Two-dimensional beam-scanning linear active leaky-wave antenna array using coupled VCOs

Two-dimensional

beam-scanning

linear

active

leaky-wave antenna

array using

coupled

VCOs

C.-C.Hu, C.F.Jouand J.-J.Wu

Abstract: An active phascd-array dcsign Tor cloctwnic Iwo-diiriciisioniil bcam ,winning using :I

phasc-sliifterless linear aac-dimcndonal i d v c Ic;tky-wavc L I I ~ ~ C I I I I ~ I array is demonsllatccl. 'l'lic

wractor-tuned voltage-cc-coiitrolled oscillators (VC:Os) ~ n d coupling i~clwirrk arc iniplcnientcrl in this arriry. TIirce coupliiig networks are dcsignd rind coiiqxircd for oplimutn coupling slrcngih between

cach VCO elctnent. 'l'lie tneasutrd pattern of ;i 4 x I wlivc lcaky-wavc ~iltctlliil i i m y shows that the

niaiii bcam can hc continuously sciiiiiicd from 68" to 40" i n elevation: a s 1hc Gcqiiciicy viirics Trim

X.24GI 17 10 9. I5Gt17. This is cquivalcnt to ovcr 10'X~ clet.troiiic luniiig IxiiidwidLh. I3y luriiiig Lhc Ilw- riinning frcq iiciicics nl' tlic ciid ctciiicnts, thc in;iin bcilIli 2 a i i bc continuously SCRIIIINI li.nin -26" to -1-

Io"

1 Introduction

Quasi-op~ical power coriihiriing aiid Imim .witnning using

arrays of cnuplcd oscillators havc rcccnlly bccn uiirier

investigwtian by m:my groups [1-5]. In thcit. appiwiichcs, il sct of p h a r antenna-loaded iictivc microwavc oscillalor circuits tire htbricatod io a pcriodic arrangcmcni. Tlic oscil-

lators in thc array i r i k i ~ i c ~ with cach otha- viii l k e s p x c ,

l1.ansr~iission-liiic circuits, or cxicrixil c;iviiics, permitting llicrn to synchroiiisc 10 ii coiniiioii freq~iency through

muha1 irijection locking.

This paper ptoposcs an aclivc pliwd-arr:iy dcsign that

iritcgralcs tlic iil1icnnit systems cmploying p h n r microstrip

icaky-wavc :mtcnna tcchnologies 16 -1 01 arid thc phisee-

shil'tcrless hc;ini-scanning tcclinique [ I I , 121 (sec Fig. I).

I n 1979, Mcnzcl proposcd il s u c ~ c s s f ~ t l method for thc

excilnlion ol' llic microstrip lint lirsl tiighcr-order made [O].

111 1990, Oliticr

1x1

dimciisioiial hcani-scanning iirrily. A pcncil beam, which is a iiiirrow I m m , c;in bc sc;inned in both elevation :tiid :I&

ruutli (lwo~ditiicnsicmal scmiiiig) by cmployitig it lineiir

onc-dirnciisional phascd array or leaky-wave I i iic-sourcc antclii1ils. 1-lowcvcr, philsc sliifters are reqiiircd in 11ial dcsign.

In 1993, 1 ,ixo iIrid York [ I I , 121 proposcil :I IICW p h ~ c - shiHcrlcss onc-diinciisional be;ini-sc,caiiiiiiig tcchniqac, usitig ii p;itch antenna ;irray with wcwkly coupled oscilla- tors. A constxi1 ptiasc progression is achievcd by control- ling ~ h c kw-running frequencies of the elid clciiie~its or ilic

array in the locking bandwidth limits. Therefore tlic iziaili

hcam ciiii bc sc;i~ined i i i aziniutli. Furtherinore, tlic plinsc progression is indcpentlcnt of the number of oscillaiors.

Here, wc dcincmstrillc' il lilici~t active phiiscd-alltenli:i

array (sec Fig. I ) with ~wti-tliinciisioi~al bcam-sc;inning mpability, (Fig. 2) without using phase shil'lcrs. Tlic cou-

pling trimmission line eriliaticcs tlic I ~ L I L L I ~ ~ s y i i c l i r o n i w

tion of tlic xtivc ;mhmxi x f i i y . The conti-olliug of the frce-

running rre-equency is uchieved by luiiiiig tlic varacior's DC bias, which also gives the addilional advanlngc or linc hi-

ing lhc sc;iniiing mglc of the aIi-:iy. microstrip leaky-wave

varactor

x azimuth j -r > eleval i on scan H m couplinu

' \

-.

resonator NE424M HEMT

MA46410

8.6

8 4

signwl iterative proccclure utilising ii coinnierciiilly avail:tble CAD tool (I-11'-EEs of Libra). A short-ciieuilecl iriicruslrip

line fecdback i s used in scrics with (tie SOUIW 10 provitlc Lhc

device ncgntivc: rcsistiincc. Thc leaky-wave antenna (wivilli

I

= IOcin and IV = I .2cin) is coiinec.tcd l o thc drain 10 coin-

pens;i(c tlic negative rcsislancc under slcady-state opcix-

lion. Tlic circcuit is dcuigncd atid fitbricakd on RTlDutaiil

suhstl-ale, with a clidwtric constanl of 2.2 and tllickiicss ol'

20riiin. NGC NFA2484 low noisc I-IBMT i s iiscd a s thc

:tc.tivc dcvice, and the drain i s biased a1 2.OV with a / E l < l ' w . - ~ L ~ ~ i c w ~ ~ ~ , , d i r i w i w I'ru/!iif., Yo/, 1.17, ,Vvo. 1. f i h t i t r y 2fMf3

ATTEN lOdB

RLOdBM lOUB/ 8.93095QHz

MKR - 13.83d Bm

J

2.2

Elevation

The gcomclry arid co-ortliniilc system of thc microstlip

leaky-w;tvc aiitciiii~ strucl~ii-c arc shown in Fig. 0. This

microstrip le;iky-wavc ilnteniiit i s opcratcd in its Erst liighcr-ordcr rnodc atid in chtiriictcrkcd by a ph:ise constant

B

ol' phasc cti1ist:iiit /I tind ;ttlcnusition c1 :IS it llaiction of t'rc-

qucncy. Ttic coiiiplcx constanls R I ~ nhtaiiicd by cniploying chc rigorous (Wicnci+-I-Iopfl solution o~cn~iotietl in

[$I.

complcx propagatinn c.onstnnts represent ;I Ibrward leaky

wave rtldi;itiiig iiito tIic spiicc at an mgIe t ~ ~ , , = cosd(/Wifi).

whcte flfIf is chc anglc of thc bc;un riinximum mcasured froin lhc z-iixis. In ;idclition, the s c " g angle U,,, cim be varicd with Frequcricy. To excitc the lirsl Iiiglier-oidcr 69

mode, the microstrip Icaky-wavc antalixi is rcd

asymmctri-

g

t

ncously,

~ I I C

can

plant):). Figs. X and 9 show lhc coniparisoii or Ihc incmrc-

iiicnt rcsults and tlic llicorcM prcdiclions of thc TI-planc

paltcrn Ibr opcraling li.cqucncy at 8.24GFIz iind 8.95GHz, mpcctivcly. Tlic ~ n c a s u l ~ x l H-plnlic m i l l hcarn can bc con-

tinuously scaiiiiecl froin 6%" to 40" from (Iic z-axis. broadside coupling iroquency conlrol frequency control

2.3

A sc;intiing-array di:igr:im is showti in Fig. IO; il comprises

N coupled-VCOs, a n i e d i of theill is coupled TO its two

neiiicst neighbours. When w conslwnt phase progressioii Arp = 0, f l I + , a loiig tlie :irr:ty is establislicd, tlic t.adialioii

iliain beam is scenned to 811 :ingle

In [I I , 121; the ititer-elemerit phase progrcssim Ay is COH-

trollccl orily hy tiiiiing the frec-luiiiiing I'rcqiicticy o f the ciid elcinent. For 21 siiiiplcst cwe, wc assiimc all of Ihc oscilla-

tors are identical, such lhal ;i conslant phase progicssiori

AT citn be syiiolicsised a t it frequency w, by the tollowing

distribution of froc-riiniiitig frcqueiicies [I I]:

where Am,,, = q / 2 Q is the locking hendwidtli, F is tlic

coupling strcngtli of thc coupling circuit, iincl Q is the qutil- ity fhctor of thc oscillator etnbedditig circuits. In steady

statc, all the oscilkttors r w i i t w coiiiinoti frequency given

by thc nvcrqc or' thc frw-running Gccpiciicics

i=l

Using eqn. 2 :iod sliglitly ad,justing tlic Gwtunnirig lic-

quciicics of the end elcnients in oppositc directions by an

amount A q , , sit1 AV, we c m obtain ti constwilt phase pro-

grcssiori A q m d the ratliatioii pnttern can be scanned with

azi tnuthal symiiietry iii a coiiical scan ina~incr.

Tlic radialion patlcm of our 4 x I active pliasccl array

w i i s mcasureed on a conical surfxe coi~responding to a t i

angle 01' 45" fioiii tlie x-iixis. Licli wiractor was biiiserl individually (SLY Tablc I). Whcn the varactors wcrc siiiiul-

musl sly biased at SV, a wminon output rrequency of

8.73 I CXlx w a s ohscrvccl. Tlic h - r u n n i n g ti.cquciicy tun-

ing

or

inost varactor's UC bi:is voltagc; illis varactor bias Iuiiiiig

rmgc i s limitcd to Ilic tn;ixitnum locking txindwidth of this

array (== 120MI-1~ a1 8.73GIJz). The results obtained froiii tuning thc DC biils 01' cach var;ictor ;ire given in Table I.

Table 1; Measurement results for three different DC bias conditions

DC bias, V Output Main beam

frequency angle from

varactor 1 varactor 2 varactor3 varactor 4 [,+, G H ~ broadside

Bias

--

--

1 5 , 5 5 5 4.5 8.739 -26"

2 5 5 5 5 8.731 0

3 4.5 5 5 5.5 8.723 +10"

13ascd OH cqns, 2 and 3 , thc output freqllellcy {OJ shaultl be

ilic samc Tor. thc three difYcrcnt hias conditions. I-towcvcu,

tlicrc i.im liquciicy shifts 01' U,' diiriiig tlic :iziimitlial scari-

nitig iiiciisu~lncli1. This frqucncy shill is probtibly duc to

thc dcvicc-paratncte~ v:iriation imtl ltlc non-linear v;ir;ictot'

tuning rxigc i l C l D S S the array. Thc i ~ l i i t t i o n patterns i i i c i i s -

u r d in axitlutli arc sliowti in Figs. 1 I L l 3 . 'l'tiesc iiiciisiircd

rerulls illuslratc that tlie mtiin bciiin CNI sctin in iizinlulh

li-om -26" lo +IO" off hroadr;idc as llie var;ictor DC bias

volktgc varies froni coiidi~ion I 10 coliclition 3. This sci~n-

l i n g rangc indicates that it is possiblc to obtain 120" o f

phasc shill hc~wecn each elcnicnl in llic array, which corn- spontis lo ii plirise clifYercncc or 360" hetween lhc lirsl atid

last cletiients in thc array.

3 Conclusions

A new aclivc phmcd-array design for clcclronic (wn-dinicii-

sional beam scmning using R linear iichc lcaky-wave

antcniin Jii-ray has bwn dcmoiistrrited witlioul using pliiise-

sliiftei-s. A wris~;int phase progression Iti:it is intlcpcrirlent

01' the numbcr of oscilla1ot-s is accotnplixhcd by tuning tlie VCO DC: bias volkigc, so tliat ttie bcatii c m be scammi iti both ii7.imiilIi ;mrl cIcvat.ioii i r i a conical S Cm m w . ~ 'I'hc coupling nclwork is desigiicci to crcatc R proper coupliiig

strciigth hctween each oscillalnr. The radiritioii ptdtwns Ibr Lhc 4 x I active microstrip leaky-wevc antenna array i i i both elevation iind nziiiiulh were me;isurcri and compared with tticorc(ieal prerlictioiis. 'I'lic initial rcsuI~s show thc potcntial of using thc circuit for low-cost kinsmitters.

aclivc IIIMYS, s p a h l power combiners iIlld r&r applica- tioris.