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A PHYSICALLY COMPACT QUAD RI DGE HORN DESI GNStephen A. SorokaAntenna h RF Syst ems Depar t mentSanders Associ at es, I nc.Nashua, New Hampshi re 03061

The el ect r i cal and mechani cal const rai nt s pl acedn t hebroadband phased ar ray has r esul t ed i n the devel opment ofel ement desi gnfor a very smal l dual l i near pol ar i zedphysi cal l y compact quad- r i dge horn ant enna. Usi ng i t sl owest operat i ng f requency, t hi s ant enna hasn aper t uresi ze of l /a x 1/41 and an axi al l ength of 1 / M . Thesedi mensi ons represent a reduct i on i n si ze of approxi mat el y5 0% over convent i onal quad r i dge horn ant enna desi gns. Wt ht hi s si ze reduct i on however , t here was no si gni f i cantdegradat i on i n ant enna per f ormance. Speci f i cal l y, a t o 1operat i onal bandw dt h w t h a maxi mumSWR of 3.5 t o 1 i n t heoperat i ng bandwas achi eved. Ot her ant enna character i st i csare dual pol ar i zat i on, 7 dB m ni mum i sol at i on bet weenpor t s, ext remel y broad pat t ern shape, and a coaxi al f eedsyst em whi ch or i gi nat es f rom t he rear of t he ant enna.

Thi s paper present s both t he t heoret i cal and empi r i calt echni ques empl oyed throughout t he devel opment cycl e. Al soi ncl uded are measured dataor t he or i gi nal physi cal l ycompact ant enna and second ant enna based n a scal edversi on of t hi s desi gn.I NTRODUCTI ON

The pr i nci pal desi gn goalf t hi s t ask was o devel op adual pol ar i zed quad r i dged antenna that was approxi mat el y1/2 t he physi cal si ze of t ypi cal of f - t he- shel f hardware.Thi s was t o be accompl i shed w t h l i t t l er nosi gni f i cantdegradat i on i n el ect r i cal per f ormance when compared t oavai l abl e hardware. Usi ng the el ement s l owest . operat i ngf requency, i t was det ermned t hat t he approxi mat e si ze oft he ant enna woul d haveo be l/a x 1/ 41 x 1/3X. Thesedi mensi ons were di ct at edy bot h t he ar ray spaci ng and t hei nst al l at i on l ocat i on f the array.

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Wt h t hi s rest r i cted si ze, t he ant ennas were t o exhi bit he f ol l ow ng el ect r i cal charact er i sti cs; dual l i nearpol ar i zat i on, 10 dB pat t ern beamw dt hs of20 degrees, and aw dt h.maxi mum VSWR of . 0 t o 1 over a3.0 t o 1 operat i onal band-

antennas was hat t he t ransi t i on f romoax t o quadi dgedAnot her desi gn requi rement of t hehysi cal l y compact

wavegui de or i gi nat eat t he rear of t heant enna. Thi s wasdue t o t he extremel y cl ose ar ray spaci ng of ant ennael ements.DESI GN

The devel opment cycl e of t he l ow band physi cal l ycompact ant enna began w t h an i nvest i gat i on of possi bl eper f ormance cr i t er i a. The pr i mary concern was whether t heVSWR speci f i cat i on coul d be achi eved. There werewopr i nci pl e desi gn curves whi ch al l owed f or di agnost i c i nsi ghi nt o t he ant ennasSWR per f ormance. Bef ore they coul d beut i l i zed however , i t was necessary t o make the assumpt i ont hat doubl e r i dge wavegui de desi gn curves coul d bet ransposed f or use s quad r i dge desi gn t ool s. Thi sassumpt i on was pr i mar i l y based on t he superposi t i onpr i nci pl e. Si mpl y stated: by desi gni ng a doubl e r i dgewavegui de t o a cer t ai n set of speci f i cat i ons, an or t hogonalset of r i dges coul de i nt roduced w t h l i t t l e si gni f i canti mpact on t he desi red perf ormance.K.L. Wal t on andV.C . Sundberg [ l ] devel oped a ser i es ofdoubl e r i dge wavegui de desi gn curves f or di f f erent rat i osfb/ a, where a and b represent t he t wo or thogonal di mensi onsof t he wavegui de. Havi ng chosen a rat i o of/a = 0.9, t het heoret i cal perf ormance of t he ant enna was comput edo be22% above the l ow end cut of f of t he wavegui de,6 % bel ow t hehi gh end cutof f of t he wavegui de, and a character i st i ci mpedance of 177 ohms. Thi s dat a yi el ded evi dence t hat f ort hi s part i cul ar aper ture si ze, t he VSWR and bandw dthspeci f i cat i on coul d be met . Another f actor whi ch di rect l yi nf l uences VSWR per f ormance i s t he ref l ect i on coef f i ci ent

associ at ed w t h t he ant enna aper t ure, Fi gure. [2]I nt erpol at i on of t hi s curve yi el ded a ref l ect i on coef f i ci enof 3.0 t o 1. Thi s served as a conf i rmat i on t hat t he maxi mumVSWR of 4 . 0 t o 1 coul d be achi eved.Bef ore a prot ot ype ant enna was f abr i cat ed, anot hercr i t i cal desi gn paramet er was consi dered. Thi s was t hei mpedance t ransf ormat i on between the wavequi de and f reespace. Because the ant enna was el ect r i cal l y smal l , hadl i mted axi al l engt h, and requi red r i dge l oadi ng, i t wasnecessary t o t ermnat e t he r i dges out si de of t he ant enna

cavi ty. Whether t he t ermnat i on was o be radi al ,exponent i al , or ot herw se woul d be det ermned empi r i cal l y

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A prot ot ype ant enna was f abr i cat ed and t est ed. Af t er anumber of var i at i onsn r i dge t ermnat i ons, a maxi mum VSWRof 3.5 t o 1 was achi eved across the band. sampl i ng of t hedat a, VSWR and pat tern character i st i cs, f rom the i ni t i alproduct i on l ot have been i ncl uded, Fi guresand 3.VSWR data depi ct i ng t he hi gh band physi cal l y compactant enna has al so been i ncl uded, Fi gure. The i sol at i on

dat a has been i ncl uded f or r ef erence, even t hought was nota requi rement of t he t ask, Fi gure. Al though t he hi gh bandVSWR data was comparabl e to thatf t he l ow band ant enna, i tshoul d be noted t hat t hi s was accompl i shed w t h a great' of di f f i cul ty. Thi s was dueto a much mor e compl ex assembl yprocedure. It i s f easi bl e t hat w t h addi t i onal t i me andef f or t , t he di f f i cul t i es associ at ed w t h t he assembl yf t hehi gh band uni t coul d be el i mnated. pi cture of both thelow and hi ghband physl cal l y compact quad- r l dged hornant ennas hasbeen i ncl uded, Fi gure .CONCLUSI ON

A physi cal l y compact quad- r i dge horn antenna w t h duall i near pol ar i zat i on, and a maxi mum VSWRf 3.5 t o 1 over at o 1 operat i onal bandw dt h, has been devel oped.ore overt hese el ect r i cal character i st i cs, pr i mar i l y VSWR and band-w dt h, wer e achi eved w t h an ant enna whi ch i s approxi mat el5 0% smal l er t han convent i onal quad r i dge desi gns. Wt h t hi ssi gni f i cant reduct i on n ant enna si ze, no si gni f i cantdegradat i on i n antenna per f ormance was noted.REFERENCE1. K.L. Wal t on, V.C. Sundberg, "Broadband Ri dged HornDesi gn" , M crowave J ournal , Vol 7 , pp 96- 101, 1964.2 . Dorne, A. , "VHF Techni ques", p . 139.

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