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TIUE DOMAIN AND INFRA-RED ANALYSISOF RIDGED HORN ANTENNAS

H C Reader and S A McDonal dUni ver s i t y o f Nata l , South Af r i ca

I n the devel opment of an HP8510B net wor kanal yzer based, pc-AT contr ol l ed m cr owaveant enna t est f aci l i t y a br oadband antennast andard was sought. The ri dged horn wassel ected because of i t s wel l known bandwi dthproper t i es, l i near po l ar i sa t i on ( l ow crosspo l ar i sat i on) and h i gh, cons tant ga i n - Kerr(1). Unf o r t u nat e l y , l i t t l e l i t e r at ur e i sf ound i n whi ch the desi gn of r i dged hor ns tospeci f i c operat i on parameters i s di scussed.Ri dged hor n per f ormance i nfor mati on i s,however , present ed i n (1) and i n Wal t on andSundberg (2 ) whi l e r el evant r i dged wavegui dei nf ormat i on per t ai ni ng t o wavegui de modalcut of f and coaxi al f eedi ng t echni ques i savai l abl e i n, f or exampl e, Cohn ( 3 ) andMuenz er ( 4 ) . More r ecent t exts cover i ngr i dged wavegui de anal ysi s, such as Bal ani s(5), re f er to t hose ar t i c l es al r eadyment i oned.Ear l y on i n t he r ange st udy, a r i dged hor n wasconst ructed accordi ng t o avai l abl e publ i shedmater i a l . Cal ul at i ons f romdesi gn curves andt heor y suggest ed an approxi mat e 1,5 -7,5 GHZbandwi dth. Thi s communi cat i on at t empts t oi ndi cate the regi ons and di mensi ons of ar i dged horn that a re c r i t i cal t o antennaper f ormance. Thi s i s achi eved vi a a postconst ruct i on anal ys i s of t he ant enna i n thet i me domai n ( TD) and f r equency domai n ( FD) .Fur t her i nterest i ng dat a r egardi ng ant ennacharacter i s t i cs i s f ound dur i ng in f r a- redi nvest i gat i on of t he hor n apert ure.F i gure 1 shows a schemat i c vi ew of a r i dgedhorn t hat was bui l t f or use as the antennar ange st andard. Ti me domai n anal ysi s,obt ai nabl e through t he features of t he HP8510Bvector net work anal yzer , i s used t o probe thephys i cal behavi our of the ant enna. I npar t i cul a r , f eatures assoc i ated wi th t he f eed ,f l are and apert ure are i dent i f i ed andanal ysed. To ass i s t i n t he i dent i f i cat i on oft hese poi nts, t he ant enna i s di smant l ed at t hef eed/ f l a re j unct i on and a shor t i ng pl a te i spl aced at t he end of t he f eed. The TD Sre f l ect i on i s shown i n f i gure 2. Mar ker s 2,2 and 3 are pl aced on the i nducti ve post,cavi t y back wal l and short i ng pl ater e f l ec t i ons r e spec t i v el y . T he pos i t i v eref l ect i on at marker 1 i ndi cates that t hei nput wave f r omt he 50 ohmcoaxi al cabl e seest he post radi at i ng i nto the r i dged wavegui deas a hi gher i mpedance. The negat i veor i ent at i on of t he waveform at markers 2 and3 conf i r ms t he l ow i mpedance proper t i es of t heassoci at ed met al pl anes. Marker 4 r epr esent st he t rai l i ng base edge of t he pul se ref l ectednegat i vel y a t t he shor t c i r cui t . The base i snot a t t he OV l evel because l ow f r equencyenergy has been l ost f romthe exci tat i on pul sea s a r e s ul t o f ear l i er di s cont i nui t i es .Mar ker 5 i s pl aced on the f i r s t o f theresonant r i ngs wi t hi n the f eed st r ucture.The resonance r epresent s energy t hat does notescape t he f eed envi r onment i mmedi atel y.The f r equency domai n ( FD) S r esponse of t heshor t ed f eed appear s i n f ygure 3 . I t i s

possi bl e t o make several st atement s connecti ngt he TD t o t he FD pl ots. One of t he moreobv i ous f eatures i s t hat t he di f f erence i n theTD bet ween mar ker s 3 and 4 i s 87.5 ps. Wheni nver t ed t o g i ve f r equency th i s y i e l ds 11,429GHZ whi ch corr el ates c l osel y wi t h t he m ni mumi n t h e FD at mar ker 5 . TWO expl anat i ons canbe gi ven f o r t h i s connect i on. The f i r s t i st hat t he hal f base wi dth of a t r i angul ar TDpul se i s the i nver se of t he f i r s t FD nul l Oft hat same pul se s spectr um As expl ai nedprevi ous l y, t he negat i ve ref l ect i on at marker3 i s t he exc i ta t i on pul se re turn i ng f r omt heshort i ng pl ate. The ref l ect i on can beanal ysed as an asymmetr i c tr i angul ar pul se.Mar ker s 3 and 4 i n t he TD woul d t hen beequi val ent t o the hal f base w dth. A secondexpl anati on i s obt ai ned usi ng t he anal ogy Ofa hal f wavel ength t r ansm ssi on l i ne where t heout put i mpedance i s t r ansl ated t o the i nput.Desp i te any d i scont i nui t i es that t het ransm ssi on l i ne may present at i t s i nput andoutput , a s i gna l t hat sees the l i ne as a hal fwavel engt h wi l l pass t hrough uni mpeded,y i e l di ng a very l ow S response i f t he i nputand out put i mpedancgs ar e matc hed. TDr ef l ecti ons such as t hose seen at markers 3and 4 c oul d be obt a i ned f r o m t wodi scont i nui t i es . I n r e f l ect i on the t i me de l aybet ween t he f eat ures woul d r epr esent t heper i od of a wav ef or m t h at s ees t h edi scont i nui t i es a hal f wavel engt h apart -hence, t he associ ated m ni mum i n the FD.Ot her TD and FD char acter i st i cs are rel ateds i m l ar l y. I n t h i s way i t i s pos si bl e t odi scern what aspect s of an ant enna af f ect t hebandwi dth per f ormance i n any si gni f i cant way.By observat i on of t he TD S r esponse of t hewhol e r i dged hor n ant enna, f i gur e 4 , i t i sapparent i mmedi atel y t hat ref l ect i onsi denti f i ed by markers 1 and 2 ar e i dent i c al t othose i n f i gur e 1. Thi s i s not surpr i s i ng ast he f i xed charact er i s t i cs of t he f eed have notchanged. I nstead of t he short ed r esponse atmar ker 3 a smal l i nf l exi on i ndi cates t hat t hef eed/ f l a r e j unc t i on i s not p er f ect ( amechani cal d i scont i nui t y, i nt roduced f orcons t r uct i ona l purposes , i s r espons i bl e f o rt hi s) . The r egi on bet ween mar kers 3 and 4r eveal s t he changi ng 2 of t he f l a r i ng hor n ,but i t can be demonstr zted f r omthi s that t hef l are has a smoot h f unct i on whose cont r i but i ont o t he S FD r esponse can be cal cul ated (e. g.6). Thel horn aper ture r e f l ect i on, whi ch i ssmal l , appears j us t of f t he gi ven pl ot a t t i me1,11125 ns. Thi s means t hat f r om marker 4onwards t he horn 2 f l a re i s r egul ar andcont r i but es m ni mal l $ to t he overal l ant ennareturn l oss. The resonant r i ng i dent i f i ed bymar ker s 4 and 5 has a peri od of 256 ps. Thi scan be rel ated t o the resonance of t he l ow Qcavi t y for med by t he back wal l of t he f eed andt he i nduct i ve post . Markers 1 and 2 showt hese f eatures and t he di st ance between t hem(125 ps) woul d be a hal f per i od f or a quart erwavel ength cavi t y, hence t he connecti on t o t her esonance di scussed. I n the f r equency domai n,f i gur e 5, t hi s eff ect can be seen as a smal lpedest al cent red at 3,9 GHz. The r esonance

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ampl i t ude i s l ow, however , and does not causethe re tu rn l oss t o become l ess t han 10 dB.Thr ough consi der ati on of t he TD S r esponse oft h e r i dged hor n ant e nna i t i s n pos s i b l e t os t a te t he re f l ec ted energy f r om t he ho rnar i ses l argel y f romt he f eed regi on. The t i medomai n gat i ng capabi l i t y of t he net workanal yzer has been used t o st udy t he eff ect oft he feed and ear l y f l are reg i on. Theconc l usi on f r omt hese studi es i s that t he f eedel ement of t he r i dged hor n exam ned her edom nates t he bandwi dth behavi our of t he hor n.Even the re f l ect i on f rom the aper t ure o f t hehor n ( occur r i ng at 1, 11125 ns af t er t = 0whi ch i mpl i es a 900 MHz charac te r i s t i c ) has noef f ect on t he l owest possi b l e cut - i n f r equencyof 1, 25 GHz.Antenna Radi at i on patt ern det erm nat i on v iaknowl edge of t he aper t ure f i el d magni t ude andphase i s di scussed i n numerous t ext s ( 7 - 9) .I n an at t empt t o f i nd a si mpl e connect i onbet ween apert ure gcwer di st r i but i on andr adi at i on pat t erns, i nf ra - red photogr aphs oft he aper t ure of t he ri dged hor n were used t of or ma t emper at ure map at t he apert ure pl ane.Thi s map woul d i ndi cate r egi ons of powerconcent r ati on whi ch coul d be used i n anal ysi sof apert ure modes and possi bl y gi ve i nsi ghti nto antenna radi ati on per f ormance. Theconf i gura t i on used i s di spl ayed i n f i gure 6.The r esul t s of t hi s measurement f or t he 6- 8GHz cases ar e di spl ayed i n f i gur e 7 . A mor edet ai l ed i sot her mal map of t he hor n apert urei s shown i n f i gur e 8.I t i s seen f r omthe pat t e rn a t , f o r exampl e ,6 GHz t hat i n the H pl ane ( hor i zont a l ax i s)t he temper atur e i ncreases monotoni cal l y f romt he edges of t he aper t ure toward t he cent r e.Thi s i n tu i t i vel y i ndi cat es a TE mode di s-t r i but i on i n t hi s pl ane and one Goul d expecta smoot h r adi at i on patt ern . A r egu l ar , Hpl ane patt ern i s i ndeed measur ed. Wor k hasbeen done to cor r el at e t he measur ed aper t uredi s t r i but i on wi th t he consequent r adi a t i onpat t erns. I n some i nstances t hi s has beensuccess f ul .Ri dged hor n desi gn i nfor mati on avai l abl e i nt he l i t erature has been f ound t o be m s-l eadi ng i n i t s pr edi ct i on o f bandwi dth

FI GURE 1 Schemat i c Vi ew of Ri dged Hor n.

per f or mance. TD and FD measur ement s have beenused he re t o i dent i f y c r i t i cal r egi ons wi t hi nt h i s c l ass o f ant enna, not abl y t he f eed. Thi si nf or mat i on can be used t o i mprove desi gnmet hods. A si mpl e method of i nvest i gat i ngaper tu re f i el d di s t r i but i on i s a l so present ed.ACKNOWLEDGEMENTSThe i nfr a- r ed ( I R) measurement was based on ani dea f i r st generat ed by D E Baker and C A vander Neut . The l att er i s thanked f or use of I Rf ac i l i t i es at t he CSI R.PEFERENCES1.

2 .

3.

4.

5.

6.

7.8.9.

Ker r , J . L. , " Br oadband Hor ns. I Researchand Devel opment Techni cal Report . ECOM-3319.Wal t on, K. L. and Sundberg, V. C. , Mar ch1964, "Br oadband Ri dged Hor n Desi gn. "M cr owave J our nal . v. 7 pp. 96- 101.Cohn, s. B. , August 1947, " Proper t i es ofRi dged Wavegui de. I Proc. I nst . Radi o Eng.Muenzer , P. J . , August 1964, "Broadbandi ngCoax- To-Ri dged- Wavegui de Transi t i ons. "M cr owaves.Bal ani s, C. A. , 1989, "AdvancedEngi neer i ng E l ect r omagnet i cs. " J ohn W l eyand Sons, I nc. USA.Evans, S . and Kong, F. N. , Oct ober 1983,' ITEM Horn Ant enna: I nput Ref l ect i onChar a ct e r i s t i c s i n Tr a ns m s s i on. " I EEProceedi ngs, Vo1.130, Pt. H, No. 6, pp.403-409.Kraus, J . D. , 1950, " Antennas. " McGr aw-Hi l l Book Co. I nc.J as i k , H , 1961, "Antenna Engi neeri ngHandbook. " McGr aw- Hi l l Book Co. I nc .Cl ar ke, R. H. and Brown, J . , 1980,"Di f f r act i on Theor y and Antennas. ' E l l i sHorwood Li m t ed, Chi chest er.

pp. 783- 788.

FI GURE 2 TD s Response of Short ed Coax- t o-Ri dgzd Gui de Feed.

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5 1 1 l o g MAGREF 0 . 0 dB2 10 . 0 dB /

414

START 0 .069651740 GHzSTOP 13 .999999940 GUz S T A R T -2S0.0 p.sT6 1 .0 n.

FIGURE 3 FD S Response of Shorted Coax-to- FIGURE 4 TD S, l Response Of Ridged HornRi dg &l Guide Feed. Antenna.

511 l a p MAGREF 0 . 0 d B

INFRA REO CAMERA

MICROWAVEABSORBER(ON FIBRE CUSS) RIDGED 'ORN

STbRT 0.069651740 GHzSTW 13.999999940 GHz

FIGURE 5 FD S l l Response of Ridged Horn FIGURE 6 Infra-Red Aperture Power UeasurementAntenna. Configuration

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rnN

2 GH z 4 GH z

(ii)7 GH z

5 GH z

(iii)0 GH z

FIGURE 7 Isothermal Maps of Ridged HornAperture

3 GHz REGION BETWEENTEMPERATURE CONTOURSIS OF EQUAL TEMPERATUREFIGURES INOICATE RELATIVETEMPERATURE IN DEGREESCCLCIUS NORMALISED SO THATOUTER CONTOUR LEVEL = 0 DEGREES

H O R N A P E R T U R EI FRONT VIEW1

FIGURE 8 Detailed Isotherm al Map of Horn at3GHz