We call this ratio kb. Thus,
Pt(tag) ?? kbPr(tag): (4:8)
The value of kb depends on how good we are in designing the modulator, and exactly how
we de?¬?ne the effective modulated power, and that depends in turn on the form of
modulation employed. In an inef?¬?cient design (in which most of the available source
power is going to power the tag, so not much is backscattered), its value could be small.
It could also be small if most of the power is backscattered but not in a way that is time
varying or is a good expression of the type of modulation desired. Alternatively its value
could be up to about 1, in the case where not much of the available source power goes into
powering the tag and most is backscattered, and we are successfully using, for example,
binary phase shift keying modulation.
In practice, the read ranges always need to be veri?¬?ed using practical measurements as
practical RF propagation losses have not been taken into account in the Friis equation given
in Equation 4.3 and it is not easy to model the propagation loss in various environments
without extensive experimental data.
One simple method for evaluating tag performance in terms of tag read range in an ideal
propagation context is to use an anechoic chamber. The reader antenna can be placed at
one end of the anechoic chamber, whereas the tag is placed along the axis of maximum
radiation from the reader antenna.
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