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1、Version0.7July2000Tag-it?AntennaDesignNotesThisdocumentdescribeshowantennascanbebuiltandtunedsothattheircharacteristicsmatchtherequirementsoftheTexasInstrumentshighperformancereadersandthirdpartyRFmodules.Photo1Tag-it?InlaysNOTEIngeneral,thedistanceatwhichatagcanbereadis
2、relatedtothesizeoftheantennaanditsassociatedmagneticfieldstrength–thelargertheantenna–thegreatertherange.Integratorsareadvisedthough,thatastheantennasizeincreasesotherissuesemerge:1.Thesignaltonoiseratiowillreduce.2.Shieldingmayberequiredtoremainwithinlegallimits3.Readin
3、gholesmaydevelop4.Matchingtheantennatothereaderwillgetmoredifficultandiftheinductancegetstoohigh,mayproveimpossibleTexasInstrumentsLtd20001AllRightsReservedVersion0.7July2000ReaderRequirementsTexasInstruments’Tag-it?systemoperatesintheHighFrequencybandat13.56MHzandantenn
4、asutilisethemagnetic(H)fieldtotransferpowertothebattery-lesstagduringreading,writingorlockingoperations.[Theassociatedelectrical(E)fieldisnotused].Thereaderexpectsanantennatobetunedtoacentrefrequencyof13.56MHz,havea50Wimpedanceandwhenconnectedtothereader,havea(loaded)Qfa
5、ctorof30.Foroptimumperformance,thereadermatchingshouldhaveaVSWRratiooflessthan1:1.2.Feedercoaxcablescanbeupto8minlength.NoteThetranspondersaredownlinklimited.i.e.Providingthatthepoweringsignalatthetagreachesafieldstrengthof152dBμV/m,thetagwillalwayshavemorethansufficient
6、performancetorespondbackoverthesamedistance.ToolsRequiredForantennadevelopment,thefollowingequipmentisrecommended:1.VSWRmeter2.AntennaAnalyser3.Oscilloscope(20MHz)4.ChargelevelIndicator1.VSWRMeterThemeterisusedin-linebetweenthereaderandtheantennaandindicatestheefficiency
7、ofthematchingbyshowingtheVoltageStandingWaveRatio.(ThisissometimesexpressedastheReturnLoss.SeeAppendix1)ThemetercanalsoindicatetheoutputpowerinWatts.Thereadershould:a)Haveafullscalerangeof5Wattsb)Operateat<1Wattc)Indicatepoweroutput.Photo2VSWRMeterTexasInstrumentsLtd2000
8、2AllRightsReservedVersion0.7July20002.AntennaAnalyserAnantennaanalysercandeterminethecharacteristicsofa
