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Dual Polarized Reader Antenna Array for RFID ApplicationS. K . Padhi (I N.C . K armakar" and C.L Law"

1 Molecular Science Division, CSI RO, Bayview Avenue, Clayton, Melboume,Australia, Email : shantanu.padhi@csiro.au2: School of Electrical and Electronic Engineering, Nanyang Technological University,SingaporeA bstract -This paper presents the design and development of a compact dual linearlypolarized aperture coupled microstrip patch antenna array at S-hand for RFID readerappli cations. The 2x2 array uses two symmetric dog-bone shaped coupling aperture.The prototype array yields 14.6 dBi peak gain, 36" half-power beam width (HPBW) inboth E and H-planes, 28 dB cross-polarization levels and an isolation better than 3.5dBbetween the two polari zation at the designed frequency bandwidth. T heseperformances make the antenna very much suitable for W I D reader applications.Introductions: Now a days, there is a phenomenal growth in demand for wirelessWsystems such as wireless local area network (WLAN), WID and point-to-pointcommunications systems.WID systems are gaining popularity in manufacturing units,purchase departments, logistics and transportation etc., where identifi cation is a primeconcem. At present, some RFID systems operating at S-band (2.45 G Hz) arecommercially available; WI D systems operating in the next higher ISM band (around5.8 GH z) are under active development [ l ]. There is a great commercial demand forRFID applications near 2.4 GHz. In these applications dual polarized shared-apertureantennas are preferred to reduce cost, size and complexity. For RFID application, it isdesirable to have an isolation of more than 25 dB between the orthogonal ports. Agood isolation also contributes in achieving a good axial ratio if the antenna is used toproduce circular polarization. Formicrostrip patch antennas, aperture coupli ng [2-41 ispreferred to other feeding mechanisms as it offers greater design flexibility such ascompact size, less complexities and reduction in overall cost.This paper presents the design and development of a shared aperture dual polarizedaperture coupled microstrip patch antenna and array at C-band (5.8 GH z) and S-band(2.4 GHz) respectively. T wo coupling apertures are placed in ri ght angle under theradiating patch to generate two orthogonal modes. T o obtain opti mum retum loss(a)bandwidth and isolation the positions of the slots are varied. U pon achieving thedesired performance the antenna was fabricated and tested in an anechoic chamber.The antenna design and i ts performances are presented in following sections. The unitantenna element can be used as a circular polarized antenna in WI D tag using anexternal polarizer as described in [5]. In this work, more emphasis was given toimprove the isolation between the two orthogonal polarization (Tx and Rx).A ntenna Design: Initiall y, a shared-aperture dual polarized square patch antenna isdesigned, optimised and characterized at 5. 8 GHz. A circularly polarized version of theantenna can be used as WID tag antenna at 5.8 GHz. Having satisfied with theperformance of the antenna a scale down model is used to design the 2x2 array forreader application at lower frequency. T he design layout of antenna array is shown inFig. 1. A corporate feed network is designed for the 2x2 array using two-stageWilkinson power dividers to divide the power equally to each element. Theconfiguration of the 2x2 antenna array is shown in Fig 1 (b). T he array is designed in

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multilayer structures. A s can bc sccn, thc radiating clcmcnts (squarc patchcs) arcetched on the top substrate and dual orthogonal aperturcs (dog-bones shaped) and fccdlines arc etched on top and bottom sides of the bottom substratc. rcspcctivcly Thcparameters of the substrate matcrials are as follows: the patch and fccd substrates arc0.79 mm thick TL X with dielectric constant ( E , ) of 2.45 and loss tangent (tanb) of0.0019. A n air gap of height 3.00 mm (cr : 1.00) is maintained in bctwccn the top(patch) and bottom (feed) layers. The thickness of air-gap, the length and positions ofthe coupling apertures are the prime parameters to determinc thc amount of powcrcoupled to radiating elements. T hese parameters are optimised such that both radiatingpatch and the apertures resonate at close frequency. T o incrcasc the isolation bctwecnthe two polarizations (Rx and Tx) both orthogonal apertures are pushed off verticallywith respect to centre position. A corporate feed network is designed to make the (2x2)array. The inter-element spacings are chosen to be d r=dv=0 . 72h to obtain theoptimum gain and mutual coupli ng between the elements. T he design and simulationof the structure are performed on Ensemble platform. A fter achieving satisfactory inputimpedance, retum loss (RL), isolation, radiation pattems and gain, aprototype model isfabricated with the help of Quick Circuit milling machine.M easurements and Di scussions: The impedance characteristics of the array aremeasured with HP8510C Vector Network A nalyzer (V NA ). The far field radiationpattems and gain were measured in an anzchoic chamber equipped with HP 8530AM icrowave Receiver.A . ReturnLoss:The prototype design layouts of the unit antenna element and the array arc shown inFig. 1 (a-h). T he width of 5 0 4 ransmission line used in the feed network is 2.27 mm.The ground plane size for the unit antenna element is (80x 80) nun2.The antennaresonates close to 5.8 GHz wi th an impedance bandwidth (10 dB RL ) of 15% at 5.8GHz. Similar resonance curve is obtained for both Rx and Tx polarizations. Theisolation between the twopolarizations i s better than -35 dB in the frequency band.The measured S-parameter of the array is shown in Fig 2.As can be seen the arrayresonates at 2.42GHz for both Rx and Tx polarization. The impedance bandwidth (10dB RL ) covers the frequency range from 2.38 GHz to 2.5 GH z (-5 %at 2.42 GHz) forboth Tx and Rx polarization. T he isolation between the two ports isbetter than 35 dBover the frequency band. T hese antenna performances are more than sufficient for anyWID application at S-band.B. Radiation PatternThe measured broadside co-polar and cross-polar E-pane and H-plane radiationpattems of the array for both polarizations (Tx and Rx) are shown in Fig.3 (a-b). Theradiation patterns are directive and symmetric in both E- and H-planes. The measuredradiation patterns agree well wi th the simulated ones. The measured cross-polar levelsare 28 dB lower than the CO -polar evels. B ack radiations are also found as the couplingaperture radiates in backwards. T he measured worst-case front-to-back ratio of the eo-polar radiation pattems is around 10dB. The back radiation can be suppressed with ametallic shield placed at adistance of quarter wavelength from the feed line. The 3-dBbeamwidth (HPB W) of both E -pane and H-plane radiation patterns is approximately36".

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C Gain:The gain measurement is performed on a HP-8530A M icrowave Receiver with thesimilar set up for radiation pattern measurements in the anechoic chamber. Themeasured gain of the array is 14.4 dBi and 14.1 dBi for both Rx and Tx polarizations,respectively compared to calculated gain of 14.7dBi and 14.56dBi for both Rxand Txpolarizations. T he simulated gain data for both array and unit element is shown in f ig4. The maximum variation of 0.5dBi in gain is obtained in the frequency range of 2.4to 2.5 GH z for the array, which is used for data communication. Similarly a variationof less than 0.4 dBi in gain is obtained in case of unit antenna element, in the vicinityof 5.6 to 5.9 GHz. T he gain bandwidth is reasonable good for both unit antennaelement and array in the respective frequency band.D. ield Trial:The array was incorporated in the reader module of in-housedeveloped W I D tag setand a series of field tests was carried out to optimise the readwrite range of tag set. Amaximum rangeof 10meters was obtained by using apassive tag.Conclusion: The papers presents the design and development of a shared aperturecoupled dual polarized square microstrip patch antenna and array for 5.8 GHz and 2.4GH z frequency band. T wo symmetric dog-bone shaped coupling aperturesareused toexcite two orthogonal modes. The array exhibi ted a maximum of 5% impedancebandwidth for both polarizations. A high isolation between the two orthogonal ports(Tx and Rx) of the order of 35 dB was observed over the frequency band of interest(2.4to 2.5Ghz). The measured radiation pattems are presented and the array yields themaximum gain of 14.4 dBi, 36 3-dB beamwidth (HPBW) and 10 dB front-to-backratio. The cross polar components areon average 28 dB below the co-polar level. Thearray was satisfactorily tested with an in-house developed W I D module. Thereadwri te range was approximately 10meter when used with a passive tag. High gainand high isolation between the two input ports make the array very much suitablecandidate for W I D reader module for 2.4GH z band.Acknowledgement: The project is supported by NST B strategic research program onMEMSAIT project vide #NST B/43/11/4-3.Reference:1. M.A.Kossel, R.K ing, H.Benedickter and W. Bachtold, An Active TaggingSystem Using Circular Polarization Modulation IEE E Tr. MTT, vol: 47, Dec.1999pp: 2242-2248.2. D.M .Pozar, M icrostrip Antenna Aperture Coupled to a Microstripline,Electronics Letter, vol: 21,No. 2, pp: 49-50, 1985.3. L.Sull ivan and D.H .Schaubert, Analysis of an Aperture Coupled MicrostripAntenna, IEE E Tran. AP, vol: AP-34, pp: 977-984, 1986.4. C.H .Tsae, Y .M .Hwang, F. K ilburg and F.Di etrich, Aperture Coupled PatchAntennas with Wide Bandwidth and Dual Polarization Capabil ities, in IEE E

Antenna Propagation Soc. Symposium Dig., pp: 936-939.5. S.K .Padhi,N.C.K armakar,C. L . Law and S. Aditya, Dual Polarized ApertureCoupled Microstrip Patch Antenna with High Isolation for W I D A pplications, inIEEE A ntenna Propagation Soc. Symposium Digest 2001,

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(a) (b)Fig I (a)The geometry of unit element ofarray. ( b) The top view ofarray. Dimensions: I:Patch (50x50) mm. 11: Air gap h?=3.5mm 111: Ground plane, slot (R,=T,): (22x4) mm,slot width at edges: 8mm lot offset: 15mm(R,) and 23mm(T-), I V: bottom substrate,fccd line width: 2.27 mm nter element spacing :dx=d,=0.72ho.

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Fig. 2Mcasurcd S-parametcr of 2x2 array. Fig. 3. Measured eo-polar and cross-polarradiation pattern in H- and E-plant,: (a) for Tx polarization aand (b) for Rxpolarization. Fig4: Calculatcd gain of la ) 2x2 array and (b)unit antenna element.

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