A-Broadband-Antenna-for-Multi-Standard-UHF-RFID-Ta

ABroadbandAntennaforMulti-StandardUHFRFIDTagApplicationsRui-HuaZENG,Qing-XinCHUSchoolofElectronicandInformationEngineering,SouthChinaUniversityofTechnology,Guangzhou,Guangdong510640,ChinaE-mail:[e-mail]-AnovelbroadbandUHFantennaispresentedformulti-standardRFIDtagapplications.Withtwomodifiedmeandereddipolesandaninductivelycoupledfeedingstructure,theproposedantennacanobtainaverywidefrequencyrange(from816MHzto988MHz)tomaintaingoodconjugateimpedancematchingtomulti-standardRFIDtagchips.Aprototypehasbeenfabricatedandtested.Thesimulatedandmeasuredresultsareingoodagreement.IndexTerms-Broadbandantenna,tagantenna,impedancematching,RFidentification(RFID)I.INTRODUCTIONRadiofrequencyidentification(RFID)isarapidlydevelopingtechnologyforautomaticidentificationofobjects.Ithasattractedgreatinterestinmanyapplicationssuchasaccesscontrol,distributionlogistics,item-leveltrackingandinventorymanagementoverthelastfewyears[1].AnRFIDsystemisbasicallycomposedoftwoparts:areaderandatransponderknownastag.AtypicalpassiveRFIDtagconsistsofanantennaandanapplicationspecificintegratedcircuit(ASIC)chip.LicensedfrequencyspectraforUHFRFIDapplicationsvariesfromonecountrytoanother,suchas866-869MHzinEurope,902-928MHzinNorthandSouthAmerica,840-845MHzand920-925MHzinChina,and950-956MHzinJapan,sothattheUHFRFIDfrequencyrangesfrom840MHzto956MHz(afractionalbandwidthof12.8%).Therefore,auniversaltagantennawithdesiredperformanceacrossthesemulti-standardUHFRFIDbandwouldbebeneficialforRFIDsystemconfigurationandimplementation,aswellascostreduction.TherehavebeenmanypublicationsonantennasforUHFRFIDtags,butonlyfewworkshaveconcentratedonbroadbandantennasforthesemulti-standardapplications.Reference[2]presentedaquasi-isotropictagantennawithamodifieddoubleT-matchingnetwork.Itprovidedaconjugate-matchingbandwidthfrom848MHzto926MHz,whichisnotsufficientfortheglobalusage.Acircularloopantennawithanomni-directionalradiationpatternwasanalyzedin[3].ItsbandwidthapproximatelycoveredtheUHFRFIDbands.However,usingthecircularloopmeanderlinesinthestructureoftheantennamayincreasethefabricationcost,andincreasethestructurecomplexity.In[4],abroadbandRFIDtagantennawithcapacitivelycoupledstructurewasproposed.Thearchitectureoftheantennaisquitesimple,butthebandwidthisnarrow,whichisonly65MHz.Thesameproblemexistsinpatch-likeantennapresentedin[5].Therearetwolayersintheantennaconfigurationanditneedsshortingplatesbetweenthetwolayerstoformaconnectiontoground,whichincreasesfabricationcosts.Inadditiontheantennaistoolarge(106mm*44mm)andtoothick(4.6mm)forsomeapplication.ThispaperpresentsasimplebroadbandUHFRFIDtagantenna,whichisfedthroughaninductivelycoupledstructure.TheimpedanceoftheproposedantennacaneasilyachievegoodconjugateimpedancematchingwiththeASICchipimpedanceovermulti-standardfrequencybandwidths.Inaddition,theproposedstructuremakesitsuitabletobefabricatedusingtheusualprocessinRFIDtagmanufacturingsinceitcanbefabricatedasaseparateinlay,whichcanbemountedafterwardonthedielectricsubstrate.Therefore,manufacturingprocesssimplifyingandcostreductioncanberealized.Aprototypehasbeenfabricatedandtested.Andthesimulatedandmeasuredresultsshowgoodagreement.II.ANTENNADESIGNBecauseofcostandfabricationrequirements,tagantennamustbedirectlymatchedtothechiptomaximizetagperformance.Thechiphascomplexinputimpedancedifferentfrom50¡.Largecaptanceandsmallresistanceareusuallyobserved[6].SuchchipimpedancewithhighQmakesitdifficulttodesignamatchedantennaandnarrowstheimpedancebandwidthoftheantenna.Theantennamustalsohaveabroadbandwidthtocompensateforthefrequencyshiftduetothepresenceofunknowndielectricmaterialsnearbythetag.Asforthedesignofabroadbandantenna,thechallengeboilsdowntoimpedancematchingbetweenthechipandantennaoverawidefrequencyrange.TheproposedantennastructureisshowninFig.1.Theantennaiscomposedofasmallrectangularfeedingloopandadipole-likeradiatingbody,whichareinductivelycoupled.Theradiatingbodyconsistsoftwomodifiedmeandereddipoleantennas.Thelengthsofthetwodipolesaredifferentfromeachotherinordertoformtwoslightlydifferentresonantfrequencies.Thus,thebandwidthoftheantennacanbeincreased.Furthermore,thestep-shapeoftheradiatingbody,whichisusuallyusedinUWBpatchantennas[7],keepstheimpedanceofantennaverysteadyinUHFband.Itcannotonlyincreasethebandwidthoftheproposedantenna,butalsomakethestructureoftheantennacompact.Besides,oneofthe1898978-1-4244-5708-3/10/$26.00©2010IEEEICMMT2010ProceedingsFig.1.GeometryoftheproposedbroadbandtagantennaFig.2.AntennaimpedancecharacteristicswhenDisvariedFig.3.AntennaimpedancecharacteristicswhenLfisvarieddipolearmsisbentsoastoreducetheoveralllengthoftheantenna.Thetwoterminalsofthefeedinglooparedirectlyconnectedtothechipwithoutanyexternalmatchingnetwork.Thestrengthofthecouplingismainlycontrolledbythedistance(D)betweenthefeedingloopandtheradiatingbody,aswellastheshapeoftheloop.Theactualequivalentlengthofthetwomodifiedmeandereddipoleisabouthalfawavelength.Asimpledesignprocedurecouldthereforeinitiallyselecttheloopsizewiththepurposeofcancelingthecapacitivereactanceofthechip.Thenap