In such a scenario,
the theoretical read range of a tag with a lossless antenna may be calculated from the Friis
equation given as
Pr
Pt ?? grgt
l
4pr 2
, (4:3)
where
Pr is the available source power from the tag antenna
Pt is the transmitted power
l is the associated wavelength of the frequency used
r is the distance between the transmit and the receive antennas
gr is the gain of the transmit antenna
gt is the gain of the receiving antenna
Thus the read range of a tag may be evaluated as given in Equation 4.4 where the radiated
power of the reader and the reader antenna gain are replaced by EIRP and the available
source power required at the tag antenna is Pr(tag):
r
l
4p ?¬??¬??¬??¬??¬??¬??¬??¬??¬??¬??¬??¬??¬??¬? EIRPreadergtag
Pr(tag) s : (4:4)
Equation 4.4 is useful only if an expression can be obtained for Pr(tag). This requires careful
consideration in case of passive tag technology. Thus if the minimum amount of power
required to operate a tag is known to be PIC, and it can be assumed that the tag is receiving
that power, and the ef?¬?ciency of the recti?¬?er structure is h and km is the power transfer
factor from the antenna to the tag circuit in the presence of modulation, that is, the ratio of
the power reaching the tag circuit in the presence of modulation at the greatest mismatch to
the available source power from the tag antenna, then Equation 4.
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