RFID线圈设计

InaRadioFrequencyIdentification(RFID)application,

anantennacoilisneededfortwomainreasons:

•TotransmittheRFcarriersignaltopowerupthe

tag

•Toreceivedatasignalsfromthetag

AnRFsignalcanberadiatedeffectivelyifthelinear

dimensionoftheantennaiscomparablewiththe

wavelengthoftheoperatingfrequency.InanRFID

applicationutilizingtheVLF(100kHz–500kHz)band,

thewavelengthoftheoperatingfrequencyisafew

kilometers(λ=2.4Kmfor125kHzsignal).Becauseof

itslongwavelength,atrueantennacanneverbe

formedinalimitedspaceofthedevice.Alternatively,a

smallloopantennacoilthatisresonatingatthe

frequencyoftheinterest(i.e.,125kHz)isused.This

typeofantennautilizesnearfieldmagneticinduction

couplingbetweentransmittingandreceivingantenna

coils.

Thefieldproducedbythesmalldipoleloopantennais

notapropagatingwave,butratheranattenuating

wave.Thefieldstrengthfallsoffwithr

-3

(wherer=dis-

tancefromtheantenna).Thisnearfieldbehavior(r

-3

)

isamainlimitingfactorofthereadrangeinRFID

applications.

Whenthetime-varyingmagneticfieldispassing

throughacoil(antenna),itinducesavoltageacrossthe

coilterminal.Thisvoltageisutilizedtoactivatethe

passivetagdevice.Theantennacoilmustbedesigned

tomaximizethisinducedvoltage.

Thisapplicationnoteiswrittenasareferenceguidefor

antennacoildesignersandapplicationengineersinthe

RFIDindustry.Itreviewsbasicelectromagnetics

theoriestounderstandtheantennacoils,aprocedure

forcoildesign,calculationandmeasurementof

inductance,anantenna-tuningmethod,andthe

relationshipbetweenreadrangevs.sizeofantenna

coil.MAN678RFIDCoilDesignAuthor:YoubokLeeREVIEWOFABASICTHEORYFORMicrochipTechnologyInc.ANTENNACOILDESIGNCurrentandMagneticFieldsINTRODUCTIONAmpere’slawstatesthatcurrentflowingonaconductorproducesamagneticfieldaroundtheconductor.InaRadioFrequencyIdentification(RFID)application,Figure1showsthemagneticfieldproducedbyaana