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PublishedinIETMicrowaves,Antennas&PropagationReceivedon30thApril2009Revisedon9thDecember2009doi:10.1049/iet-map.2009.0232InSpecialIssueonAsia-PacificMicrowaveConferenceISSN1751-8725Designofanovelultra-highfrequencyradio-frequencyidentificationreaderantennafornear-fieldcommunicationsusingoppositelydirectedcurrentsC.Cho1J.Ryoo2I.Park3H.Choo41InstituteofAdvancedTechnologies,SamsungThales,Yongin,Korea2CommunicationResearchLab.,CentralR&DCenter,LSIndustrialSystems,Kyunggi,Korea3DepartmentofElectricalandComputerEngineering,AjouUniversity,Suwon,Korea4SchoolofElectronicandElectricalEngineering,HongikUniversity,Seoul,KoreaE-mail:[e-mail]:Theauthorsproposeanovelultra-highfrequency(UHF)near-fieldreaderantennathatproducesastronganduniformmagneticnearfieldoverabroadregiontodetectradio-frequencyidentification(RFID)tagsattachedtovarioustargetobjectsconsistently.Theconceptoftwocloselyspacedoppositelydirectedcurrents(ODCs)isusedtoobtainastrongmagneticnearfield,andasymmetricalinverted-LstructurefedbyacoaxialfeedwithaparasiticpatchisemployedastheantennabodytorealisetheODCswithoutanyadditionalfeednetwork.ThedesignparameterswereoptimisedusingtheParetogeneticalgorithm(GA)inconjunctionwiththeFEKOEMsimulator.Theoptimisedantennahadanaveragemagneticnearfieldof220dBA/minthereadingregion(30×30×10cm3),whichissuitableforcommercialsmart-shelfapplications.Finally,weexaminedtheeffectonourproposedantennaofplacingobjectsinthereadingregion.Theresultsshowthatourantennaisappropriateforsmartshelveswithitem-leveltagging.1IntroductionRecently,radio-frequencyidentification(RFID)systemsforultra-highfrequency(UHF)near-fieldcommunicationshavebeenusedforitem-leveltagging,astheycandetecttagsonvarioustargetobjectsmoreconsistently[1–4].Near-fieldUHFRFIDsystemsdramaticallyincreasereadingstability,becausetheRFpowerofthereaderispredominantlydeliveredtopassivetagsusingthemagneticnearfield,whichpassesthroughdielectricmaterialswithverylowsignalattenuation[5].Inaddition,near-fieldsystemsallowconsiderablysmallertagsthanthoseusedinfar-fieldsystems,broadeningtherangeofapplicationsofRFIDtoextremelysmallitems[6,7].UnlikeRFIDsystemsforfar-fieldcommunications,innear-fieldRFIDsystemsthereaderantennahastogenerateastrongmagneticfield.Forexample,identifyingacommercialGen2micro-chiptagusinga0.5-cmradiusloopantennarequiresthecomponentofthemagneticfieldnormaltothesurfaceoftheantenna,Hz,tobeatleast220dBA/mwhichisobtainedfromseveralmeasurementsandwillbeconfirmedusingAmpere’slawinthenextchapter[8,9].InadditionHzshouldbeuniformlydistributedtoremovenullsinthereadingzoneandincreasereadingstability.Amongthevarioustypesofantenna,single-ormulti-turnloopantennasarethemostcommonlyusedinnear-fieldreaderantennas,becauseoftheirabilitytogenerateastrongmagneticfield.However,single-andmulti-turnloopantennasoftenexhibitnullsinHzwhenthecircumferenceoftheloopisgreaterthan0.5l,decreasingreadingstability.Ontheotherhand,loopswithasmallerIETMicrow.AntennasPropag.,2010,Vol.4,Iss.10,pp.1543–15481543doi:10.1049/iet-map.2009.0232&TheInstitutionofEngineeringandTechnology2010fielddecreasesrapidlywithdistancefromthemidpoint,resultinginalimitedreadingarea.Weproposeanovelnear-fieldreaderantennaforUHFRFIDsystems.Theantennaisbasedontheprinciplethattwocloselyspaced,oppositelydirectedcurrents(ODCs)generateastrongmagneticfieldoverabroadsurfacearea.Incontrasttosingle-ormulti-turnloops,theproposedantennaallowsnullsinthemagneticfieldtoberemovedeasilyandtheamplitude,lengthandlocationoftheODCstobetunedtoobtainauniformlydistributedmagneticnearfield.Theantennabodyisasymmetricalinverted-LstructurefedbyacoaxialfeedincombinationwithaparasiticpatchtoproducetheODCs.Ina30×30×10cm3readingregion,averageHzisabout220dBA/m.Testsoftheproposedantennainsmart-shelfapplicationsdemonstratedthatitisafeasiblenear-fieldreaderantenna,withcommercialapplications.InSection2,theuseofODCstoobtainastrongmagneticfieldisexplained.Section3describestheimplementationandoptimisationoftheODCideausingParetogeneticalgorithm(PGA),andSection4presentstheexperimentalverificationoftheoptimisedresults.2ODCsforUHFsmartshelvesTheobjectiveofthisstudywastodesignanear-fieldantennaforsmart-shelfapplications,basedonasurfaceareaof30×30cm2andareadingdistanceofatleast10cm(readingvolume.30×30×10cm3).Itwasassumedthatmultipletargettagswouldbeplacedonthesmartshelfsimultaneously,andthattheywouldbeattachedtotargetsthatadverselyaffectEMpropagation(forexample,ahighdielectricmaterialsuchasplasticbottleswithliquidcontents).Toimprovereadability,evenundersuchdifficultconditions,near-fieldreaderantennaspredominantlydelivertheRFpowertopassivetagsusingthemagneticnearfield,Hz(wherethez-directionisnormaltotheantennasurface).Thereforeunlikesystemsdesignedforfar-fieldcommunications,anear-fieldreaderantennacangenerateabroad,stronganduniformmagneticfield.Forexample,tooperateaGen2micro-chipwithPchip,minofabout210dBm[10–12]nearasmallloop0.5-cm-radiusantenna,Hzmustbeatleast220dBA/m.Toachievestablereadability,Hzsh