In reality, its magnitude is between 0 and 1 depending on the circuit design.
The squared dot product of the polarization vectors j^pT ^pRj2 is called the polarization loss
factor (PLF) and re?¬‚ects the loss due to the mismatch of the polarizations of a transmitter
antenna and a receiver antenna. When readers have a circular-polarized antenna, the PLF
is 0.5 no matter what polarization the tag antenna has [3]. Finally, the transmitter and the
receiver antenna gains are determined by their orientations as de?¬?ned by the spherical
coordinates (uT,fT) and (uR,fR), respectively.
In Equation 10.2, the antenna gain is not a constant. Rather, it is a function of the
antenna??™s own orientation unless the antenna radiates power isotropically. Antennas of
different types differ in their own radiation patterns, which leads to different values of
GR(uR, fR) and GT(uT, fT) in Equation 10.2. In this section, for purposes of illustration we
will use a half-wave dipole antenna for the tag and a patch antenna for the reader. Patch
antennas are chosen for RFID readers because they are less sensitive to tag orientations.
Similarly, passive backscatter tags with half-wave dipole antennas are common in far-?¬?eld
applications and usually have longer read ranges than inductive-type tags.
De?¬?nition 3: The reader axis is de?¬?ned as the line joining the centers of the tag antenna and
the reader antenna (which are r units apart).
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