These gains are dependent on the simple relative physical orientations of the
two antennas for energy supply and energy harvesting. The polar coordinates of the two
antennas are shown as functions of u and f for both tag and reader. The maximum energy
transfer normally occurs when the center lines of the planes associated with each antenna
are coincident. As the angles change from this most favorable orientation, the respective
gains will begin to decrease. Thus, in order to evaluate the amount of power that is really
available at the tag, it is necessary to know the relative physical (structural) orientations of
the reader and the tag antennas.
Reflection coefficients
Antenna gains
PR = PT
G T(q T,f T) GR(q R,f R)l
2
(4??r )2
Polarization
(1??’????“T???2)(1??’????“R???2)???pT ??? pR???2 ??§ ??§
FIGURE 1.1
The Friis equation showing key elements.
Physics and Geometry of RFID 5
In addition to the antenna gains which deal with the physical (structural) antenna
orientations, there is a question of the polarization of the two antennas again as indicated
in Figure 1.1. Polarization is concerned with the relative orientations involving the electric
?¬?elds generated by the conducting elements of the interrogator and tag antennas. The
polarization is a somewhat complicated facet and will be considered in a separate section
to follow.
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