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SUBIR GHOSH

COM DEV, 155 Sheldon Drive, Cambridge, Ontario, NiR 7H6, Canada

ARSTRIES

This paper describes a new monopulse feed system that enhances the tracking accuracy and operating bandwidth of a space-fed radar.The feed comprses a compact 'mode triplexer' and a multimode corrugated horn. Signal strength of the two higher order modes withrespect to the dominant mode at the triplexer output ports furnishes an estimation of angular co-ordinates in azimuth and elevationplanes. Efficient microwave networking in the mode triplexer improves the operating bandwidth. The hom launches multiple higherorder modes to optimize the feed aperture field distribution, simultaneously, for the Land A beams. Beam dependent effective size ofthe aperture in the multimode horn enhances the tracking accuracy of the radar. Based on measured data, a net 70% improvement oftracking accuracy and 6 dB improvement of difference beam sidelobes are achieved over a 7% bandwidth in a space-fed radarretrofitued with the new feed system.

n flODUCI

To provide accurate estimate of target location in two orthogonal angular co-ordinates, a monopulse radar aensna g ats a set ofthree panems, ulusl. The anten apetr distribution for first of these patters, designated the I (sum) beam. is designed to

imize the gain of the antenna along the boresight axis, and to satisfy sidelobe objectives in other dimctions. The aperturedistribution for the second patten, designated AEL (difference - elevation) beam, is designed to produce a null response along theazimuth plane including the boresight axis while maximizing slope of the vector field response with increasing angle from boresightaxis along the elevation plane, and to satisfy gain and sidelobe level objectives in other directions. Lastly, the aperture distribution forthe third pattern, designated AAZ (difference-azimuth) beam, is designed to produce a null response along the elevation plane includingthe boresight axis while maximizing slope of the vector field response with increasing angle along the azimuth plane, and to satisfy gainand sidelobe level objectives in other directions. Generally, a comprehensive measure of performance of a monopulse radar antenna isexpressed through a large number of parameters. Performance rating of certain important radar parameters, e.g., gain ratio, nornalizedmonopulse difference slope, spiUover loss, sidelobe levels are predominantly influenced by the antenna apertr distributions for eachof the three beams individually. The antenna aperture distributions for the three beams are provided by a feed system which distributesthe rf excitation over the aperture in a prescribed fashion. There can be two classes of feed systems employed based on the type of theradar antenna:* Corporate Feed System may be employed for phased arrays. In this case the rf excitation proceeds from the beam ports through

branching transmission line or waveguide network to the individual elements of the radiating aperture.* Optical Feed Sytem may be employed for reflectors or phased arrays. In this case the rf excitation proceeds from the primary

feed through space radiation to the individual elements of the ndiating aperure.

The design of a monopulse optical feed system for an antenna employing a reflector, a lens or a transmission lens array, is complicatedby the fact that there are two aperures to consider, the antenna aperwre and the feed aperture. Optimum feed apenure requirementdiffers for the three beams, and there are fewer degrees of freedom available for controlling antenna aperture rf excitation for the treebeams simultaneously. Hence, in the past, certain important performance parameters, such as, gain, normalized monopulse differenceslope and spiliover efficiency, had to be compromised in the radar design. Previous attempts to overcome those deficiencies have onlysucceeded partially and that has been achieved at the expense of a significantly complex constructional design of the feed system. Thispaper describes a new monopulse optical feed system which has successfully eUminated all the prevailing deficiencies. This feed.comprising a novel modal extraction network and a single aperture multimode corrugated horn, achieves an optimally independentcontrol on rf excitation of the antenna aperture for the three beams, sinultaneously,while maintaining a simple structure.

Figure 1: Space-fed Antenna Characterisics When a Baine Monopulse Feed is Employed1122

To provide optimum illmination of a given antenna apetue, the size of the feed apertue differs in the case of Sand A beams. Whenthe feed aperture is chosn to provide best rf excitation of the anena aperme for the £ beam so ta the requird gain and sidelobeperfonnance in this beam is achieved, the antenna excitation for the A beams have a high spillover loss (Figure 1) that causes lowangular sensitivity in the A beams (-6 dB gain separation between S and A beams). On the other hand, an attempt to optimize theperformance of the A beams using a larger feed so that the spillover efficiency from the A beam excitation at the antenna aperture can beimproved, results in sacrifice of gain in the S beam due to under illumination of the antenna aperture for this beam. Therefore, a majorsource of mprovement in the desip of monopulse optical feed system calls for a beam dependent effective size of the radiating feedaperture. An ideal feed aperture excitation for the three beams were first described by Hannan 11_in 1961 as shown in Figure 2. Sevral attempts to improvise this excitation at the feed apertu k diEAhave been made in the past which only approximated the ideal excitation by a combination ofmulti horn and multimode confi'gutions with icreased complexity in design and construction. -AZIn this paper a single aperture multimode corrugated horn has instead been considered which has \Yrequired degrees of freedom to achieve a very close to the idealized excitations of Figure 2 at thefeed aperure and of Figure 3 at the antenna aperture, in a most efficient manner. Note that thegain separation between S, and beams in this case is only 3 dB. I _ _ l |A brief comparative study of the concepts underlying he prior designs and the one described inthis paper is taken up first In Table 1 feed aperure excitation for the three beams and resultantantena aperture illination characteristics are shown in respect to hes four designs of theoptical feed sytem:* Four-Horn Baseline 0 Corrugated Single-Hom Baseline* Four-Horn Triple Mode 0 Conrugated Single-Hom Multimode

FeedAperer

lgure 2: An Ideal Monopulse FeedApertre Exdtaton

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-3dB"3d

Figure 3: Span-fed Antenna Charc cs When an Idelizd Monopule Feed Is Employed

Also included in this table are the schematics of the cicuitry associated with these feeds and the far field performance of the thre beamsin a radar employing them. Comments are included to highlight the strengths and shortcomings of each design. Clealy, the new feedsystem, based on corrugated single-horn multimode design concept, outperforms other cases in every respect

;Y T Y Operating Band

p ~~TEY21 TEY31 TEY32 TEY41I I I Ik I I --

I iEmY1 TEY12

FIgure 4:

I ITEYI, TEY23

IITEY14

Cut-off Spectrum for the Hybrid Modes in a Co ted Rectnguar Wavegulde

A further insight into the underlying principle of operation of this new feed design may be gained from the dispersion diagram of thehybrid modes of a corrugated rectangular waveguide shown in Figure 4. Note from this diagram that the feed utlizes a total of as manyas seven hybrid modes of the corrugated structure to achieve desired independent beam shaping for the three beams simultaneously.However, this does not necessarily limit the useful bandwidth of the design while attempting to appropriately phase a large number ofmodes having distinct dispersion behavior. In fact, only one higher order mode at a time is utfized for shaping the pattern in a specificplane of a particular beam. This is further explained in Figure 5. Hence, up to a 7% bandwidth is easily available from this design tosatisfy the requirements of most radar systems.

The triplexing of the three modes, TEor, LSEI, and TE%, to provide requte AML and AAZ outputs, respecdvely, is achievedthrough a novel microwave circuitry. It employs a mode selective iris filter to provide a reflection plane for TE,, and LSE,, modeswhile allowing TEFrn mode to propagate unattenuated through the fldter. Transducers for the relevant modes are situated on either sideof the filter. Essentially, this approach facilitates a very compact low loss, high isolaton and a widened bandwidth triplexing of thethree modes.

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-t

FEED TYPE FEED NETWORK AND ANTENNA APERTURE._____________ =APERTURE EXCITATION EXCITATIO - FF RADIATION PATTER

FOUR - HORN M.BASELINE i _

* Non optimum excitaion for 0 Non optimum excitation * Low gainroCOMMENTS: al th beams 0 Large spillover loss * LOw diffence slope* Compact desig 0 Sensitive to asymmetry 0 g sidlobs i A beams

w~~~~~~~~~~~~~~~~~~~V EL1[I11DT~~~ ~~I1Iae 1TI -

CORRUGATEDSINGLE - HORNBASE I NE \ 1 l f,'riIIIil l1111L

5~~ ~ ~ ~~ow A S E11 R I N Ek wR11 LIIr

* Non optmum etaton S Non optimum excitation * Low gain raioCOMMENTS: forA beams 0 L spillover los 0 Low diffCe slope* Compactdesign * Sensitivetoasymmetry Hig silodebesinA beams

I~~~~~~~~~~~~~~~~~ IV TW IItc TI X 11111li 1

FOUR HORN I H lwTRIPLE MODE K 2 U l

I I IFiLAlfflItliUI I II -

_ &,-[1 11sspt 'h I .; 1 lIZ'h.* Improved excitation for * Optimum excitation 0 High gain rado

all the beans 0 Low spillover loss * High diEference slopeCOMMENTS: 0 Intricate networkdng 0 Less sensitive to 0 Low sidelobes for all

* Less compact design asymraetry the beams

CORRUGATED N I -1 ± 1.|2'SINGLE - HORN I L 'I)

MULTIMODE AZ

l I r I);i}!.l0,!l,.~~~~~~~~O"1-t4ff4lS Optimum excitation for 0 Optimum excitation S High gain ratio

al the beams 0 Low spillover loss 0 High difference slopeCOMMENTS: 0 Simple networking * Less sensitive to 0 Low sidelobes for

_- * Compact design asymmetry all the beamsTable 1: A Compaative Study of Different Optica Feed Systebm for Space-fed Radar An _hn

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AAZ AEI

TEY12 TEY,| TEY21 TEl23l

Itit +4liiii~ ft+0Figurem 5:. lanfwd Feed Aperture Disiribudoin for the Triple Beam Uding Highe O *rd Modem

A black darnand cutaay pesetve iw of the dgnwith all ceanse of the circuitry inertdin a split block c n aisshown in Figure 6 whkhiglgt the key fena of this deincomCpr

KEY FEATUREtS4 ~~~~~~~~~~~~~~~~~~~~~~~~Independentcontrol on Gain and

eanmwidth of r & A patternsindependent control at E&H plan

2)uys y,n*LowsiddelofL.3) Iy,al meS bw *High aperUMeffiienc.

4)*LOW insertion lossL5) ~~~~ ~ ~ ~ ~ ~ ~ ~ ~~~~~~*Mkdinbe RF &rwloy.6) UUSU6 W*Spltebkockintegrated construction

7) NaScT *Co..p'. low won .s euged.

FIgur 6: A Sdpernl a Cut-awayPerspecTV View of the Feed SYsteMs

The principle of opaon of the feed described above wel depicts the involved natu of the design. It is essntial to prebcislyestimateand optimie the influence of various design pamete in each component vs-a-vis at the integrated feed Wsstem leveL Tehorn design with the higher order mode lanhr,the mode couplers, the mode selective f'dtens, each calls for precisonmoelntechnique based on electrom fild theory modal analysis. Integrted splt block constuion with limited scope for experimentaloptimization furthersenfre a precison design approach. The entire feed system has been modelled accurately based on modalanlysis tt beg at the input ports and proceeds through the stucture up to the radiated far field amplitde and pa c t s.As many as, 2500 modes are required at some instances to accurately account for the discontinuities, specially, in the horn section.Therefore, a very large CPU time is required on a dedicated worksaion to fully evaluate the chactristics of thre beas at onegiven frequency. The optmztion of the overal feed design is an involved process and calls for a good insight into the design conceptas well asa signiicant hardware experience. However, once the design has been optimized, the fabrication lmay proceed dircty withvery high confidence in the results to be achieved as per the CAD simulations; provided good tolernces in fbiaonaremanied

A number of prMototypeshaIVe been built based on the designs optimized by the above mentioned procedure at feunybands ofoperation rnigfrom microwave up to mllimetre-wave bands. In Figure 7 a photograph of such afeed system oeaigat K-band isshown. The in Figures 8 & 9 are shown to compare the cae wher independent control on the excitation for theantenna aperture for tehrebeamns 'has not been' and 'has been', respectively considered. Given a subtended edge angle for theazimuth and elevation planes, the spillover beyond these angles is clealy far less for the case with independent control on iluintoof antenna aperture for the thre beams. An excellent correlation between measured and predicted characteristics is observed in theseexamples. In regard to the phase characteristics, notice the flatnes of response within the subtended angie of the antenna apertur.This has been achieved by employing a profiled flaring section in the corrugated horn to yield a nearly coincident phase centre locationfor all the t beams, simultaneously. The overAll results achieved for such feed systems at various frequency bands of operaion aresummarized in Table 2. A separate investigation, retrofitting the new feed in a space fed arry radar antenna. showed a 70%improvment in the nettracking accuray and 6dB improvement in the differn beam sidelobes.

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Figure 7: Fully Developed Engineering Model of the Feed SystemMeasured. Predicted::tof: Measured: - _ Predicted::,*sI TI I I I

60£

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-90 -W -30*Figure 8:

- - _- - --

I 1.30' -9 -30' 30* 60' 90

Radiaton Patterm of Corrugated Single-Horn Baseline Feed System

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-90'

Figure 9:

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l. r -*v ' I

3(r .6W 9(_-0W -W -30' 30' 90'

Radiation Patterns of Corrugated Single-Horn Multimode Feed System

1126

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Table 2: Feed System Performance Sunmary

A new monopulse feed system has been described which featues a rf circuitry for efficient extaction of twacing signals and amultimode corrugated horn for compatible 'X vs A' illumination of antenna aperture. The feed design tools have been developed basedon precision em field theory modal analysis to ascertain full optimization and integrated compact construction. Achieved results arereported depicting the advantages of the new design and precision design procedures. This compact feed provides a significantenhancement of taking accuracy in radars and readily retrofits most existing radar systems,

Esouragement from COM DEV management d assi fm C}ecy Lee-Yow, Jean Mas and Michel Lelei are tulkfnlyacknowledg

(1] P. Hannan, "Optimum Feeds for All Three Modes ofa Monopulse Antenna in Theory and Practice", IRE Trans. AntesPropagat, vol. AP-9, no. 5, September 1961, pp. 444-461.

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PARA_M BASELINE DESIGN MULTIMODE DESIGNX-Band Ka-Band W-Band X-Band Ka-Band W-Band

%Bandwidt_ 9% 7% 5% 7% 5% 3%VSWR(AnyPon) 1.5:1 1.5:1 1.5:1 1.5:1 1.5:1 15:1

lIsertio Loss, dB (Any Port) O.25 SO.5 <1.0 .90-25 T .0.5 .1.0Pk Power. kW (SumPortOny) 100 5 0.5 100 5 0.5Port o Port IsolaIo dB >35 >30 >25 >35 >30 >25

Rlative Gain dB (A ) -2.5 -2.5 -3.0 0.2 0.2 -0.1Size, cm$ 25x15x12.5 15x5x4 6x3x2.5 35x18x12.5 20x6x4 7.5x4x2.5Mass, ggms 2400 250 75 2900 325 90Coasintion Modular Splitbiock Modujw ----= ock Splktko-k