= (1)

3.1 Electromagnetic (em) Wave in Free Space

Electromagnetic waves are disturbances to the electricaland magnetic fields. A changing electric disturbance producesa changing magnetic field at right angle to the electricfield.

Antenna placed at nose of the aircraft is a part of guidanceRADAR system, which will guide the aircraft. Various jammingantenna are placed on different parts of aircraft for jammingthe enemy signals. Antenna placed at the belly of theaircraft for data link application. All these antennas areoperated on different frequency bands, so care should betaken that to avoid the interference of radiation patternof all these antennas. Also when these antennas are placedon the aircraft body, its radiation pattern gets distorted,so one should design an antenna such that it will meetour application.

3. ELECTROMAGNETIC WAVES

Before understanding the concept of antenna one shouldknow what are Electromagnetic wave and its propagationmodes in free space. The full Electromagnetic spectrum isshown in Fig.4. Antennas dimensions are dependent onwavelength of the signal being transmitted. From Fig.4, it

Figure 2. Propagation of EM waves.

Figure 3. Application of airborne antennas.

(b)

(a)

Figure 4. Electromagnetic spectrum.

Figure 5. EM wave in free space.

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Electromagnetic Wave originates from a point in freespace, spreads out uniformly in all directions and it formsa spherical wave. An observer, however, at a grate distancefrom the source is able to observe only the small part ofthe wave in his immediate vicinity and it appears to himas plane wave just as the ocean appears flat to a personwho can only see a few miles around him. Thus at a largedistance from the source the wave has similar propertiesto the plane waves in the strip line and so by analogy ofstrip line the properties of EM waves in free space asfollows:1. At every point in space, the electric vector field E and

the magnetic vector field H are perpendicular to eachother and to the direction of propagation as shownin Fig.5.

2. Velocity of EM wave in free space is given byc=1/(μ0å0)1/2 = 3 × 108 m/s (2)

3. E and H oscillate in phase and ratio of their amplitudeis constant being equal to 120ð or 377 Ohm or (μ0/å0)12.

4. Whatever may be the frequency, the EM waves travelsin space with the velocity of light.

5. EM wave propagates in free space as Transverse ElectroMagnetic waves (TEM mode).Equation of EM waves in free space is given by:

(3)

(4)

3.1.1 Polarization of Electromagnetic Wave

The Polarization of Electromagnetic wave is definedas the orientation of electric field vector in space withrespect to time. There are three types of EM wave polarization:

1. Vertical Polarization-

When E field vector of EM wave is perpendicular tothe earth, the EM wave said to be Vertically Polarized..

2. Horizontal Polarization

When E field vector of EM wave is parallel to theearth, the EM wave said to be Horizontally Polarized.

2f

ωπ

=0 0

1

fλ

μ ε= 2πβ

λ= 0

00

EZ

H= 0

00

Zμε

=

3. Circular Polarization

When E and H field of the EM wave are of sameamplitude and having a phase difference of 90o, waveis said to be circularly polarised..

3.1.2 Properties of Electromagnetic Waves

1. Reflection and Refraction:EM waves gets affected from Reflection and Refractionsame as that of light wave. Due to Reflection andRefraction the polarization of the EM wave get changed,so care should be taken that the designed antenna willtransmit or receive the EM wave of desired polarization.

2 2

2 20 0

1x xE E

t zμ ε∂ ∂=∂ ∂

( )0

j t zxE E e ω β±=

2 2

2 20 0

1y yH H

t zμ ε∂ ∂

=∂ ∂

( )0

j t zyH H e ω β±=

Figure 6. Vertical polarisation.

Figure 7. Horizontal polarisation.

Figure 8. Circular polarisation.

Figure 9. Reflection and refraction of EM wave.

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3.3.2 Open and flare up wave guide: Aperture (Horn) antenna

4. RADIATION PRINCIPLE OF ANTENNA

One of the first questions that may be asked concerningantennas would be “How are the electromagnetic fieldsgenerated by the source, contained and the guide in thetransmission line and antenna, and finally detached fromthe to form a free-space wave? “ The best explanation canbe given as follows.

Let us consider a voltage source connected to a two-conductor transmission line, which is connected to anantenna as shown in Fig. 11. Applying a voltage sourceacross the two-conductor transmission line creates an electricfield between the conductors. The electric field associatedwith it electric line of force, which is tangent to the electricfield at each point and the strength, is proportional to theelectric field intensity. The electric field forces the chargecarriers to be displaced which constitutes the current andhence creates magnetic field intensity. Associated with themagnetic field intensity, the magnetic line of force, whichare tangent to the magnetic field.

• Reflection,

Reflection coefficient:Depends on media, polarisation

of incident wave and angleof incidence.

• Refraction,

if both media are lossless

3.2 Guded Electromagnetic Waves

Electromagnetic Wave also exists in guided structurelike:Cables : Used at frequencies below 35 GHzWaveguides : Used between 0.4 GHz to 350 GHzQuasi-Optical Systems : Used above 30 GHz

In above structures propagating modes of EM wavegets changed like in waveguide EM wave propagates inTransverse Electric (TE) and Transverse Magnetic (TM)modes.

3.3 Launching of EM Waves

EM wave launched into the free space by means ofantennas and the selection of antenna is depending onthe guided media:

3.3.1 Open up the cable and separate wires: Monopole & Dipole antenna

r iθ θ=

r

i

E

Eρ =

1

2

sin( ) sin( )t i

ηθ θη

=

1 1

2 2sin( ) sin( )t i

μ εμ εθ θ=

When a.c. signal is applied to the line from sourcetime varying electric and magnetic fields are created. Thecreation of time varying electric and magnetic fields betweenthe conductors form electromagnetic waves which travelalong the transmission line as shown in Fig. 11. Theelectromagnetic waves enter the antenna and have associatedwith them electric charges and corresponding currents. Ifwe remove part of antenna structure as shown in Fig. 11,free space waves can be formed by connecting the openends of the electric lines. The free space waves are alsoperiodic but a constant phase point moves outwardly withthe speed of light and travels a distance of wavelength/2 in the time of one half of a period.

Before we attempt to explain how guided waves aredetached from the antenna to create the free space waves,let us draw a parallel between the guided and free spacewaves, and water waves created by the dropping of apebble in a calm body of water or initiated in some other

Figure 10. Launching of EM wave from open cable and separated wires through dipole antenna.

Figure 11. Launching of EM wave from waveguide through aperture antenna.

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manner. Once the disturbance in the water has been initiated,water waves are created which begin to travel outwardly.If the disturbance has been removed the waves do notstop or extinguish themselves but continue their courseof travel. If the disturbance persists, new waves are continuouslycreated which lag in their travel behind the others. Thesame is true with the electromagnetic waves created byan electric disturbance. If the initial electric disturbanceby the source is of short duration, the created electromagneticwaves will travel inside the transmission line, then intothe a antenna, and finally will be radiated as free spacewaves, even if the electric source ceased to exist. If theelectric disturbance is of continuous nature, electromagneticwaves will exist continuously and follow in their travelbehind the others.

When the electromagnetic waves are within thetransmission line and antenna, their existence is associatedwith the presence of the charges inside the conductors.However, when the waves are radiated, they form closedloops and there are no charges to sustain their existence.This leads us to conclude that electric charges are requiredto excite the fields but are not needed to sustain them andmay exist in their absence. This is in direct analogy withwater waves.

5. ELECTROMAGNETIC WAVE PROPAG-ATIONMODES:

Electromagnetic wave can propagate into the free spaceby three modes:1. Ground-wave propagation2. Sky-wave propagation3. Line-of-sight propagation

5.1 Ground-wave propagation

The ground wave is a wave that is guided along thesurface of the earth just as an electromagnetic wave isguided by a waveguide or transmission line. Surface wavepermits the propagation around the curvature of the earth.This mode of propagation exists when the transmitting andreceiving antennas are closed to the surface of the earthand is supported at its lower edge by the presence of theground.

• Follows contour of the earth.• Can propagate considerable distances.• Frequencies up to 2 MHz.• Example

– AM radio

5.2 Sky-wave Propagation

The sky waves are of practical importance at mediumand high frequencies for very long distance radiocommunications. In this mode of propagation electromagneticwaves reach the receiving point after reflection from theionized region in the upper atmosphere called ionosphere-situated between 50Km to 400 Km above earth surface-under favorable conditions.

• Signal reflected from ionized layer of atmosphere backdown to earth.

• Signal can travel a number of hops, back and forthbetween ionosphere and earth’s surface.

• Reflection effect caused by refraction.• Frequency: 2-30MHz.

Examples– Military Comm.– Amateur radio

5.3 Line-of-sight propagation

In this mode of propagation, electromagnetic wavesfrom the transmitting antenna reach the receiving antennaeither directly or after reflections from the ground in theearth’s troposphere region. Troposphere is that portion ofthe atmosphere which extends upto 16Km from the earthsurface. Frequency: More then 30MHz

Figure 12. Ground wave propagation.

Figure 13. Sky wave propagation.

Figure 14. Line of sight propagationa

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• Transmitting and receiving antennas must be withinline of sight– Satellite communication – signal above 30MHz not reflected by ionosphere– Ground communication – antennas withineffective line of site due to refraction

• Refraction – bending of microwaves by the atmosphere– Velocity of electromagnetic wave is a function

of the density of the medium– When wave changes medium, speed changes– Wave bends at the boundary betweenmediumsExamples:TV, satellite, optical comm.

6. ANTENNA CLASSSIFICATION

Antenna can be classified on the basis of:1 Frequency - VLF, LF, HF, VHF, UHF, Microwave,

Millimeter wave antenna2 Aperture - Wire, Parabolic Dish, Microstrip

Patch antenna3. Polarization - Linear (Vertical/Horizontal),

Circular polarization antenna4. Radiation - Isotropic, Omnidirectional,

Directional, Hemisphericalantenna

6.1 Frequency Basis

Examples of Antenna on Frequency basis

1. Very Low Frequency (VLF) & Low frequency (LF)antenna:Vertical Radiators, Top-loaded Monopoles, T and InvertedL antennas, Triatic antenna, Trideco antenna, Valley-span antenna.

2. Medium Frequency (MF) antennas:Radiators (monopoles and dipoles), directional antennas.

3. High Frequency (HF) antennas:Log periodic antenna, conical monopole and InvertedCone antennas, Vertical whip antenna, Rhombic antenna,Fan dipole antenna.

4. Very High Frequency (VHF) & Ultra High Frequency(UHF) antennas:Yagi-Uda antennas, log periodic antennas, Helical antennas,Panel antennas, Corner reflector antennas, parabolicantennas, discone antennas,

5. Super High Frequency (SHF) & Extremely High Frequency(EHF) antennas:Parabolic antenna, pyramidal horn antennas, disconeantennas, monopoles and dipoles antennas, Microstrippatch antennas, fractal antenns.

6.2 Aperture Antennas

Aperture antennas transmit and receive energy fromits aperture.• Wire antennas• Horn Antenna• Parabolic reflective antenna• Cassegrain antenna

6.2.1 Wire Antenna

A wire antenna is simply a straight wire of length ë/2 (dipole antenna) and ë/4 (monopole antenna), where ëis the transmitted signal wavelength. A wire antenna canbe a loop antenna such as circular loop, rectangular loop,etc. Basically all vertical radiators are come in to wireantenna categories. A whip antenna is the best exampleof wire antenna.

6.2.2 Vertical Monopole antenna

• Length < 0.64l• Self impedance: ZS = Z

ANT+R

GND + R

REF

• Efficiency: η = |ZANT

| /|ZS| η ranges

from < 1% to > 80% depending on antenna lengthand ground system

• Efficiency improves as monopole gets longer andground losses are reduced

RADAR, ExperimentalExtremely High Frequency (EHF)

30-300 GHz

Airborne RADAR, Microwave Links, Satellite Communication.

Super High Frequency (SHF)3-30 GHz

Television, satellite communication, radiosonde, surveillance RADAR, navigational aids.

Ultra High Frequency (UHF)300-3000 MHz

Television, FM broadcast, air traffic control, police, navigational aids.

Very High Frequency (VHF)30-300 MHz

Telephone, Telegraph and Facsimile, amateur radio, ship-to-coast and ship-to-aircraft communication.

High Frequency (HF)3-30 MHz

AM broadcasting, maritime radio, coast guard communication, direction finding.

Medium Frequency (MF)300-3000 KHz

Radio beacons, Navigational Aids.Low Frequency (LF)30-300 KHz

Navigation, SONAR.Very Low frequency (VLF)3-30 KHz

Typical serviceDesignationFrequency Band

RADAR, ExperimentalExtremely High Frequency (EHF)

30-300 GHz

Airborne RADAR, Microwave Links, Satellite Communication.

Super High Frequency (SHF)3-30 GHz

Television, satellite communication, radiosonde, surveillance RADAR, navigational aids.

Ultra High Frequency (UHF)300-3000 MHz

Television, FM broadcast, air traffic control, police, navigational aids.

Very High Frequency (VHF)30-300 MHz

Telephone, Telegraph and Facsimile, amateur radio, ship-to-coast and ship-to-aircraft communication.

High Frequency (HF)3-30 MHz

AM broadcasting, maritime radio, coast guard communication, direction finding.

Medium Frequency (MF)300-3000 KHz

Radio beacons, Navigational Aids.Low Frequency (LF)30-300 KHz

Navigation, SONAR.Very Low frequency (VLF)3-30 KHz

Typical serviceDesignationFrequency Band

ë /4 Vertical Monopole: (Fig.16)

Figure 15.

Figure 16. ë /4 Vertical monopole

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• Length ~ 0.25l• Self impedance: ZS ~ 36 - 70 W• The l /4 vertical requires a ground system, which acts

as a return for ground currents. The “image” of themonopole in the ground provides the “other half” ofthe antenna

• The length of the radials depends on how many thereare

• Take off angle ~ 25 deg ë /4 Vertical Monopole: (Fig.17)

one end and open at the other end. If flaring is done inone direction, then sectorial horn is produced. Flaring inthe direction of Electric vector and Magnetic vector, thesectorial E-plane horn and sectorial H-plane Horn are obtainedrespectively. If flaring is done along both walls (E and H)of the rectangular waveguide, then pyramidal horn is obtained.By flaring the walls of a circular waveguide, a conical hornis formed.

Figure 19. Corrugated conical

horn antenna

Figure 20. Pyramidal and

conical horn antennas.

Figure 17. ë /4 Vertical monopole.

Figure 18. Rectangular loop.

• Length is approximately 0.48l• Self impedance ~ 2000 W• Antenna can be matched to 50 ohm coax with a tapped

tank circuit• Take off angle ~ 15 deg• Ground currents at base of antenna are small; radials

are less critical for l/2 vertical

The Rectangular Loop: (Fig.18)

• The total length is approximately 1.02 l.• The self impedance is 100 - 130 W depending on height.• The Aspect Ratio (A/B) should be between 0.5 and

2 in order to have Zs ~ 120 W.• SWR bandwidth is ~ 4.5% of design frequency.• Directivity is ~2.7 dBi. Note that the radiation pattern

has no nulls. Max radiation is broadside to loop• Antenna can be matched to 50 Wcoax with 75 W l /

4 matching section.

6.2.3 Horn Antennas

A horn antenna maybe regarded as a flared out oropened out waveguide. A waveguide is capable of radiatingradiation into open space provided the same is excited at

6.2.4 Parabolic Reflective Antenna

A parabola is a two dimensional plane curve. A practicalreflector is a three dimensional curved surface. Thereforea practical reflector is formed by rotating a parabola aboutits axis. The surface so generated is known as “paraboloid”which is often called as “microwave dish” or “parabolicreflector”. The paraboloid reflector antenna consists of aprimary antenna such as a dipole or horn situated at thefocal point of a paraboloid reflector. The important practicalimplication of this property is that reflector can focusparallel rays on to the focal point or conversely it canproduce a parallel beam from radiations originating fromthe focal point.

6.2.5 Prime Focus Paraboloid Reflector antenna

• Shaped reflector: parabolic dish, cylindrical antenna.–Reflector acts as a large collecting area andconcentrates power onto–a focal region where the feed is located

Figure 21. Prime focus paraboloid reflector antenna.

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6.2.6 Cassegrain Antenna

In cassegrain antenna primary feed radiator is positionedaround an opening near the vertex of the paraboloid insteadof at focus. Cassegrain feed system employs a hyperboloidsecondary reflector whose one of the foci coincides withthe focus of paraboloid. The feed radiator is aimed at thesecondary hyperboloid reflector or sub-reflector. As such,the radiations emitted from feed radiator are reflected fromcassegrain secondary reflector which illuminates the mainparaboloid reflector as if they had originated from thefocus. Then the paraboloid reflector colliminates the raysas usual.

6.2.7 Advantages of cassegrain antenna• Less prone to back scatter than simple parabolic antenna• Greater beam steering possibility: secondary mirror

motion amplified by optical system• Much more compact for a given f/D ratio.• Reduction in spill over and minor lobe radiation.• Ability to get an equivalent focal length much greater

than the physical length.• Ability to place the feed in a convenient location.• Capability for scanning or broadening of the beam by

moving one of the reflecting surfaces.

6.2.8 Microstrip Patch Antenna

In spacecraft or aircraft applications, where size, weight,cost, performance, ease of installation, and aerodynamicprofile are constraints, low profile antennas are required.In order to meet these specifications Microstrip Patch antennasare used. These antennas can be flush mounted to metalor other existing surfaces and they only require space forthe feed line which is normally placed behind the ground

plane. The major disadvantages of patch or microstrip antennasare their inefficiency and very narrow bandwidth whichis typically only a fraction of a percent or at the most afew percent.

6.3 Antenna Classification on Polarization Basis

Antenna polarization is governed by the polarizationof Electromagnetic waves. Based on that:1. Linearly (Vertically/Horizontally) Polarized antenna.2. Circularly Polarized antenna.

6.3.1 Linearly (Vertically/Horizontally) polarizedantenna

If antenna is transmitting/receiving Vertical E fieldvector, then antenna is said to be vertically polarized antenna.

If antenna is transmitting/receiving horizontal E fieldvector, then antenna is said to be horizontally polarizedantenna.

Figure 22. Cassegrain antenna.

Figure 23. Microstrip patch antenna.

Figure 24. Various shapes of patch antenna.

Figure 25. Examples of linearly polarised antennas.

6.3.2 Circularly Polarized antenna

If the antenna is able to transmit or receive E fieldvectors of any orientation, then antenna is said to becircularly polarized antenna.

Figure 26. Examples of circularly polarised antennas.

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6.4 Antenna classification on Radiation Pattern Basis

On the basis of radiation pattern antenna can be classifiedas:1. Isotropic antenna.2. Omnidirectional antenna.3. Directional antenna.4. Hemispherical antenna.

6.4.1 Isotropic Antenna

An isotropic antenna is a fictitious antenna and isdefined as a antenna which radiates uniformly in all directions.It is also called as isotropic source or omnidirectionalantenna or simply unipole. An isotropic antenna is ahypothetical lossless antenna, with which the practicalantennas are compared. Thus an isotropic antenna is usedas reference antenna. Although sometimes, a half-wavedipole antenna is also used as reference antenna but thesedays use of isotropic antenna as reference antenna is preferred.Let us assume that practical antenna is having a gain of3 dBi means that gain of practical antenna is three timesmore than that of isotropic antenna when both the antennaare connected with same source.

6.4.2 Omnidirectional Antenna

Omnidirectional antennas are those antennas whichwill cover equally well in azimuth direction and havingsome angle in elevation direction. Basically most of thewire antennas are having omnidirectional radiation pattern.Examples are Whip antenna, Dipoles antennas, etc. Theradiation patterns of omnidirectional antennas are shownbelow.

6.4.3 Directional Antennas

Antennas which directs its energy in one particulardirection is said to be directional antennas. These antennasare having very high gain and directivity to cover largewireless distance. Examples are paraboloid reflector antenna,Yagi-Uda antenna, Log periodic antenna, etc. Radiationpattern of these antennas are shown below.

6.4.4 Hemispherical Antenna

Antenna whose radiation pattern will cover the onehalf of the hemisphere either upper hemisphere or lowerhemisphere is said to be antenna with Hemispherical Radiationpattern. Such types are antennas are implemented on aircraftbody to cover the lower hemisphere for data link purpose.Examples are all Monopoles antennas with large groundplane. The radiation pattern of these antennas are shownbelow.

7. ANTENNA CHARACTERISTICS

Before designing an antenna one should know itsperformance parameters or characteristics of antenna forparticular applications. The beam pattern of any antennais shown below in Fig.29 and 30.

Figure 28. Directional radiation pattern.

Figure 29. Upper hemispherical radiation pattern.

Figure 30. Antenna pattern showing main beam and side lobes.

Figure 27. Omnidirectional antenna.

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The performance parameters of the antennas are discussedbelow:

7.1 Radiation Pattern

The radiation pattern of any antenna determines itscoverage area in free space. The radiation pattern of anyantenna looks like as shown in Fig.31.

7.2 Gain (G)

Gain of an antenna without involving the efficiencyis defined as “the ratio of maximum radiation intensity ingiven direction to the maximum radiation intensity from areference antenna produced in the same direction withsame power input”.

Gain is also defined as the increase in signal strengthas the signal is processed by the antenna for a givenincident angle– Usually expressed in dB– Can be negative

An isotropic antenna has unity gain– 0 dBA general Gain equation is given by-G ç (4ð/ë2) Apwhere

ç – efficiency of the antennað – wavelength in metersAp– the physical area of the aperture in m2

7.3 Directivity (D)

Directivity of an antenna is defined as the ratio ofMaximum radiation intensity to its average radiation intensity.

Relation between Directivity and Gain of antenna-G ç Dwhereç – efficiency of the antenna

7.4 Antenna Efficiency (ç )

The efficiency of antenna is defined as the ration ofpower radiated to the total input power supplied to theantenna and is denoted by ç . Thus,

Antenna Efficiency, ç =Power Radiated/Total InputPower

In terms of resistances,

ç = [Rr/(R

r+R

l)] × 100

where, Rr = Radiation resistance; R

l = Ohmic loss resistance

of antenna conductor

7.5 Beam Area and Beam EfficiencyBeam area :

7.1.1 Properties of Radiation Pattern of antenna

• Always measured in Far field.

2DFar field: r 2

λ> D: largest dimension of the antenna

• Field intensity decreases with increasing distance, as1/r .

• Radiated power density decreases as 1/r2.• Pattern (shape) independent on distance.• Usually shown only in principal planes.

7.1.2 Antenna Regions

Far-Field (Fraunhoffer) Region 2D

r 2λ

>

– Where D is the largest linear dimension of the antenna– This is the region where the wavefront becomes

approximately planar– The apparent gain of the antenna is a function only

of the angle (i.e., the antenna pattern is fully formed)

Radiating Near-Field (Transition region)2D

< r <22

λπ λ

– The region between near and far field– E and H are equal, but inverse square law does not

apply– The antenna pattern is not fully formed

Reactive Near-Field r<2

λπ

– Gain is not a meaningful parameter here– E and H are not equal– Reactive components 10% or more of radiating components

may cause error in field measurements

Figure 31. Antenna Parameters definitions are based on the geometry of the antenna gain pattern.

Figure 32. Antenna radiating regions.

2

0 04

( , ) sin( ) ( , )A n nP d d P dπ π

π

θ φ θ θ φ θ φΩ = ⋅ = Ω∫ ∫ ∫∫

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Main Beam area :

Minor lobes area :

Main Beam Efficiency :

7.6 Effective Aperture and Aperture Efficiency

Effective aperture of the antenna is that aperture thatwill actively take part in transmission and reception ofelectromagnetic waves. The relation between physical andeffective aperture of the antenna is given by-

Effective Aperture=K × Physical Aperture, 0< K <1Receiving antenna extracts power from incident wave: rec in eP S A= ⋅

Aperture and beam area are linked:2

eA

Aλ=Ω

Aperture efficiency can be defined:e

app

A

Aε =

7.6.1 Radiation Resistance

The radiation resistance is a hypothetical resistanceand does not correspond to a real resistor present in theantenna but to the resistance of space coupled via thebeam to the antenna terminals.Antenna presents impedance at its terminals, A A AZ R jX= +

Resistive part is radiation resistance

plus loss resistance, A R LR R R= +

( , )M n

Mainbeam

P dθ φΩ = Ω∫∫

min

( , )m n

orlobes

P dθ φΩ = Ω∫∫

MM

A

ε Ω=Ω

range in units of frequency over which the antenna operates– Often stated in percentage bandwidth

7.8 Beamwidth (èB, ÖB)

The “n”-db beamwidth (èB, ÖB) of an antenna is theangle defined by the points either side of boresight atwhich the power is reduced by n-dB, for a given plane.– For example if èB, represents the beamwidth in the

horizontal plane, ÖB represents the beamwidth in theorthogonal (vertical) plane.

– The 3-dB beamwidth defines the half-power beam.

7.9 Polarization

The polarization of an antenna defines the orientationof the E and H waves transmitted or received by the antenna– Linear polarization includes vertical, horizontal or slant

(any angle)– Typical non-linear includes right- and left-hand circular

(also elliptical)

7.10 VSWR/Return loss

VSWR or Return Loss determines the matching propertiesof antenna. It indicates that how much efficiently antennais transmitting/receiving electromagnetic wave over particularband of frequencies.

7.11 Impedance

Antenna must be terminating with 50 Ohm impedancein order to transfer maximum power from transmitter intofree space.

8. ANTENNA MEASUREMENT

Antenna must be undergoing various measurementsbefore installing on the system. Basically there are twotypes of measurement conducted on antennas:1. Passive Measurement/Laboratory Measurement

• VSWR/Return Loss• Impedance Bandwidth

2. Active Measurement• Radiation Pattern (Elevation And Azimuth)• Gain• Directivity• Half Power Beamwidth• Cross Polarization

8.1 Passive Measurement/Laboratory Measurement

VSWR/Return Loss and Impedance Bandwidthmeasurement can be done on Vector Network Analyzer.Antenna port is connected to one port of the networkAnalyzer and can see its VSWR/Return Loss and ImpedanceBandwidth directly on the screen of the Network Analyzer.

8.2 Active Measurement

In active measurement, the following properties ofantenna can be tested:

• Radiation Pattern (Elevation And Azimuth)• Gain

7.6.2 Frequency Coverage

The frequency coverage of an antenna is the rangeof frequencies over which an antenna maintains its parametricperformance– Antennas are generally rated based upon their stated

centre frequency– Example: 9.85-10.15 GHz, fc = 10.0 GHz

7.7 Bandwidth (B)

The bandwidth (B) of an antenna is the frequency

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Figure 34. Set up for measuring VSWR/Return loss and impedance of antenna using vector network analyser.

• Directivity• Half Power Beamwidth• Cross Polarization

8.3 Radiation Pattern Measurement

• Open field– Outdoor Elevated Range– Ground Reflection Range

• Anechoic chamber– Rectangular Anechoic Chamber– Compact Antenna Test RangeOpen Field

Anechoic Chamber

Radiation Pattern of Some Antennas

9. ANTENNA APPLICATIONS

9.1 Astronomical Antenna

Radiation Pattern of Mobile antennas

Helical Antenna1. Highly DirectionalA n t e n n a2. Circularly PolarizedA n t e n n a3. Use in Radio Astronomy

9.2 Defence Antennas

Paraboloid Grid Reflector Antenna

A close-up view of the conical high-frequency Dipoleantenna mounted on the bow of the Ship

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A close-up view of the antenna masts andbridge structure aboard the guided missilecruiser as seen from off the ship`s starboardbow.

A view of the antenna rig aboard the guided missilefrigate USS DOYLE (FFG-39).

A view of the antenna array on the island structure ofthe nuclear-powered aircraft USS Theodore Roosevelt(CVN-71) .

A view of the AN/SPN-46(V) radar antenna for theautomatic carrier landing system (ACLS) aboardthe nuclear-powered aircraft carrier USS AbrahamLincoln (CVN-72).

Antennas by Drdo

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