Design of a Planar Slotted Waveguide Array Antenna for X-band ...

JOURNAL OF THE KOREAN INSTITUTE OF ELECTROMAGNETIC ENGINEERING AND SCIENCE, VOL. 11, NO. 2, JUN. 2011 JKIEES 2011-11-2-05DOI : 10.5515/JKIEES.2011.11.2.097Design of a Planar Slotted Waveguide Array Antenna for X-band RadarApplicationsRashid Ahmad Bhatti 1 ․Byeong-Yong Park 1 ․Yun-Taek Im 2 ․Seong-Ook Park 1AbstractA planar slotted waveguide array antenna has been designed at 9.37 GHz for X-band radar applications. The antennaconsists of multiple branchline waveguides with broadwall radiating shunt slots and a main waveguide to feed thebranch waveguides through a series of inclined coupling slots. The antenna feed point is located at the center of themain waveguide. Element weights in the array have been calculated bysampling a continuous circular Taylor aperturedistribution at the 25 dB sidelobe level in both the E and Hplanes. A commercially available electromagnetic (EM)simulation tool has been used to characterize the individual isolated slot and that data hassubsequently been used todesign the planar array. The array is finally analyzed in a CST Microwave studio and the measured and simulatedresults have been found to be in good agreement.Key words : Antenna, Slot, Waveguide.Ⅰ. IntroductionSlotted waveguide array antennas are widely used forradars and communication systems [1]. They offer significantadvantages in terms of weight, volume, and radiationcharacteristics. These antennas are very attractivedue to their planar, compact, and rugged construction. Ametallic waveguide is a low loss structure and it canhandle high power. The dimensions of the slots in thewaveguide walls can be controlled to realize the desiredpattern shape. Longitudinal shunt slot arrays are also attractivedue to their very low cross-polarization levels.The design of a resonant slotted waveguide array antennais generally based on the procedure published byElliot [2]~[4]. In these design procedures, inter-slot distanceis one-half the guide wavelength and the waveguideend is short circuited at a distance of a quarter-guidewavelength from the center of the last slot.Linear arrays are placed adjacent to each other to realizeplanar arrays. A feed waveguide is placed beneath thebranch waveguides containing radiating slots and poweris coupled from the main waveguide to the branchwaveguides through the series of inclined slots. The feedwaveguide is terminated in the short circuit located at adistance of one-half of the guide wavelength from thecenter of the last inclined coupling slot. External and internalmutual coupling effects can be fully incorporatedinto the design procedure. Other major research worksonlinear and planar slotted waveguide antennas are [5],[6]. Isolated slot characterization is required in the designof slotted waveguide arrays. Isolated slot admittancecan be measured experimentally [2] or computednumerically [7]~[9]. Care must be exercised in calculatingthe individual slot data as minor errors might severelyaffect the overall array design.In this paper, the design of a planar slotted waveguidearray antenna based on the procedure in [3] is discussed.The design procedure includes bothexternal mutual couplingeffects and internal mutual coupling using commercialEM simulators to approach more exact results.The planar antenna has been designed at 9.37 GHz withshunt slots acting as radiatorsin the broadwall of a reducedheight waveguide to suppress higher modes. Theplanar array consists of twelve linear arrays of shuntslots, which have been placed side-by-side with truncatedcorner slots to fit the antenna in a circular boundary.A main waveguide is run across the branch waveguidesto feed them through the inclined-series couplingslots. Antenna feed is located at the center of the mainwaveguide.Ⅱ. Isolated Slot CharacterizationIn order to use the design procedure [3], it is im-Manuscript received February 1, 2011 ; revised May 16, 2011. (ID No. 20110201-005J)1 Department of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea.2 Department of Information & Communications Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon,Korea.Corresponding Author : Yun-Taek Im (e-mail : imyuntaek@kaist.ac.kr)97

JOURNAL OF THE KOREAN INSTITUTE OF ELECTROMAGNETIC ENGINEERING AND SCIENCE, VOL. 11, NO. 2, JUN. 2011portant to generate isolated slot admittance as a functionof slot length and the resonant length of the slot as afunction of its offset. For feed slots, the resonant lengthof the slot as a function of its tilt angle is also requiredforthe design of the array. A commercially available EMsimulation tool, HFSS, has been used to generate the basicslot data subsequently used in the array design. Inthe HFSS simulations for the shunt slot, the roundended 2.5 mm wide slot was created in a 1 mm thickbroadwall and admittance was recorded at a distance ofone-half the guide wavelength from the center of theslot; the waveguide was terminated in a short circuitplaced at a quarter-wave distance from the center of theslot. Slot length was varied and at each step of normalizedcomplex admittance, conductance (G/G r ) and susceptance(B/G r), was recorded and was normalized withresonant conductanceof the slot. The design curve thusgenerated is shown in Fig. 1. The real and imaginaryparts of the admittance are denoted as a h 1(y) and h 2(y),respectively, where y is the slot length normalized bythe resonant length, l/l r , which can be written as( , )Y x lG0( )= h ( y) + jh ( y) g( x)1 2The waveguide width and height are 21.5 mm and 5mm, respectively. Next, the slot offset is changed and ateach step, the slot resonant length is calculated wherethe imaginary component of the admittance was zero.The resulting resonant length of the slot as a functionof offset is shown in Fig. 2.We will denote the curve shown in Fig. 2 as v(x),where x is the offset of the slot from the center-line ofthe waveguide.Normalized resonant conductance, g ( x) = GrG0, ofthe isolated shunt slot as a function of offset is plottedFig. 2. Resonant length of the broadwall slot versus offset.in Fig. 3. The curve g(x) can be approximated as follows[7],2 æ p x ög ( x) = K sin ç ÷è a øa b 2 æ pb10 k0öK = 2.09 cos ç ÷b10 k0è 2 ø , (1)where a and b are the waveguide width and height respectively,and b10 is the propagation constant of TE 10mode and k0 = 2p l0.The coupling slot was also characterized in HFSS forits resonant resistance as a function of the tilt angle. Inthe simulation model, waveguide was terminated at thedistanceof the half-guided wavelength from the centerof the slot and resistance was recorded at the port locatedat thedistance of the half-guided wavelength fromFig. 1. Normalized admittance of isolated shunt slot versusslot length.Fig. 3. Normalized conductance of isolated shunt slot asa function of offset.98

BHATTI et al. : DESIGN OF A PLANAR SLOTTED WAVEGUIDE ARRAY ANTENNA FOR X-BAND RADAR APPLICATIONS( b / )( )( / l ) 3K = j k k b a3 10 0k0lmn- jk0R1- jk0R2æ z'ömn 1 e egmnrscos4lk lmn/ 4lmn/ k R k R0 mn0 î íì éù= òç÷ · ê + úè l ø l-0 ë 0 1 0 2 ûk0lrs- jk0Ré 1 ù æ z'örs e ïü+ ê1- ú · cosdz'rs ýdz'mn ,4lrs/ç04lk lrs/÷ë l òûk R- 0 rsè l0ø 0where R is the distance from the point Pmn( 0,0, z ¢mn )measured in local coordinates at the center of the mn thslot to the point Prs( 0,0, z ¢rs ) measured in local coordinatesat the center of the rs th slot. l mn and l rs representthe slot length of mn th and rs th , respectively.ïþFig. 4. Resonant length of the series coupling slot as afunction of tilt angle.the center of the slot. At each tilt angle, the slot lengthwas varied to make the imaginary component of the slotimpedance equal to zero. The resulting curve is shownin Fig. 4.Ⅲ. Planar Array Design ProcedureThe antenna design rests on the following designequation [2].YGamn0where,Kand,f1mn= K f1mn( )VVsmnm0 10( k k )1 20=j a l hG ( b / k)( ka)( kb)( 2 ) cos( 10 )2 2( p 2kl) - ( b k )p klmn b lmn æ p xmnö= -Ksin ç ÷ .è a ømn10(2)éR x x z zë2( mn pq ) ( mn pq )æzz öùç ÷ úèøûpq= - +mnê - + -k0 k0R 2 is the distance from P mn to the left end of the slotPrs( 0,0, - lrs) and R 1 is the distance from P mn to the rightend of the slot, Prs( 0,0, lrs ) and is given as,2 é( ) ( )zmnùR1= xmn - xpq + zmn zæpqlöê- + ç -kpq ÷ëè ú0 øû2 é( ) ( )zmnùR2= xmn - xpq + zmn zæpqlöê- + ç +kpq ÷ëè ú0 øûThe typical configuration of the two slots in an arrayis given in Fig. 5.In (3), MC mn is the mutual coupling term for the mn thslot, which incorporates interactions from all of the slotsin the array.The aperture distribution has been calculated by samplingthe continuous aperture at slot locations for the —25dB first sidelobe level.The planar array is depicted in Fig. 6. It consists ofa twelve branch waveguide witha feed waveguide placedZpq222In equation (2), V m is the mode voltage in the m thsbranch waveguide and Vmn is the slot voltage of themn th slot. Y is the active admittance of the mn th slot.YGamn0=amn2fmn ( xmn lmn)( )22 fmn xmn,lmn02 ,Y xmn lG( , )mn+ MCmn(3)x mnξ2l mnςR 2RR 12l pqxwWaveguide center lineVMC K g x l x lR S srsmn=3å å s mnrs mn mn rs rsr= 1 s= 1 Vmnrs ¹ mn( , , , )Z mnFig. 5. Two slots in a planar array.99

JOURNAL OF THE KOREAN INSTITUTE OF ELECTROMAGNETIC ENGINEERING AND SCIENCE, VOL. 11, NO. 2, JUN. 2011Fig. 7. Aperture distribution for the planar array withtruncated corner elements.Fig. 6. Planar slotted waveguide array antenna.behind.For the mn th slot in the array, the distance from theorigin is calculated as [9],100( 2 -1) ( 2 -1)ææ m dz ö 2 æ n dx ö2 örmn= ç ÷ + ç ÷ç è 2 ø è 2 ø ÷è ø ,where dx and dz are the inter-element spacing along xand z directions.The aperture distribution is then calculated as follows[9],whereA g p( )mn=0 mn , (4)p mnpr=amn2 S Jg( p)= åpSn -1n 02 20 J0( pmn)( pm )n -12mÕ 1-2n=1 unm= J0( pmm )èn -12æ mmÕ 1-2n 1 è unm nm 0næ m öç ÷øöç ÷= ø¹>m are zeros of ( ), S0= 1 ,where mJ1p u .The calculated aperture distribution is depicted in Fig. 7.A 3-D array factor plot corresponding to the sampledaperture distribution is shown in Fig. 8.Fig. 8. Array factor corresponding to the calculated aperturedistribution.After calculating the array weights, the following procedureis used to calculate the slot dimensions (offsetsand lengths).A set of initial offsets and lengths is assumed for theradiating slots. Resonant lengths and zero offset are areasonable choice for starting the design. These offsetsand lengths are used to calculate the following mutualcoupling termin equation (3),VMC K g x l x lR S srsmn=3å å s mnrs mn mn rs rsr= 1 s= 1 Vmnrs ¹ mnfor every mn in the array.( , , , )Asearch is then conducted to find the couplet ( xmn,lmn)( , )xmnlmn that satisfies the following relation,

BHATTI et al. : DESIGN OF A PLANAR SLOTTED WAVEGUIDE ARRAY ANTENNA FOR X-BAND RADAR APPLICATIONSéùê 22 fmn ( xmn,lmn) úIm ê ú = - Imê Y( xmn,lmn ) úêëG ú0 û[ MC ]where Y/G 0 is calculated from the design curves, as( , l )Y xmnG0mnmn( h1 ( ymn ) h2( ymn )) g ( xmn)= + .Once this is satisfied, then active admittance of theslot become spure real. During this search process, acontinuum of couplets that satisfy this relation is found.Similarly, for the pq th slot, the same search is conductedand the continuum of couplets that make the active admittancepure real is found. But for a given acceptablecouplet for the mn th slot, there will only be one coupletfor the pq th slot that will satisfy the following equation,Yapq2sf ( , )0 pqxpq lpq VpqVma 2smn mn ( mn,mn ) mn pG= .Y f x l V VG0This procedure provides a set of acceptable coupletsfor each slot. However, which couplet to choose will dependon the required admittance level in each branchline.The final acceptable couplets in a branch-line shouldsatisfy the relationship,YN ( m)amnå = Cm,n=1 G0where C m is the required admittance level in the m thbranch waveguide.The entire procedure needs to be iterated as the newslot data will give an improved mutual coupling calculationand the solution will converge after 3 or 4 iterations.After calculating the radiating shunt slot data, the seriesfeed slot data is then calculated according to the procedurereported in [9].The sum of the series of slot resistances is given by,Må r = W ,mm=1m= and m=1, 2…M,r m is the normalized resonant resistance of the m th slot,B is the slot excitation function, K is the excitation normalizationconstant, and W=1 for the end-feed or W=2for the center feed. B m is given in terms of the aperturedistribution.where r KB 2( m),(5)(6)(7)(8)N m2 2( ) ( , )B m = å A m n ,n=1where N m is the number of shunt slots fed by the m thseries slot.Using these resistance values, the corresponding slotangles are calculated using the design curves or the followingrelationship,(9)2R b10 l é l0g ù= 0.131 êIqsinq+ JqcosqúR0k ab ë 2aû , (10)where theta is the tilt angle of the series slot, andæ p ö æ p öcos K1 cos K2I ç ÷ ç ÷q 2 2=è ø±è øJ 1- K 1-Kq22 21 2K1 b10 l0= cosq± sinq.K k 2aOnce slot resistance as a function of the slot tilt angleis known, we can calculate the slot length for differentslot angles from the design curve shown in Fig. 4. Theapplied slot length (l) is 16.5 mm and the offset lengths(x) are 2.1, 1.85, 1.6, 1.0, 0.8, and 0.7 mm after the calculationand CST-simulation.The feed waveguide is fed at the center through astandard X-band waveguide. The width of the feed slot isoptimized in CST to get the desired match at 9.375 GHz.A 3-D exploded model of the antenna assembly isshown in Fig. 9.Fig. 9. Exploded view of antenna assembly.101

JOURNAL OF THE KOREAN INSTITUTE OF ELECTROMAGNETIC ENGINEERING AND SCIENCE, VOL. 11, NO. 2, JUN. 2011Fig. 10. Surface currents on the aperture.Fig. 12. Comparison of the measured and simulated E-planepatterns.Fig. 11. Simulated 3-D pattern of the antenna.The surface currents on the plane containing the radiatingslots are depicted in Fig. 10. It can be observedthat the amount of current on the slots decreases withincreasing distance from the center of the antenna, thatexhibits amplitude tapering.A simulated 3-D pattern of the antenna is depicted inFig. 11.A comparison of the measured and simulated patternsis given in Fig. 12. The first sidelobe level of the antennais 25.6 dB down and the 3-dB beamwidth of the antennais 8.2 degrees. The measured gain of the antennais 26 dBi.A comparison of the measured and simulated reflectioncoefficients of the antenna is given in Fig. 13.The antenna shows a bandwidth of 200 MHz for the —10dB input reflection coefficient. The radiation efficiencyis calculated as,2Gain ´ lEfficiency = , Area ´ 4pand efficiency of the antenna is 46 %.(11)Fig. 13. Comparison of the measured and simulated inputreflection coefficients.Ⅳ. ConclusionsA planar slotted waveguide antenna has been designedat 9.37 GHz using broadwall radiating slots. Theantenna design is based on the commonly used Elliot’sslotted waveguide antenna design procedure, using thecommercially available EM simulation tools, HFSS &CST MWS. The antenna has been designed for a —25dB sidelobe level and a beamwidth of 8.2°. Comparisonsof the measured and simulated far-field patternsand input reflection coefficients have been reported, andshow amaximum 10 dB error within the measuredbandwidthdueto loss or leakage in thewaveguide adapter.102

BHATTI et al. : DESIGN OF A PLANAR SLOTTED WAVEGUIDE ARRAY ANTENNA FOR X-BAND RADAR APPLICATIONSThis research was financially supported by the Ministryof Education, Science Technology (MEST) andKorea Institute for Advancement of Technology (KIAT)through the Human Resource Training Project forRegional Innovation.References[1] Phillip N. Richardson, H. Y. Lee, "Design and analysisof slotted waveguide arrays," Microwave Journal,pp. 109-125, Jun. 1988.[2] Robert S. Elliot, L. A. Kurtz, "The design of smallslot arrays," IEEE Trans. Antennas Propagation, vol.AP-26, pp. 214-219, Mar. 1978.[3] Robert S. Elliot, "An improved design procedure forsmall arrays of shunt slots," IEEE Trans. AntennasPropagation, vol. Ap-31, pp. 48-53, Jan. 1983.[4] Robert S. Elliot, W. R. O’Loughlin, "The design ofslot arrays including internal mutual coupling," IEEETrans. Antennas Propagation. vol. Ap-34, pp. 1149-1154, Sep. 1986.[5] H. Y. Yee, "The design of large waveguide arraysof shunt slots," IEEE Trans. Antennas Propagation,vol. 40, no. 7, pp. 775-781, Jul. 1992.[6] Alan J. Sangster, A. H. I. McCromick, "Theoreticaldesign and synthesis of slotted waveguide arrays,"IEE Proc. H. Microwaves, Antenna and Propagation,vol. 136, no. 1, Feb. 1989.[7] George J. Stern, R. S. Elliot, "Resonant length oflongitudinal slots and validity of circuit representation:theory and experiments," IEEE Trans. AntennasPropagation, vol. Ap-33, no. 11, pp. 1264-1271, Nov.1985.[8] Lars G. Josefsson, "Analysis of longitudinal slots inrectangular waveguides," IEEE Trans. Antennas Propagation,vol. AP-33, pp. 1351-1357, Dec. 1987.[9] Robert S. Elliot, Antenna Theory and Design, No. 6,Prentice Hall, Inc, 1981.Rashid Ahmad Bhattiwas born in Bahawal-Nagar, Pakistan, onJuly 14, 1975. He received the B.Sc. degreein electrical engineering from Universityof Engineering and Technology, Taxila,in 1999 and the M.Sc. degree in electricalengineering from National University ofsciences and Technology (NUST), Rawalpindi,Pakistan, in August, 2005. Currentlyhe isworking toward the Ph.D. degree in electrical engineering atthe Korea Advanced Institute of Science and Technology(KAIST), Daejeon, Korea. From September 1999 to December2005, he worked as an RF and Antenna Research Engineer atAdvanced Engineering Research Organization (AERO), Pakistan.Here, he has been involved in the EMI/EMC testing anddesign of slotted waveguide array antennas for trackingradars.His current research interests include the design of MI-MO antennaarrays, multiband antennas for portable communicationdevices, slotted waveguideantennas and numerical methodsin electromagnetics.Byeong-Yong Parkwas born in Daejeon, Korea. He receivedthe B.S. degree in division of electricalengineering from ChungNam NationalUniversity, Korea, in 2008, the M.S. degreein electrical engineering from Korea AdvancedInstitute of Science and Technology(KAIST), Daejeon, Korea, in 2010, and iscurrently working toward the Ph.D. degreein electrical engineering atKorea Advanced Institute ofScience and Technology (KAIST), Daejeon, Korea. His currentresearch interest is the design of MIMO antenna arraysfor portable communication devices.103

JOURNAL OF THE KOREAN INSTITUTE OF ELECTROMAGNETIC ENGINEERING AND SCIENCE, VOL. 11, NO. 2, JUN. 2011Yun-Taek Imwas born in ChungNam, Korea, on June7th, 1978. He received the B.S. degreeindivision of electrical engineering and computerscience from Hanyang University,Korea, in 2005, the M.S. degree in electronicand electrical engineering from thePohang University Science and Technologyat Pohang (POSTECH), in 2007, andis currently workingtoward the Ph.D. degree in electrical engineeringat the Korea Advanced Institute of Science andTechnology (KAIST), Daejeon, Korea. He was engaged in developmentof RFID antennas and millimeter-wave beam-formingsystems from 2005 to 2007. His current research interestsare MIMO antenna systems and radar systems including antennadesign.Seong-Ook Park(M’05) was born in KyungPook, Korea,in December 1964. He received the B.S.degree from KyungPook National university,in 1987, the M.S. degree from KoreaAdvanced Institute of Science and Technology,Seoul, in 1989, and the Ph.D.degree from Arizona State University, Tempe,in 1997, all in electrical engineering. FromMarch 1989 to August 1993, he was aResearch Engineer withKorea Telecom, Daejeon, working with microwave systemsand networks. He later joined the Telecommunication ResearchCenter, Arizona State University, until September 1997.Since October 1997, he has been with the Information andCommunications University, Daejeon, first as an AssociateProfessor, and currently as a Professor at the Korea AdvancedInstitute of Science and Technology, IT convergence Campus.His research interests include mobile handset antenna, and analyticaland numerical techniques in the area of electromagnetics.Dr. Park is a member of Phi Kappa Phi.104