A DUAL BAND MICROSTRIP PATCH ANTENNA WITH CIRCULAR POLARIZATION NAZIA HASAN Research Scholar, Electronics and Communication Department Uttarakhand Technical University, Dehradun Dehradun, India nazia_hasan6@yahoo.co.in S.C.GUPTA Electronics and Communication Department DIT University, Dehradun Dehradun, India sureshprem1938@gmail.com Abstract Thispaperproposesdesignofamicrostrippatchantennahavingcircularpolarization.Toachievecircular polarizationtwo slots arecreated at 45o and135o in Xshape, at centreof the patch. This antenna hasdual band characteristicsat3.6GHzand3.5GHz,thepolarizationat3.6GHziscircularwhileat3.5GHzislinear.A comparisonbetweenMPAhavingsingle-slottiltedat45degreeandX-slotisdone.Theantennawithsingle-slot gives 170MHz bandwidth, while X-slot antenna provides 200MHz. Keywords-Circular Polarization, microstrip patch, X-shape slot 1.IntroductionMicro-stripantennahavealwaysbeenanattractive choicefortheresearchers,becauseofitsnumerous advantagessuchaslightweight, lowvolume,lowcost, conformalconfigurationcompatibilitywithintegrated circuitsandsoon.Usuallymicro-stripantennasare designedwithlinearpolarization,butinsome applicationssuchassatellitecommunication,circular polarizationisdesiredbecauseofitsinsensitivityto transmitter or receiver orientation. Amicro-strippatchisoneofthemostwidelyused radiatorsforcircularpolarizationgeneration.Circular polarizedoperationandpolarizationdiversityare becomingmajordesignconsiderationsforpractical applications of micro-strip antennas. Circular polarization can be achieved by using circular, pentagonal(Haneishi1989),equilateraltriangle(Tsang 1998),ring(Suzuki1988)orellipticalpatches(Shen, Long1981)truncatingthecornersofasquarepatch (Choi2002),feedinganearsquarepatchdiagonally fromanyvertexwithmicrostripfeed,feedingapatch withcoaxialcableatapointondiagonalofthesquare patch(ShaLiu2008),usingdualorthogonalcoaxial probefeed(Sharif2005),usingdualmicrostripfeed along with a 90o phase shifter (Shakeeb 2010), etching a slot tilted at 45 degrees [2], or by using slots of different geometries and other methods. In this paper, design and simulation of a micro-strip patchantennadesignedat3.6GHzwithlinear polarization is presented. Two slots crossing eachother inshapeofXarecreatedatcentreofthepatch antenna.Asaresult,thelinearpolarizationofthe antennachangesandcircularpolarizationisachieved. Cross slot has been used earlier in shape of Plus sign [3] andwithcoaxialprobe,butinthispaperX-shapeis usedandmicro-striplinefeedalongwithaQuarter Wave Transformer is used. Thisantennahasdualbandcharacteristicsand showstwotypesofpolarization,at3.5GHzthe polarizationislinearwhileat3.6GHziscircular.This Conference on Advances in Communication and Control Systems 2013 (CAC2S 2013) 2013. The authors - Published by Atlantis Press 314Nazia Hasan, Dr. S.C. Gupta antenna will be referenced as X-Slot Antenna or XSA in this paper for convenience. 2.Antenna Design Table 1 below, shows various dimensions of micro-strip patchantenna,list&propertiesofmaterialsusedand resulting polarization. Table1. Various dimensions of Patch antenna S.no.Parameter Value 1.Frequency3.6 GHz 2.Dielectric constant of substrate Duroid(tm) 2.2 2.Width of patch22.58 mm 3.Length of patch22.58 mm 4.Widthofquarter wave transformer 0.582 mm 5.Length of QWT20.8325 mm 6.Widthof50ohm transmission line 4.84 mm 7.Length of 50 O TL15 mm 8.Slot length8.6mm 9.Slot width0.86mm 10..PolarizationLinear Fig.1(a) Microstrip Patch Antenna at 3.6 GHz Thepatchantennawasdesignedonthebasis oftransmissionlinemodel(TLM).Thewidthofthe patch is calculated first, given by the formula,

(1) wherecristhesubstratedielectricconstant,Wisthe width of the patch and h is the height of the substrate. The patch we have used in our model is square patch, so thelengthandthewidtharethesame.Thedimensions of the patch are extended to account the fringing effect; the extension is given by,

(2) Since the length has been extended by A on each side of the patch, the effective length is given by, (3) Patch resonant length L is given by,L = Leff 2A (4) UsingtheformulasgivenbyTLM approximation,parametersfortheantennawere calculatedat3.6GHz frequency.The material used as substrateisDuroid(cr=2.2)withheighth=1.57 mm. AQuarterwavetransformer(QWT)isusedtomatch theimpedanceofpatchantennawiththatof50Ohm transmission line. QWT is a useful and practical circuit forimpedancematchingandalsoprovidesasimple transmissionlinecircuitthatfurtherillustratesthe properties of standing waves on a mismatched line. Therefore, when it is desired to match a load resistance RLwithafeedlineofcharacteristicimpedanceZ0,a pieceoflosslesstransmissionlineofcharacteristic impedance Z1 and /4 length is used to connect them,to make the reflection coefficient I=0 looking into the /4 matchingsection.ThematchingimpedanceZ1isgiven by, (5)

Fig. 1(b) Quarter wave matching transformer The length of the transformer is /4 or 20.8325 mm and the width of the conductor is given by the formula (6) for Z0(cr)1/2 >89.91, that is A > 1.52 and for Z0 (cr)1/2