Many RFID tags today are some form of a
dipole. Figure 1.3 is an illustration of an ideal dipole. The dipole can be thought of as two
collinear wires end to end with a small space between them. The two wires would be
oriented along the Y axis of Figure 1.3. Figure 1.4 illustrates an interrogator patch antenna
and the tag antenna simultaneously in the most favorable relative orientation. It is assumed
that the Y and Z plane of the tag antenna illustration is parallel with the plane of the
interrogator antenna. It is further assumed that the X axis of the tag antenna is perpendicular
to the plane of the interrogator antenna at the (0,0) origin of that plane.
Thus, in Figure 1.4, the most favorable orientation of the interrogator and tag antenna
implies that GR(uT,fT) and GT(uT,fT) are at their maximum values. That is the best you can
do for the antennas speci?¬?ed. Thus, the distance, r, is the relative positional difference
along the X axis as discussed earlier.
y x
Phi
Theta
z
FIGURE 1.3
Radiation pattern of an ideal dipole.
y
z
Theta
x
Phi
FIGURE 1.4
Interrogator radiation pattern (left), an ideal dipole (right).
Physics and Geometry of RFID 7
From the energy pattern of Figure 1.5, it can be understood that the energy along the Y
axis is essentially 0, thus backscatter from the tag has very little if any practical energy in
that orientation (Figure 1.
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