The orientation issue in RFID is normally addressed by providing a circularly polarized
antenna on the interrogator and a dipole antenna on the tag. Thus, the dipole will receive
energy more favorably through virtually any possible orientation where the plane of the
dipole if rotated in such a manner that the plane in which it is rotated will be parallel to
the plane of the circularly polarized transmitting antenna. Due to the phasing and dual
feed con?¬?guration requirements, the tag antenna cannot practically be circularly polarized
and is typically some form of a dipole antenna.
The use of a circularly polarized antenna for the interrogator with the tag antenna
orientations maintained as parallel planes insures suitable tag performance in many
circumstances. However, the relative angles of the planes on Figure 1.15c vary, when the
tag plane is rotated through the angles indicated as a-variations in ef?¬?ciency will exist.
Consider the circularly polarized and dipole antennas of Figure 1.16. The pattern of the
dipole on the right allows it to be rotated about the Y axis while maintaining a favorable
orientation with the circularly polarized antenna on the left. In addition, the dipole can
be rotated about the X axis of the dipole diagram.
Hence, in summary, a circularly polarized antenna reduces the number of unfavorable
reader=tag antenna orientations allowing an increased success of reading the tag.
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