However, when RFID tags are placed
on an item, it is most dif?¬?cult to ?¬?x the orientation of the item throughout the item life cycle
at all possible locations where the tag may be read. Thus, the relative polarizations will be
at the best as in Figure 1.9, and at the worst as in Figure 1.13, with any other orientations in
between these two extremes. Thus, except in very special circumstances, one does not want
both the RF interrogator transmitter and the RFID tag to be dipole antennas due to the
possibility of the orthogonal or other unfavorable orientations as illustrated in Figure 1.13,
when transmitter or receiver may not be permanently aligned.
While radio transmitting antennas are typically mounted vertically for vertical polarization,
television antennas are horizontally polarized. Before the days of cable and satellite
TV, rooftops were adorned with antennas formed with multiple horizontal elements
resulting in what is termed a Yagi antenna. The plane formed by the parallel elements
A A
(a) (b)
B
B
FIGURE 1.13
Two orthogonal dipoles.
a
A
E Field
Time
B
b
FIGURE 1.14
Linearly polarized antennas with most favorable orientation.
Physics and Geometry of RFID 11
was (is) parallel to the earth, that is, horizontally polarized. Thus although radio and
television do not share the same frequency bands, there is still little or no interference
due to the respective polarizations.
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