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The E ?¬?eld with a circularly polarized antenna.
10 RFID Handbook: Applications, Technology, Security, and Privacy
the scope of this chapter. However, in general, the means by which this can be accomplished
is to use a ?¬‚at planar antenna, typically circular, with two feed attachments where
the two attachments are electrically 908 out of phase. The feed points can be coupled to the
antenna surface through the air with appropriate spatial distance and orientation.
The reason why the two antenna polarizations are important in RFID is because of the
various possible orientations of the item that is tagged relative to the interrogator antenna.
For example, consider the view from antenna A to antenna B under the condition where
antenna B has been rotated by 908 in space as shown in Figure 1.13.
From Figure 1.13, it is obvious that very little of the E ?¬?eld vector as pictured in Figures
1.8 (E ?¬?eld) and 1.11 (E vector) from antenna A will impinge on antenna B. Thus, the
amount of energy delivered from antenna A to antenna B will be minimal. In general, this
means the RFID tag will likely not have received suf?¬?cient energy from the continuous
wave (CW) of the interrogator antenna to power the RFID chip on the tag (Figure 1.14).
In the traditional use of radio, the station transmitting antennas are typically vertical,
and most radio receivers are going to be placed on a ?¬‚at surface parallel to the earth in
which case the manufacturer will include the antenna with the proper polarization with
respect to vertically polarized transmitting antennas, which are mounted vertically to be in
the same polarization as the transmitting antennas.
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