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DEFENSE DOCUMENTATION CENTERFOR

SCIENTIFIC AND TECHNICAL INFORMATION

CAMERON STAT;ON. ALEXANDRIA. VIRGINIA

UNCLASSIFIED., I.'+• C c ?,.+']++

NOTICE: Uber gcverment or other dra•ings, speci-fications or other data are used for any purposeother that in connection vith a definitely relatedgovernment procurment operation, the U. S.Covernment thereby incurs no responsibi.ity, nor anyobligation vhatooerer; and the faet that thU Govern-ment may have formulated. furnished, or in any wysupplied the said dravings, specifications, or otherdata is not to be regrxded by implication or other-vise ac in any manner licensing the holder or avyother person or corporation, or conveying any rightsor permission to manufacture, use or sell anypatented invention that my in any way be relatedthereto.

I~

SEDL-G220 DA 36-039 AMC-00088(E)SVOLLIF:

-l Noncommunication Expendable

_ Jammer Investigations

= ;= VOLUME 2- L=

no Omnidirectional Antennas

CHARLES F. HUMMEL

rJUL61%DDC-IRA C

S • I\ \M I \ L C o4N IC sYSTErN's

G6NE/I T£LEPHONE A, ELCTNIT OI

n PREPARED FOR THE UNITED STATES ARMY

A A

PPR FOTUNDSASR

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BestAvai~lable

copy

DEFENSE DOCUMENTATION CENTER AVAILAZILITYNOTICE:

QUALIFIED REQUESTORS MAY OBTAIN COPIES OFTHIS REPORT FROM THE DEFENSE DOCUMENTATIONCENTER. FOREIGN! ANNOUNCEMENT AND DISSEM-INATION OF THIS REPORT BY DDC IS LIMITED.

EDL -tZZOVolume 2

ELECTRONIC DEFENSE LABORATORIES

P 0. Box Z05

Mountain View, California

REPORT No. EDL-GZZ03 April 1964

NONCOMMUNICATIONS EXPEI'DABLE JAMMER INVESTIGATIONS

VOLUME 2

OMNIDIRECTIONAL ANTENNAS

Charles Hummel

Approved for publication . Harvey JohnsonManagerTactical Electronic Warfare

Robert StoneHeadFuze ECM

Prepared for the U S Army Electronics Research and DevelopmentLaboratories under Contract DA 3b60U9 AMC -0UUtjE).

SYLVANIA ELECTRIC PRODUCTS INC.

EDL-G220Volume 2

C O XNT E N T S

Secton Tatie Page

I. INTRODUCTION ............................. 1

2. PRELIMINARY INVESTIGATIONS................ 2

3. OMNIDIRECTIONAL ANTENNA TYPES CONSIDERED 3

4. DEVELOPMENT OF THE STACKED-DISCONEARRAY ANTENNA ..................... 5

4. 1 Discone Geometry ... ......................... 54.2 Stacked-Discuae Array ....................... 74. 3 Radiatior Pattern . ....................... . . 11

5. GROUND EFFECTS .......................... 13

5. 1 Theoretica: Considerations ............. ..... 135.2 Computation Reults ....................... 20

6. RESULTS OF INVESTIGATION ............... 28

7. REFERENCES . ............................ 29

EDL-G220

Volume 2

ILLUSTRATIONS

Figure Title Page

I Dizcone Geometry . ... ...... ................. 6

2 Sinele-Element Di.cone Prior to Asbembly ...... 4

3 A~sembled Single-Element Discone .9.... ..

4 Assembled Stacked-Discone Array withRadome Removed .. ..................... 10

5 Four-Element Stacked-Discone Array

Antenna with Radome Assembled ............... 12

6 Radiation Pattern -- Ele~ation..... 14

7 Radiation Pattern-- Azimuth ........... ........ 15

8 VSWR as a Function of Frequency ...... .. . . . . 16

9 Vector Relationship . . . . . .......... 18

10 Geometry of a Microwave Antenna andits Mirror Image ........................ 19

11 Power Level as a Function of ElevationLevel and Antenna Height ...... ...... . ...... 21

12 Power Level as a Funct-on of ElevationLevel and Antenna Height ..... .. ................ 23

13 Power Level as a Function of ElevationAngle and Antenna Height ...... .... . .......... 24

14 Dower Level as a Fun'tio, nf FlevattonAngle and Antenna Orientation .... ............ .. 25

15 Power Level of a single Element as a

Function of Elevation Angle andAntenn¢. Height ...................... 26

16 Power Level as a Function of Antenna

Height and Antenna Orientation forLo% Ele ation Levels (Flat Earth). . ...... 27

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EDL-G220Volume 2

NONCOMMUNICATION EXPENDABLE JAMMER INVESTIGATIONSVOLUME 2 - OMNID1RECTIONAL ANTENNAS

Charles F Hummel

I INTRODUCTION.

Adequate electromagnetic coupling between antennas is essential to the successfultactical operation of a microwave )ammer against a target. in an idealized case,%,here reflecting surfaces are very r.: note from the antennas (or where thesurfaces are near but have reflect.cn co, fficients which are negligible atmicrowave frequencies). electromagnetic k-oupling between antennas can bereadily established from free-space radiation patterns and the distance between

antennas. Coinplications arise, however, when the antenris are in relativelyclose proximity to a large reflecting surface whose reflection coefficient is not

r, ,,ihible. Such is the situation usually encountered in various practicala. , aitions and, in particular,, in the case of tactical deployment of an' expendablemic rowave jammer whose antenna is in relatively close proximity to the earth'ss-- rface and the target is near the horizon (i.e. . at a very low elevation argle).

The type of reflecting surface, such as tirregular terrain, its moisture content,

tnow cover, etc.. add further complications to the general electromagneticcoupling problem particularly when various types of obstructions and vegetation

are involved. The heart of the antenna coupling prv'blem, however, centersaround one important antenna-system parameters:, antenna height above ground

The following discussion has a two-fold purpose.

(a) to present engineering data related to the development ofan B-db (relative to isotropac) omnidirectional (in the azimuthplane) antenna breadboard model and its measured free-spaceradiation- pdtte rn cha acte ri stic s,

Ib) to presert data in graphical form. based upon theoreticalground-effect considerations, which show the importanteffect that height above ground has upon the antenna's eifectiveradiated power at low elevation angieb

The data presented herein do not include effects due to irregular terrain.obst ructions. .r vegetation. Only flat terrain is assumed and the data shouldbe 1 on.idered representat.ve only of the idealized terrain Lundition. This"was du-e in order to explore the magnitude of the ground effect wtthoi.t unduetormpl;cation Actual conditions encountered in the field can be expec ted tocause quite wide variations.

EDL-GZZOVolume 2

2 PRELIMINARY INVESTIGATIONS.

Early in the program it appeared desirable to consider an azimuth omnidirectionalmicrowave antenna configuration which could he an inte-,ral part of the structureof the expendable-jammer vehicle (such as a cavity-backed slotted array, oras a portion of the tail structure) For a description of the expendable jammerthe reader is referred to EDL-M622. It soon became apparent that theexpendable jammer's microwave antenna performance would be quite dependentupon the vehicle's vertical attitude and the antenna height after impact. Thevehicle's inability to control its penetration depth (over wide variations in typeof terrain such as hard soil to loosely packed sand) means that a relativelylarg.,,. riatiou, - .ntenna-to-ground height must be considered.

Another problem pertaining to environment presented itself during the preliminaryinvestigations Under typical field conditions, he orientation angle, off vertical,that the longitudinal axi.s of the vehicle and antenna can be expected to assumeafter impact could not be predicted or controlled. A realistic solution to thisproblem was deemed important since a relatively large off-vertical angle ofincidence could seriously reduce the effective radiated power by a directiveantenna whose beam is narrow in the elevation pattern.

Also considered in the preliminary investigations were various types of azimuthomnidirectonal antennas which could be developcd, separate from the vehicularstructure, that would lend the-nselves to being elevated from the aft portionof the vehicle after impact Briefly considered was the possibility of utilizinga gimbal-type assembly to maintain vertical or near-vertical antennaorient.tion regardless of the vehicle's orientation with respe t to the terrain;for the case where the vehicle might impact on the side of a hidl, however; thegimbal assembly could not be expected to orient the beam of the antenna towardthe target

The need for a suitable microwave elevation direction-finding device to solveelevation orientation problems when using a high-gain antenna became apparentThe only simple and practical alternative was to consider an antenna which hada broader beamwidth and then to compensate for the corresponding reductionin antenna gain by increasing the antenna height above ground A solution totho lattcr approach 's deemned less complex and possibly more reliable inoperation, so breadboard developmental work was initiated

I See list of references in Section 7

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EDL-G220Volume 2

3. OMNIDIRECTIONAL ANTENNA TYPES CONSIDERED.

Initial electrical design effort centered around various types of omnidirectionalin azimuth, vertically polarized, S-band antennas which would provide free-space directive ga-as ot at least 6 db over the frequency range of 2. 7 to 2. 85

Gc with antenna power handling capabilities of 2-kilowatt peak (20 wattsaverage)

The first antenna type considered was a curved-surface radiator of the taperedmoncpole variety with a dielectric-ring type of lens above a metallic groundplane. 2 Preliminary tests of this type oi omnidirectior.al antenna contigurationdisclosed, however, that the dielectric ring's beam-shaping effect in elevationwas not as pronounced as was expected.

The next antenna type considered was a simple serias-fed collinear array offour dipoles.3 Seemingly identical breadbca-d nmodels exhibited ratherpronounced variations in electrical characteristics; the relatively smallcylindrical confLguration was found co be critical with regard to aligment,

spacing, tolerances, etc. In addition, test results at these microwavefrequencies could not be repeated readily with routine handling of the breadboardmodels. A parallel-fed collinear dipole array with a coaxial-within-coaxialfeed offered hope of improvement over the series-feed arrangement, but provedto be quite complex in feed arrangement.

The last antenna type considered wsa ac. indwviclually fed st-tcked-disconearray consisting of four radiating elerm.ents connected via spiraling coaxialcable routings to a power divider.4" , r. When this configuration wasbreadboarded and tested electrically, it was found to be much less criticalto physical misalignments and other nonuniformities than the other antennatypes investigated.

The la.st conf-guration, the stacked-dtscone array, appeared to hatisfy the

need for a relatively simple, .iechanicaliy reliable structure %hich couldlend itself to modular construction (the radiating elements are individuallyfed from separate ports of the power divider). The modular aspect was

considered desirable for razious reasons. For instance, in later radiationmeasurements. patterns could be obtained for a single elemnent as well asfor the array of four :.ements and comparisons could be made convententlybet%%een wide-beam and relatively narrow-bearm. effects over terrain at differentorientation angles. Consequently, a breadboard model was deiigned andconstructed in the modular c nfiguration for system testing within thelabo rato ry.

It snould be noted that the antenna breadboard effort centered ,round electrical

characteristics physical requirements relating to size, weight, vbratiurn.shock. and other environmental factors were not sresbed in the breadboard

-3-

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l -

EDL-G2Z0Volume 2

4. DEVELOPMENT OF THE STACKED-DISCONE ARRAY ANTENNA.

The radiation pattern of a discone element is similar to that of a dipole; theenergy is radiated in patterns which form a solid of revolution with a figure-eight cross section. The 6peratlonai frequency range of a discone, however,exceeds that of a dipole. With the discone's axis oriented vertically, theradiating element provides vertically polarized patterns which are, forthe idealized case, perfectly omnidirectional in azimuth. (Slight deviationsin the omnidirectional characteristic were encountered in this applicati..n,due to slight aperture blocking by feed lines. This is discussed later.)

4. 1 Discone Geometry.

The discone consists of a cone (i. e. , a conducting conical surface) and a coaxialfeed line that is coincident with the axis of the cone. The outer conductorof the coaxial is connected to the cone and the inner conductor of the coaxialis permitted to pass through the slightly truncated apex of the cone and connectto the center of a relatively small conducting disk. The disk is positionedperpendicular to the cone's axis and at a predetermined spacing.

The cone's flare angle and slant height are based upon electrical requirements(relating to such characteristics as bandwidth, impedance. and desired radiationpattern characteristics) in addition to physical requirements which relate tomaximum over all size. Disk size and disk-to-cone spacing are chosen foroptimum impedance match to the coaxial-line feed.

With reference to Figure 1, the geometry of a discone can be expressed thus:

tan- ' r)h

C

-%here total flare angle (as measured from the truncated apexof the cone),

r. .: maximum radius of the cone,

r U minimum radius at the slightly truncated apex.

h - height of the cone (as measured from the truncated apex).

Optimum "alueb for disk size and disk-to-cone spacing were establishedexperimentally; disk radiu., (rd) was set at 70 percent of the maximuis coneradius (r, ), and disk-to-cone spacing(s) was selected on the basis of minimumimpedance mizinatch over the operational frequercy range. The cone'sslight truncated apex forms a part of the antenna feed; a value was assigned

-5.

MDL-GZZtOVoIlxme 2

0 SK

h€

Figure 1. Ihsctlt. Geometry.

Vo)lure 2

4 1 -- Continued

to r., the minimum radius at the truncated apex, such thit it would beessentially negligible in the geometrical relationship expressed above (i.e.a value of approximately 10 perceat or less of the maximum cone radius).

With reference to Figure 1, design compromises were neeeded in the geometrical

ItvuUt of the discont, to achieve desired radiation-pattern characteristics and-,ntable impedance matches consistent with physical requirements relating

to maximum permissible size i. e. , in terms of flare-angle (*) and height (he)]

Geometrical values selected for the application were as follows:

h : 1. 62 inches (0. 372 to 0. 390 ý over the operational frequencyrange)

59 degrees

r, : 1. 00 inch

rd 0. 70 inch

s 0. 122 inch

ro • 0. 100 inch.

One of the discone elements, prior to assembly, is :.Nown in Figure Z. Thethree large dielectric disks (with slotted edges) provide support for the coneand small disk and provide a guide for the coaxial cable routing of otherdi ,t ont. elemnents after assembly. The dielectric rods "erve as spacers},tc,*' decks Figure 3 shows the modular, discone element after ;asstenbly

\%tit its individually fed coaxial line which conne ts to onte of the ports in a power,hvidcr

Elcttrical measurements made upon the single element disclosed a VSW\ ,fI 25 ±O 2 over the operational frequency range, with elevat,,in -pat'ern half-pw,%,,'r beamw-i cth approximately 50 degrees (i. e. . in frl,'. sppace). The fre..

-);t.t.- di rec tive gain was 3 1 db relative to isotropic at the mi dband frequent'v,,1 2 7; Cc

I .Sta_ keci-Disone Aca .

I-'i.urv c4 shows the assembled, stacked-discont array. Four identical di.-comtniodnles are stacked, one above the other, with thair individual coaxial feedlincs .pi raling downward and connecting to the power divider. T'he rndiating

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0

E

U

U

14

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0E

EDL-GZ20Volume 2

c0

0

90

EDL-GZ20Volmne 2

63100703

Ficure 4. As~cmblh-d Stacked Di.cone Array with Radomt. Removed

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3DL-G220Volume Z

4 2 -- Continued

elements are 270 electrical degrees apart and are fed in phase by equallengths of coaxial line. The lengths of coax for the lower elements are woundaround the base support of the power divider. The larger dielectric disks, inaddition to serving as guides for the spiraling coaxial lines, also serve asgudes for mounting the radome (shown to the right in Figure 4)

The Wour-element stacked-discone array antenna with radome assembled isshown in Figure 5. This antenna array has an overall height of 20 inchesincluding the power divider. The height of the radorne is 15-5/8 incheswith an outside diameter of approximately 5-1/16 inches. Maximumoverall diameter at the base of the assembly is 5-3/4 inches. Total weightof the antenna array is 6. 25 pounds. (No attempts were made to restrictweight in this breadboard model. The experimental discone elements wereconstructed of brass, which is very easy to machine.)

4 3 Radiation Pattern.

For a vertical array of discone elements, maximum free-space radiationoccurs in the horizontal direction when all of the radiating elements areoperated in phase. The elevation pattern characteristic of a verticallydiscone array may be expressed thus4 .

sin nIt sin$ - 1F (,)z ( ( n -2

.;I sin 'P-

•here - elevdtion angle

ni number of discone elements

- spacxng (electrical degrees) between di coneh

phase difference between adjacent di! cones.

The individual feed of this antenna array makes !t convenient to shift the axis ofthe mAin beam slightly (i.e. . up or down in elevation) by altering the indivi-dual lengths of coaxial line The electrical phase difference betweenAdjacent discones ig). as expressed It& t".e above relationship. can be variedin accordanci with the following relationship

whhere B, the desired angle of beam tilt

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EDJ.-G2ZO

Volume 2

63100702

Figure 5. Four-Elenment Stackcd Discone Array Antenna with

Radome Assembled.-12-

Vol'.r,:e

. 3 -- Cont:nued

Equal lengths of coaxial line were used to feed the di scone elements inphase for initial system testing. A problem was encountered in the preciseplacement of the separate coaxial transmission lines in order to achievethe least d) s'.urbance in azimuth pai*ern characterist.ic As in most','erlically oriented omnidi rectional structures, the supporting mast or.in this case, the multiple transmission line feeds have currents induccdin !hem by the radiating elements; these induced currents (and theirresultant reradiated energy) cause 3lightly distorted or scalloping effectsin the omnidirectional radiation pattern. In this application, patterndistortion was minimized by spi raling the coaxial lines past the disconeapertures to spread the effects of aperture-blocking more evenly in a.zimuth.The radome ensures that the rather critical positions of the coaxial cableswill not be disturbed by handling the model. Radiation-pattern measurementsdisclosed that maximum deviations from omnidirectional approximated2 to Z 5 db

Typical free-space radiation-pattern measurements are shown in Figures6 and 7 Figure 6 shows the free-space elevation pattern at midbandcforthe stacked-discone array. Half-power beamwidth is about 17 degrees;di rective gain relative to isotropic is approximately 8 db. Typical devia-lions from the perfectly omnidirectional characteristic 4tre shown in theazimuths! radiation pattern of Figure 7 Here, a deviation of 1 5 dbcan be observed at a midband frequency of 2. 75 C4.. Cross-polarizationapproximates minus 12 5 db maximum.

Over the frequency range of 2. 7 to 2.85 Gc, fruc - space directive gainof the four-element stacked-discone array is abot.: 7 5 to 8 db Wi th an

,.Vet-4gt, IWt~anivw, dt h tf 17 degret I,• Maxiyinrin 41(i, !,,he It'Ive] ,vvr ths

I '. qjency rainge. is mi nrs 9 db

VSWR values over the ,perational frequency range arte atbou. I f or ) 2 fu"

the array. Fi'urtv 8 provides a comparison of VSWR ,vaus the ,rr-tyul fittia eltei.t '.r.ts a.nd for ai single -l ei nt

G , ROUND EFFECTS

S1I 1 "ieu rt-tical Considerations

'I i. tt1nportpni e ft-ci that ho 'ght aboy,. v rund has tipon the raddiatl.d pow.tu.r

Iceve! of a irnic row:,,e ant,,nna can b64 ,a•.cribed mat.herrm ;ti ally , s. ng ,h.

follwing theoretic'il ctineo|pts. For an ant•inn~t's free-space radiativnpIal'trn it is assumed thi t a rme! c' rtjg -u rfa e tor S rf&:, •s) ii nlfin: tely t,'Ar

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EDL- 0220Volume 2

GA 114 r~. 4-EL) p ;IVI t' trdRO I ~. C,

iSCG~f-Adii

EDL- G22')Volume 2

DCCIA*FAY

COSCS-POt&ItZATION: -12.5 db

Figure~ -. Radiatimii IPatterm- Aximutl

EDL,-GZ220Volume 2

ARRAY

1 .2

2.6 2.7 2.8 2.9

FREQUENCY -- Gc

Figure 8. VSWVR as a Ywwtictim of Freq~tencv.

EDL -GIZIVolume..J

5. 1 -- Continued.

away from the antenna or that the surface's reflection coefficsent isnegligible. In the presence of perfectly flat earth infinite in extent andat a distance of many wave-lengths from a microwave antenna, fieldintensity (relative to that in free space) may be expressed thus

E +I' +i

U

where field intensity relanve tu that in free space

o' magnitude of the reflection coefficient

phase difference between the incident and reflec'ed

waves

Other terms are considered to be negligible here (i. g. . surface wave.secondary ground effects, induction field. etc V

In this generalized relationship, the first term (which has a vdue Ufunity) represents the chrect wave; the second term represents the reflectedwave Rewriting the expressior.z in terms of relative power level weobtain

00 E 0L 1o o-• • r I . • ; (cosA £& sin } 1)

The term contained within the brackets is known as the ground effect andtends to Alter the antennals free-space radia':un-pattern power level (P Iin accordanc, with the magnitude of the reflection coefficient (I p9 ) andthe difference in phase which ex.sts between the incident and refl.ected wavei.-.(L1

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£L)Lt,-GJ•1O

Volume Z

.I -- Continued.

, 1 2 4 21picos (I8o-.)-

Figure 9. Vector Relationship.

From the vector relationship shown in Figure 9. the ground-effect term isthe vector sumn or resultant of the incidedit and reflected waves. as establ-ished by the law of cosines:

t +, I; (cos.L +j sin L.)] (le +/ -214 Cos (180

S[I+• +2I' C•s L

The difference in phase (a) which exists between the incident and reflectedwaves can be expressed thus:

:= (a - 6),

whe re If phase angle of the reflection coefficient (i e.. thephase change occurring at reflection)

I= phase angle associated with the effective change inpath length (i. e., between the antenna's direct waveand the reflected wave, av expressed in wave-length).

In~

£•DL -G;'4UVulunie 1

.- -CunhnuLd.

OF THEDIFFERENCEANTENNA IN PATH-LENGTH

BETWEEN DIRECT & REFLECTED WAVES

(2 h COS e)r 2r. _hcos_____

=ELECTRICAL LENGTH ( (--)C2 h COS 9) =

Figure 1O. Geometry oi a Microx, ave Antenna and its Mirror Inmage.

Difference in path length between the direct and reflec:ted waves can be

conveniently determined by assuming a mirror-Image beneath the intenna

as shown in Figure i0. Expressed in electrical length:

S=(Zhcu•. G) (--)

IMAGE

4( h cus 0)

6hErL h RIntCnna height ,±buve the ground plane

Fgr= d10t.oe try et eon the A ntenna and its mirror-Image

D- = a ngle uf the antennth direct ad av e ct meadw ured cnithrespect to v'rtical

Hence.p 6 2h 4 hcub e

-19-

4 r~ h -u

EDL -G,20Volume 2

5 1 -- Continued.

Substituting this term into the generalized relationship.

P + ~+ 21 0 Co p [+022 P1 Co (ohrs

From this relationship it will be noticed that P , the power level of theantenna's free-space radiation pattern, is moditfied in accordance with acosine function invol-vng antenna height above ground, expressed -n terrm:

of wavelength Maxima and minima are introduced within the ground-elfect term A maximum occurs when the direct and ground-reflectedwzves are in phase, a mimnmum occurs when the reflected wave it. 130degrees out of phase or nearly so. with the direct wave

5 2 Computation Results

The results of computations based upon theoretical concepts presented inSection 5 1 are shown graphically in Figures 11 through 16 The datado not include effects due to irregular terrain obstructions, or vegetationOnly flat terrain is assumed; the data should be considered representativeonly for the idealized-terrain condition. Actual conditions encounteredin the Ifield wLl vary widely from this basic assumption.

In theae comzputations. values assigned to the complex reflection coelf -cient fi e for average ground') ranged from 0 995 ASo 0 • oj2_ 770 atelevation angles between 0.05 and 2.0 degrees. respectively A frequentyof 2 75 Gc 1i e , midband) was assumed throughout the ca'culziiuoi, Itshould be noted that all calculations involving the antenna array and itsground effect were based upon the measured free-space elevation -pa-terncharacteristic shown in Figure 6 Therefore, a free-spacf directivegain of 8 db (relative to isotropic) serves as the basis upon whicn compar-sons may be made with radiated power levels over flat terrain

Elevation angle 1:. e , 9tP - 6) versus power level as a function of antennaheigh* above ground is shown in Figure 11 for vertical antenna-ariavorientation At a very small elevation angle, say 0 05 degree, the radiatedpower level can be increased 6 db by doubng the antenna height. For

instance, a power level of minus 8 db at an antenna height of 5 feet can be:ncreased * r.�inus Z db by doubling the height to 10 feet As elevatiunangle i! permitted to increase, the radiated power levelb gradually incrv.i-and Ifur this, the theoretically flat-terrain case' eventually reach ir.axI;nmbetween plus 12 9and plus 13.8 db for antenna heights of 2-1/2 and 1Z-/112feet respectively. Firat maximum and first minimum can be recdilyob.erved in Figure II for the vertically oriented case A maximum levt"of only plus 8 db can be expected in situations where variations in terrain

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EDL-GZ20"Volume 2

15

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3

0.05 0.1 I 5.0ELEVATION ANGLE -- DEGREES

Figure 11. Power Level as a Funaction oi U.e;vat:on i.vvcl an~d

Antenna Height.

.41

EDL-G220

\Tolume 2

5.2 -- Continued.

contour are almost comparable to that of the antenna's height.

Figure 17 shows radiated power-level characteristics for an antenna-array

orientation of 5 degrees off vertical. A radiated power level of minus Q. 3db now exists at an antenna height of 5 feet; minus 3. 3 db exists at 10 feet

for 0.05 degree elevation. Therefore, a 1. 3-db reduction occurred between- ertical orientation and 5 degrees off vertical. Tilting the beam upward

and downward produces an appreciable difference in radiated power level

in the vicinity of 3 to 5 degrees in elevation. When the beam is pointeddownward 5 degrees at an antenna height of 2-1/2 feet a null in excess of

minus 10'db exists at 4 degrees elevation; when the beam is pointed upward5 deg.-ees. however, the level of the null increases to plus 4. 5 d, At

elevation angles less than 2 degrees, upward and downward beam tilts

produce no noticeable difference in radiated power level.

Figure 13 shows the case of a stacked-discone array oriented 10 degreesoff vertir.!. Since the free-space half -beamwidth is 8.5 degrees, muchless power now radiates at small elevation angles. For an antenna heightof 5 feet, for instance, a radiated power level of minus 14. 5 db now exists

at 0.05-degree elevation; correspondingly, at 10 feet, a radiated powerlevel of minus 8.5 db exists. A reductiorn of 6. 5 db has occurred betweenvertical orientation and 10 degrees off vertical orientation. Tilting thebeam upward and downward produces a difference in radiated power levelabove i-degree elevation, as can be seen it1 Figure 13. L.low I-degreeelevation, upward and downward beam tilts produce no noticeable difference

in radiated power level.

A composite zet of curves for an antenna-array height of 5 feet at various

orientations appears in Figure 14. At an elevation angle of 0 28 degree.the radi,.ed power level for vertical orientation is plus 6. 5 db; for 5 degreesoff vertical, the radiated power level is plus 5 db; and at 10 degrees offertical, the power level is 0 db relative to isotropic.

Figure 15 shows 2 set of curves for a singlc-discone radiating element Infree space the single element's half-beamwidth is 25 degrees; orientationangels for vertical to approximately 15 degrees off vertical do not signifi-cantly affect the power level near the horizon and are, therefore, accomm-

odated in this set cf curves. Measured free-space directive gain of thesingle-discone element is approximately plus 3 db (or 5 db down from thefour-element array's free-space gain of plus 8 db). This reduction in radiatedpower level can be readily observed from the curves in Figure 15 For*:nstance, at an antenna heigh, of 5 feet and at 0.05 degree elevation, th.radiated power of a single-discone element is minus 13 db for any orientationfrom vera'- iI to about 15 degrees off vertical.

E-DL-G2ZOVolume 2

15

i '-- ~~T TITEED•

o

-> 0 ISOTROPICj7Z 0LEVEL

-5 00 /•,•"

"a -. S.

... ,. z.

in t I at

0.05 0.1 0.5 1.0 .0

ELEVATION ANGLE -- DEGRES

Figure 12. Power Level as a Fw:tcoicn of E1evation Le-el andAntenna Heigh.

EDL-GZZOVolmute Z

TI LTED

0

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0.05 0.1 0.5 1.0 5.0

ELIVATION NIGLC -- DEGREES

Figure 13. Power Level as a Function of Elevation A^iXe and AntennaHeight.

20 .4

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Volume 2

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Figure 1-1. Pouwer , ..vcl as a Function of Elevation Angle andAntenna Orientation.

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Volume 2

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Figure 15. Power Level of a Single Element as a Function ofElevation Angle and Antenna H~eight.

226

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Figure 16. Powcr Level ais a Function of Antenna Height and AntennaOrientation for Low Elevation Levels (Flat Earth).

- 7-

ZDL -G220Volume 2

5. 2 - -Continued

For final compariscris, Figure 16 has been plotted for two small .;vvationangles only, 0. 05 and 0. 20 degrees. Here the graphical data have beenplotted such that the solid lines represent characteristics of the four elementa rray (i. e for various orientations discussed earlier) and the dashedlines represent characteristics associated with the single-discone element.It should be not'ed that in this illustration antenna height, instead of elevationangle, has been plotted along the abscissa Perhaps the most significant.feature to obs',rve in this set of curves ij that a compromise must. even-tually be reached between maximum allowable antenna height above groundand minimum elevation angle for a specific radiated power level. ForCxamnple. froin Figure 16 it can be seen that the radiated power levels foreither an array or a single element at, 0. 2-degree elevation and 2 5 -footant enna hei ght a re comnpa rabl e t~o the radiated l evel whi ch occu rs at t~hereduced elevation angle of 0 05 degree and cc-respondingly in-reattedantenna height of 10) feet

o RESULTS OF INVESTIGATION,

A compromise must be reached betweer. various system requi rements toachieve a specified effective radiated power output from a microwaveomnidi rectional (in azimuth) antenna which is in relatively close'proximityto flat earth If maximum effective radiated power is requi red at thelowest, possible elevation angle. antenna height above ground must beinc reased accordingly

Should maximum antenna orientation, tf vertice.i. pose serious problemns itiradiated power level for a particular elevation ingle and antenna height,d; rec'.ive gain of Ohe antenna may have t.o be sacrificed for broader beamwidth to help solve the antenna -orientation problem. From the foregoingdata approximately 5 db in rzdiated power level must be sac rificedwhewn itI appea rs desi rablte to substitute a broad -beamwidtbi single -di sconeelcirnen'. fur t vert.ically oriented four element, stackcd-discune array .Hloweve r, fu r 10 deg rees off -vertical orientation, 2 db i n radiated powe rlev'e! -an actudllv be gained by substituting the single. discone element fur 'hetuu r-elem en, a rrayv

The heart of the antenna coupling problem is the important effect tha' heightabove ground has upon the antenna's radiated power level at low elevationanglet, Fo r the successful tactical ope ration of a mic rowavt- laminteragainst a fit -ge'. near the I arizon, antenna coupling problems essentiallyredoce to a so ri es of coniproini ses i n antenna and systemn requi rcmeats

udr va ri outs envi runniental conditions

-2.8

EDL-0220Volume 2

7. REFERENCES.

1. Andrew D. Fliss, et al, "Noncommunication Expendable Jammer

Concepts -- Technical Feasibility Studies (U), " Technical Memor-andum EDL-M622, Electronic Defense Laboratories; 15 March]1964. (SECRET publication)

2. Prof Dr. Hans Meinke, "Research on Broadband Antenna Design,"

Institut fdr Hochfrequenz - Technik der Technischen Hochschule,

ANLtidhen (no date), WADC USAF ARDC European Office, Contract

AF (052) - 41; ASTIA AD255-858.

3 Henry Jasik, Ed. , "Antenna Engineering Handbook," 1st edition,pp 2 2 - 2 8 , MzC~raw Hill; 1961.

4. A.G. Kandoian, W. Sichak, and R.A. Felsenheld, "High Gain v.-iliDiscone Antennas," Electrical Communication, , 25, pp 139-147;June 1948.

5. W. P. Allen, Jr. , "Design and Development of the AFCS PositionFixing Ground Station Antenna," Vol. 1, 11th Annual Symposiumon USAF antenna R&D, Monticello, Ill.; October 1961.

6 J. J, Nail, "Designing Discone Antennas," Electronics, pp 167-169;August 1953.

7. H R. Reed and C.M. Russell, "Ultra High Frequency Propagation."Wiley; 1953.

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