The term from the Friis equation that most people are familiar with is the distance, r,
between the interrogator and the tag. This is important both for the powering of the tag
and the ability of the receiver in the interrogator to see the changes in radar cross section of
the tag antenna. In the case of powering, the PR must be suf?¬?cient to operate the tag
where the interrogator is supplying PT. For the interrogator to read the backscatter from
the tag, the change in the tag re?¬‚ection coef?¬?cient, GT, alters the power received by the
interrogator, PR, which is used to decode the data/number from the tag.
There are differences between tags, which operate close to the interrogator based on the
wavelength of the frequency being used. The closer is termed the near ?¬?eld and the more
distant operation is the far ?¬?eld which is the object of this presentation. This discussion is
concerned with the far ?¬?eld. In particular, in the case of far-?¬?eld ultrahigh frequency
(UHF), there is always an effort to reduce the power required by the chip and thus the
tag. Each time the matching impedance within the tag is changed to provide the modulation
of the backscattered RF, the re?¬‚ection coef?¬?cient is changed. Thus, power harvested by
the tag is changed (reduced) by this change in re?¬‚ection coef?¬?cient indicated in Figure 1.1.
The re?¬‚ection coef?¬?cient is again somewhat more complicated and will be discussed in a
later section dedicated to that subject.
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