Modification of Horn Ant for Low Sidelobe Levels

7/27/2019 Modification of Horn Ant for Low Sidelobe Levels

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TRANSACTIONS OS AXTENNAS AKD PROPAGATION VOL.-4P-14, O. 6 SEPTEMBER 1966 605

[ 1 -&) cos (T, . + cos (s,E) dl?.1(6) and (9) are the equations to beolved. I n

I' is the contour of the parabola and is

We need only the values of t he cosines. The normalis a vector

i d y - d xdI 'n =

i and j are the unit vectors along the X and Y

(R, a) are the coordinatesof 9 . Hencer = i (R- ) +j(u - ) (12)s = (i os 0+j sin0)s. (13)

ThusR - - (g - )-x ]-y (14)r r d yr

and(15)S

The d y / d I ' in the two expressions mill combine with thectr in the integral to convert to an integration over yfrom y = --a to y = + a, theextent of the reflector.Finally, R - x / r is the value f cosa n the gain(a, ) .

ACKNOWLEDGMENTOur thanks to 1%'. C. Danforth, Jr., who did the pro-

gramming for the IBM090 computer.

Modifications of Horn Antennasfor Low Sidelobe Levels

Absfracf-A modified horn anten na with significantly reducedes over nearly a two-to-one frequency band is discussed.

horn has a well-defined phase center at its apex, and the Ee patterns are nearly identical over the frequency band if

circular) aperture. Horns considered coverof flare angles and nclude one which fits the criterionthe optimum horn a t the lower end of the operating frequencyVSWR is less than 1.2 over most of the frequency band.

INTRODUCTIONIS WELL KKO i 3 T that in the region of th e

main beam and first few sidelobes the radia tion pat-tern of a horn antenna may be computed from its

However, utsidehis regiongy diffracted a t th e edges of the horn determines

[ l and the radiation pattern can be

d here was supported in part by Contract AF 30(602)-Manuscript received March 1, 1966; revised May 15 , 1966. Theew York, and The Ohio State University Research Foundation.

The authors are with the Antenna Laboratory, The Ohio State

computed with more accuracy by other means. First-order edge diffraction calculations, which are adequateforalmostanyhorn for aspectangleswithin he in-cluded horn angle, are easy to make, requiring at mostan adequate table f Fresnel Integrals.

In a pyramidal horn antenna the electric field vectoris perpendicular to one pair of aperture edges, desig-nated as E-plane edges. I t has been shown tha t mos t ofthe backlobe structure of a pyramidalhorn esultsfrom energy diffracted by these edges. In fact, the en-tire E-plane pattern of a particular horn antenna hasbeen calculated accurately by tre at ing the diffractionfrom such edges as well as the geometrical optics field

The pertinent first-order radiating mechanisms [2] ,[3] for the E- and H-plane horn geometries are illus-trated inFig. 1. Higherordermechanisms [4] ausedby multiple reflections of energy diffracted by the edgetoward heoppositehorn wallbecomesignificant oraspects outside the included angleof th e horn when thehorn has a small apertu re. The far-field patterns of the

[2 I .


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606 E E E TRANSACTIONS ON mTENNAS Ah?) PROPAGATION SEPTEMBER

Fig. 1. Radiating mechanisms for horn antennas.

horn antenna can be obtained by summing he com-puted fields of these individual radiating mechanisms.Studies of this type have shown that the principalsource of backlobes is th e field caused by edge diffrac-tion in the E-plane. Furthermore, the fields caused bydiffractions at the E-plane edges are the source of theirregularitiesoftennoted n heE-planepatterns ofhorn antennas. Diffracted fields of the H-plane walls donot yield a significant contribution to the E-plane radi-ation pattern of horn antennas. Consequently he re-duction of the E-plane edge illumina tion would simul-taneously educe hebacklobesandeliminate he r-regularities in the E-plane pattern. The entire E- andH-planepatterns [2 ] , [SI , [4] of hornantennashavebeen calculatedaccuratelywithout he ntermediatestep of determining an aperture d istribution.

Thispaperdescribes womethods of reducing hesidelobes and backlobe levels of a horn by controllingthe llumination of the E-plane edges. The basic ap-proach is to prevent illumination of the E-plane edgesby electrically modifying he walls of the horn having anE-plane edge as an element. Th e first modification sachieved by introducing a series of quarter-wavelength-deep choke slots nto thewalls of the horn near the aper-ture.The secondmodification sachieved by ntro-ducing a reactive surface in the form of a cutoff corru-gated urface a t he walls of thehorn. Of course th ehornwallsmustbe hickenough nitially oacceptthese modifications. The second modification may alsobe used to radia te circularly polarized wave by placingcorrugations in the orthogonalwalls. When such a hornisexcitedwith a circularly polarizedsignal, it wouldyield an ante nna with good axial ratio for angles equalt o or less than the horn angle. The same principle maybe applied to the circular horn simply by extending thecorrugations around thewall of the horn.

THECHOKE-SLOT ORKSciambiandFoldes 5]havesuggested he use of

choke slots to shield edges and thus reduce radiationleakage around these dges. This technique has een ap-plied by Peters and Rudduck [6] who obtained a back-lobereduction of 24 dB using a 6-slot fram e and byKritikos et al. [ i ] who obtained a reduction of 21 dBusing 2 slots. Kay [S ] developed a circular horn withbent-chokelotseparated y pproximately 0.31X(which he designated as the scalar feed) with backlobesmore tha n 30 dB below pattern maximum .

Th e choke-slot horn described in this paper was con-structed with an aperture 3.5 inches square and with aflare angle of 92O. At 10 GHz the slots are $-wavelengtdeep with a spacing of 6 slots per wavelength. Six slotswere used with the spacing between slots the same asthe width of the slots. I t is necessary to have an un-modified antenna for comparative measurements. Withthe choke-slothorn this iseasilyandaccuratelyob-tained by placing stripsf aluminum tape over the sloand painting the tape with silver paint. The behaviorfthe backlobe level of the antenna relative to the maxi-mum s of primary nterest. These data, plotted as afunction of frequency nFig. 2 , are compared o herelative backlobe level of the control horn. The lowestfrequency used was 6.6 GHz; the choke depth a t thatfrequency is X/4.

Th e effect of the chokes on the main lobes comparedlvith the control antenna in Fig. 3 , where beamwidth isplotted vs. frequency. I t is felt that due to irregula ritiein the patte rn of thecontrolhorn at th e higherfre-quencies, th e 6-dB beamwidth is a more representativeparameterhanhe -dB eamwidth. I t has eenshon-n [2], [ 3 ] hat the irregularities in the E-plane paterns of hornantennasarecausedby he adiatingmechanisms illustrated in Fig. 1.

iT7ithin it s included angle the magnitude of the radi-ation field of a horn antenna depends upon the relativephase of the far fields of th e different radiat ing mecha-nisms. Thus, the introduction f choke slots can resultna decrease or an increase in the axial radiation field and,consequently, heon-axisgaindepending on whetherthe edge diffracted fields are more nearly in-phase or oof phase with the geometrical optics ield. Furthermore,any loss due to curren ts lowing on the walls of the hornis probably reduced since the fields are now detachedfrom the surface, resulting in increased efficiency.

Th e effectiveness of the chokes n rejecting energyfrom the E-plane walls can also be seen by probing the3.5 inch aperture in both the - and H-planes. T he aper-ture distribution for the control horn is shown inFig. 4and may be comparedwith th at of thechokedhornshown in Fig. 5. The aperture distributions shown wereobtained a t 7.1 G H z . The chokes produce a symmetricaldistribution in theE- and H-planes; this s substantially


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LAWRIE A N D PETERS: HORN ANTEhThTAs FOR LOW SIDELOBE 607-x1 - m D l l Cr mk c d m a- - - - - C 4 n t o , Horn-25

*-a~ - .___ 1___ - *e---_'--J-------

8 -35

-436 8 9 0 I 2rvea"Cn3 ,Wl

Fig. 2. Ratio of backlobe to mainlobe level of the chokeslot horn and the ontrol horn.

3. 6-dB beamwidths of the choke slot horn and control horn./ I \I I I I I \ I0 2 3

0 , 3 * r n c e P l D r n ccnres 0 1 I l o . u . e ,le.",

4. -4perture field distribution of the control horn at 7.1 GHz.

5. Aper ture field distribution of the choke slot horn at 7.1 GHz.

Fig. 6. Normalized E-plane patte rns of the controlhorn and the hoke slot horn at 6. 6 GHz.

Angle Of R o d l a t i o n (81Fig. 7. Normalized E-plane patterns of the control

horn and the choke slot horn a t 12.0 GHz.

the same s the H-plane distribution f the control horn.Figures 6 and 7 compare the E-plane pattern of th e

control horn and the choked horn a t 6 .6 GHz and 12.0GHz, espectively.Note that a t 12.0 GHz hemainbeam of the control horn has begun to split into twolobes. This spl itting represents a limitationn the band-width of the horn. Splitting of the main beam of t hechoked horn does not occurt 12.0 GHz. Thus the split-ting may be attributed to edge effects which have beeneliminated b~7 the chok elots.

THECORRUGATEDORNWidely spaced choke slots as used in the choke-slot

antenna yield results which arerequencyensitive.Consider two such slots in a planar surface which areseparated by a distance S and which are illuminated bya wave propagating over the surface. The phaseof thesignal scattered by one slot relative to the phase of thesignal scattered by the other slot is

where o is the angular frequency. This frequency de-pendent mechanism may be avoided by reducing thespacing between the slots.

If the spacing between choke slots is reduced untilthere are 10 or more slots per wavelength these yield asurface that may be defined as a corrugated surface. Ahorn with sucha surface is shown in ig. 8.

The analysis of an infinite corrugated surface may be


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66s IEEE TRANSACTIONS ON ANTEhWAS AND PROPAGATION SEPTENBERsimplified considerably by makinghe following assump-tions.1 ) The slot walls (teeth) are vanishingly thin.2) Only the TEM mode in the slo ts is reflected from

th e base of t he slots. The higher order modes areattenuated before reaching the base.

The second assumption is equivalent to requiring thatthe slot width, , be small compared with both the free-space wavelength and t he sl ot depth d. For such a sur-face, results obtained by Elliott [g] can be used to showthat the reactance of the surface is given to a good ap-proximation b37

provided th at g / ( g + t ) = 1.This condition is satisfied if t 4 g / 1 0 and the second

assumption is validor g


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LAWFiIE AND PETERS: HORN MVTEhWM FOR LOW SIDELOBE 60 9

Fig. 8. The corrugated horn geometry.

Fig. 9. Ratio of backlobe to mainlobe levels of the small !> = '--.. .corrugated horn and the control horn. ,I ; P 3 W FAr - > ' \

A n g l e Of Rod im i on (8 )Kormalized E-plane patterns of the large corrugatedhorn and the control horn at 10 GHz.

7 8 9 IO I1 12 13 14 I5Frequency (gc lFig. 10. 3-dB beamwidth of the small corrugated hornand the control horn.

Fig. 11. Normalized E-plane patterns of the small corrugatedhorn and the control horn at 10 GHz.

:I

-0

E X P E R I M E N T A L0 o a o C A L C U L A T E DPOINTSr20' 40'

A N G L E O F R A D I A T I O NFig. 13 . E-plane pattern of the large corrugated horn with lens.

IA- I1.3-

1.2-1.11.0-

\ MEASURED WIT H X-BANDSLOTTED GUIDE

6 7 8 9 IO $1 12 13 ISFREQUENCY IGcIFig. 14. VSWR of the large corrugated horn.


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610 IEEE TRkWSACTIONS O N ANTENNASNDROPAGATIONpatterns coincide and are located at the thro at of t hehorn.

The calcu lated direc tivity of t he large control horn is21.5 dB , while tha t of the large corrugated horn is 22.5dB (measured gain =22.35 dB). Thechange in directiv-ity iscausedby hereduct ion of theE-planebeam-width an d by the removal of the saddle in the E-planepattern of the control horn. The saddle s attributed toedge effects. I n other words, the fields diffracted by theedges of the control horn are not in-phase with the geo-metrical optics field resulting in the on-axis minimumshown n Fig. 12. These edge diffracted fields are sig-nificantly educed andconsequently heminimum iseliminated and the axial fa r ield is increased by theuseof the corrugated surface.

Th e backlobe of the large corrugated horn is 57 dBbelow the main beam. Th us it is 27 dB better than thecontrol horn.

Th e presence of the corrugations close to th e wave-guide feed can affect th e impedance of the horn. How-ever, a low- VSWR can be obtained over a broadbandprovided the corrugations are introduced t a small dis-tanceaway rom hewaveguide feed. Th e measuredVSWR of the large corrugated horn, with the corruga-tions beginning 2.5 inches from the waveguide feed, isshown in Fig. 14. Since the VSWR over the frequencyband isreasonably ow, the corrugated horn may beused in high power systems. I t is conceivable th a t th enarrowgaps of thecorrugated urfacemightcausecorona or other breakdown phenomena. However, thelarge corrugated horn described here has been energizedby a source with 2 0 kW of peak power a t 10 GHz andno evidence of any suchbreakdownphenomena hasbeennoted.

Initial measurements of the E-plane pattern of thelarge corrugated horn indicated severe interference ef-fects throughout the back hemisphere.t was found h atthe primary sourceof the interfering signals was leakagethrough the waveguide joints and components, i.e., de-tector, attenuator.A rearrangement of components andthe judicious applicationof aluminum tape and metallicpaint greatly reduced the interference. The remaininginterference s attribute d o cattering romvariousstructures in the v icinity of the pattern range.

CONCLUSIONSTh e use of choke slots or a corrugated tru ctu re in the

walls of a horn antenna have been demonstrate d to beeffective methods of reducing the backlobe level of th ehorn. The use of corrugated surfaces produces a greaterimprovement than the choke slots. The attainable re-duction in backlobe level is limited by diffraction from

the wedge formed by the waveguide and the all of th ehorn t o the edge of the opposite wall, similar to th ediffraction illustrated in Fig. l (b) for the H-pla ne. I t wasfound that theuseable bandwidth of themodified hornsis at least as great as the bandwidthf th e transmissionline feeding th e horn.

The type of modified horns discussed n th is papermay find applications such as use in pattern ranges anradar cross section ranges. As Kay [8 ] notes, the application of this type of antenna as feed mill result in thgood low-temperatureperformance equired in manymodern systems, andwill also be useful in he reductionof interference between various systems. Since its phacenter is a well defined ingle point, hecorrugatedhorn should also find application for lens and reflectorante nna systems. One can envision furt he r uses for cut-off corrugated surfaces, such as application to screeningences,or theeductionfnterferencendground clutter n radar systems. Furthermore, corru-gated surfaces might find application in the isolation ofan ante nna from surrounding surfaces, such as an airframe; however, such applications require further stu

REFERENCES[l ] L. Peters, Jr., and R. C. Rudduck, RFI reduction by controof antenna sidelobes, I E E ET r a n s . on Electromugnetu Cmpatibil i ty, vol. EMC-6, pp. 1-11, January 1964.121 P. 3.1.Russo, R. C. Rudduck, andL. Peters, Jr., A method forcomputing E-plane patterns f horn antennas, I E E E T ra ns . onAntentaus and Propagation, vol. AP-13, pp. 219-229, March 1965[3] J. S. Yu and R. ,C. Rudduck, TheH-plane radiation pattern ofhorn antennas,AntennaLaboratory,The Ohio Stat e University Research Foundation, Columbus, Rept. 1767-5, May1965, prepared under Cont ract A F 30(602)-2711 for Rome AiDevelopment Center, hew York.[a] J. S. Yu , R. C. Rudduck, and L. Peters, Jr. , Comprehensive

analysis for E-plane horn antennas by dge diffraction theory,I E E E T r a n s .on A nte nn as a d Propagation, vol. AP-14, pp. 138-149, March 1966.[ 5 ] A. F. Sciambi and P. Foldes, A critique of advanced high performance antenna feed systems, RCA Missile and Surface[6]L. Peters, Jr. ,and R. C. Ruddick,Application of electroRadar Division, Moorestown, p. J., 1964 (discussed in [7]).magneticabsorbingmaterials as interference eduction tech-niques, AntennaLaboratory,The Ohio Sta te University

prepared under Con tract AF 30(602)-2711 for Rome Air DeResearch Foundat ion, Columbus, Rept . 1423-7, November 1963velopment Center, Air Research and Development Command,Griffiss AFB, New York.[7] H. Kritikos, R. Dresp, and K. C. Lang, Studiesof antenna sidelobe reduction, Rome Air Development Center, Griffiss AFBNew York, Rept. RADC-TDR-64-355, vol. 1, October 1964.[8 ] A . F. Kay, The scalar feed, TRG Inc., E ast Boston, Mass.March 30, 1964, Sci. Rept . 5 , preparedunder Contract A FOffice of Aerospace Research, USAF, Bedford, hlass.19(604)-8057, Air ForceCambridge Research Laboratories[9] R. S. Elliott, On the heory of corrugatedplanesurfaces,IRE Trans. on Antennas and Propagation, vol. AP-2, pp. 71-81,April 1954.[lo] R. A. Hurd, The propagation of an electromagnetic wave alonan infinite corrugatedurface, Canad. J . Phys. , vol. 32

[11] D. R. Rhodes, An experimental investigation of the radiationpp. 727-734, December 1954.patterns of electromagnetic horn antennas,P ro c. I R E , vol. 36pp. 1101-1105, September 1948.

Modification of Horn Ant for Low Sidelobe Levels

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