10.2.1 Powering Region versus Read Range
Regardless of whether RFID deployment is at the pallet level or at the item level, RF has
limited radio range. This is especially true for passive RFID tags, where such distance may
range from a few centimeters for near-?¬?eld inductive coupling to several meters for backscatter.
It is also worth noting that the desired read range is case dependent and it is not
necessarily always desirable for it to be as large as possible because in some cases, longer
read range can be detrimental due to detecting unintended or irrelevant tags.
It is important to realize that read ranges are based on a path loss model that
considers that the power received by an RFID tag is only related to the path that the signal
traverses. Equation 10.1 shows the free space Friis??™ transmission equation used in such
applications:
PR ?? PT
GTGRl2
(4pr)2 (10:1)
In Equation 10.1, PT is the power from the transmitting (reader) antenna while PR is
the power received by the receiving (tag) antenna. GT and GR represent, respectively, the
antenna gains of the transmitting and receiving antennas, r is the distance separating the
two antennas, and l is the wavelength (in the same units as r). To use the above formula,
both antennas have to be perfectly aligned, that is, at the most favorable orientation. The
read range obtainable at some other relative antenna orientation is dependent on the
radiation pattern, which might differ based on the speci?¬?c design used for the antenna.
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