Radiation characteristics of switchable ferrite microstrip array antenna

LDixit and PKSPourush

111 llic casc of extr:iortlinaty morlc, ~ h c propagution COII- stant dependence on basic paramctcrs is givcii 21s:

It is seen ~ I I R ~ , wllc~l is negative, thc cxll+iiditla1y wwc is iltwying cvtii i T the inateri:il is lossless. Thc rrecluency riiiigc for ncgnlivc jrcjy is:

[Wrl(W{1 4- W.J+ < !d < (do t iJnJ ( 7 ) 'rlil: frequency iimils dcfiric thc approximate ra.ngc withill arid around which tlic h r j l c s cxhibit interesting iiiicrowavc characlcristics. The use OF ii biased lidd to control the pi:opa.h of the extlaonliiiery wavc iwults in n o extcrrdly swilchablc aiiteuiia. 'The ;tntcnna is 'olr when tm :Ittenwit- irig cxiraordinniy wiivc in thc fcrritc propagttes iiiid IIICI-t: i s l i t t le rsidiation and is 'OII' wlicii !lL:fl is positive and the ordinary wnvc is propagating.

I n clic picsent paper, tlic conwpl or switchahle nntennti has I-iccri cleveloped by t;iking ii 4 x 4 clcincnt circular array gcoriictry printed 011 bixcd YIG h i t c suhstnrte in HII S- barid (3GHz) of microwuvc ficqucrlcy range.

2 Theory

The array gcoinclry awl its co-ordinitte syslclm arc ,sl~owtl in Fig. I

4

It consists of I 6 i d e n h l elcmciits of radius 'CI' printed 011 Y IC+ ferrile stibshxtc of thickness h = 1.27 niin md sub- strate pcrinitlivily = 15. 4xMV = 1720 Gtiuss, I/ , , = 20000~. Each patch ctin bc cxcilcd by B niicrostrip lint coiinccted to the edge 01' hy ;I coaxial line from the hick a l the plsne .$ = 0. Rias licld is taken nonixil 10 thc phmc or siibstixte. Thc dispcrsioii curve for ii grotintlcti Y [G I'crl.iic slxb is shown in Fig, 2 .

It is evident from this Figiirc (hat 4, is ncgttive or kigf is ncgativc Tor [m. t l (q l + q,,)]lgT < o .= (04) + q~ according to which cxtranrrlii~ai~y wavc is dec;iyiiig iii lhis rangc. TIE cut-off and resotimcc limil arc round to he at 10.4 and 7.63Gl Iz, icspcctivcly, and within h is rangc thc aiitciina i s cKccclivcly oll' R S :L latliator. 'I'hcrciirIc, hy pi.opcrly selccting and changing thc valuc of bi;is l i c ld , IIW designed illltenllii

can bc uscd as a switclxible anlennii.

IS2

Tlic dimension oi' each clcincnl is calcLdatccl using the I'ollowiiig qii:ilioii [&I:

I b rloiiiinan( mode, in = 1, I I = 1 K,,,,, = 1.84118.

dong the cdgc or llic itsoilator ;mi is given iis [8]: tr,, is (he c l k c h c t.aclius :~ccount for the f'ringing liclds

150"

1 BOu

30"

0" -10 -20 .30 -40 -50 -60 -60 -50 -40 -30 -20 -10

\li 150" 3 0"

100" a" -10 -20 -30 -40 -50 -60 -60 -50 -40 -30 -20 -10

Table 1: Measured values of pattern characteristics of array geometry

E plane

unbiased case biased case Pattern characteristics

Direction of max. radiation (major lobs) 42" 100

Half powsr beam width (HPBW1 32" 5"

Dircction of max. radialinn (minor lobo) 8" 3 4"

Half power beam width (minor 1abe)lHPBW) 8" 3 2"

SCL (dbl - -3.0

__ -. __ .- -. - . . .- -

H plane

unbiased case biased cam

50" IO" 42" 8"

8" 3 6 na 22 - -2.6

. ~- ~

6 Efficiency

Efficiency rj of' an imtcnna cati he c;iIcul;ilcd i is [HI:

( 2 0 ) P,. q ( % ) = - x 1ou I j y

whcre P , is the total powcr, wliich iiiclucies thc pnwcr iadi- ated wtid power allcnuatcd in the disk resoilator owing Io the hnik conductivity ol thc disk rcsonator atid impcrl'wt subsll*ak. Thus:

Pp 1:. 'I' r,: + r,, (21 1 (2% p c - -- 1.68 1[)- ln j - 3 ~ ~ ;

and

P,! = 0.805 x IW4 x 11. i:;inhI?li/f (23)

Hcrc f ' i s in GI+ and tiin 8 is cqual to 0.0002, and is ttic loss larigcnt of the subs(i~atc.

7 Bandwidth

The batidwidth (RW) or an aiitctiti:i is cslculaicd as [X, IO]:

8 Discussion and conclusion

In this pi~pcr wc liwe developed ii co~iccpt OF switcllablc iiiitcnii:i and it liws bccii ilndyscd :it 3GHz or microwave iicquency rirngc by (alting 4 x 4 eleiiients of circular patch iiiicinstri)? antcnna gcomclry. 'The pmiiielcrs uscd for the study un a hiasccl ferrite substriilc arc 4,2n.1, = 1720 gauss r i i i d bias ficld &, = 2 0 0 0 0 ~ whilc, lor iinbirisctl h + c ~ T M , ~ -1 0: Ho = 0. 'I'lic piugi,cssivc phase cxcitalion dill'cr- cncr: bctween the clcincnls or array is taken as {Ix = p , = n/2 and d4. 'The mcliation p~tlci-ns, p:ittern chariiclcris~ics, di>i t ion contluclanzcl. d i iw l iw gain, efficiency and band- width of t inny gmmetry are compulcd and teporlctl in Figs. 3 m r l 4 and ii i Tables 1 and 2, rcspcclively.

Lt is cvitlcnt IYom the ilispmion c l l c ~ t on ferrite inaterial khat tlierc should bc R propagating ant1 a uoiipropagating rcgioii for A ti iintciiiia. Tlicrc is H fi-etluciicy fiiiigc bounded by limits, naniciy cut-off limit iind rcsunniice liinit.. Iti this region wlicrc pe,J i s iicgitivc, tlic cxtraorrlinai-y wiivc is Iiighly atlenualing aiid tliereforc h c mlcnna is ckctivcly off tis ii radialor.

Some Y;ilicnl fcaturcs of t h i s iirr;iy gcomdry are summa- rised :IS follows; (i) On biasing, thc rndiiition pxtlcrn hccnmcs directive in natoi-c and thc nurnhcr of lobcs im Ihmd to he g m b r than ttiitt of lhc anbiased UISC. Thcrchrc. it can be won- cludcti thal (hc hiascd circulnr piitclics have to he excited in higliet order nioilcs instcacl of iii the doininaril m e [l]. (ii) 11 i s cviclent from thc tiispcrsioii ciirve thtit, h r tlic given piiraiiickrs, tlic cut-oA limit is IO.4Cil-Iz ancl ltic resonancc l inii l is 7.63C.il-Iz. Wittiiii t h i s rmgc pLyj' is negative ;ind Ilic extraordinary wave is dcciiying s o 11ic antcnna is efficc(ivcly 011' ii!, a radiator in this fitiigc, Tliiis, by propcrly sclccliiig atid changing thc bias field, thc switchnblc mteniia can he dcsigncd, atid we ciin find OLI( tlic optiinurn pzerl'olmancc of' thc anleima systcoi. (iii) Wlicn thc ankima is bi:iscd with ii DC magtietic licld, gain ancl bmdwidth :ire both incrcnscrl. 111 thc GISC oi' = p,, = d2, the valuc ol' l l ic gain is 12.9dU ;i.nd tlic eiihaiice- incnl in bandwidth i s 2h.5'%1 Iot. h i : d Eerritc whilc, fnr & = ( I , = m'4, gain is 13.7dB and tlic cnhaticcnicnI in bmd- widlh is 21.8% Our rcsults for the gain ;itid hindwidth are significanrlp better (tian rcccii~ results reportctl by Yiiug [3] ~ i i d Stiiraj et d, I 121. (iv) Thc size of pitch is ctmsidaablp red~rccd when dcsigiicd on 21 fcrrite subslralc, TliIs rcdudioti would wr- Iiiinly haw n wide iisc in ercating miiiiaturis;ition or m mLciiiia system, which h a s U potential tipplicittim in spice ;aid ~ l l ~ l a i ~ cointnunica~ion [I ] . (v) Efficiency of iintcniia is iiicreased when biasing is pro- vidcd. I11 the case or p.y = p,, :: xi2 tlic efficiciicy is iiorcd as 74.8% and 24.W) tbr biased :ind Luibiascd ferrite, rcspcc- tivcly. I n mother Ci1Se wlicrc fi, 7 fly -- n/4: we have recorrlctl iiii cniciency of 73.4% I'm hiuscd and 26.9%, liir

(24)

(25)

Thc values of radiation conductance, directive gain, c f k &nc.y and bandwidth have been cslculakd Ibr the array geometry using lhc above expressions lor unbiased and bi;iscd h i k material by taking Iho s m c input pxraiiidcrs Tot two values of progressive phase cxcitalion dill'ci-cnce hctwwn the mteiinu clcmeiib (i.e. 13, = fiy = rd2 and "4). The inrcgral involved in the abovc cquations haw bccn solvcd u4ng the 1111 mericd inclliod [I I ] . Calculated valucs :ire given in Tahlc 2.

It is observed koni the 'Table t h t thcm i s a sigtiificmt chxigc in (tic valucs of anten~ia ~~ar;inic(crs on biasing thc Ferritc s u b s k m as well as 011 ctiaiiging the pi-ogi-essiw phasc cxcitalion differcnw.

Table 2 Calculated values of antenna parameters of array geometry

/I, = py = n/2

unbiased case biased case unbiased case biased case

oh m-' ohmf ohm-' ohm-'

p, = p,, = d4 S. No. Antenna parameters ~

1 Radiation conductance IG) 5.88 x 6,218 x 6.56 x 104 4.986 10-4 ~-

2 Directive gain( Dgl 7.5 dB 12.9 dB 9.0 dB 13.7 dB

3 Efficiency (h) 24.8% 74.8% 26.9% 73 4%

4 Bandwidth IBW) 16.2% 26.5% 16.7% 21,8%

1 ss