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
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