The power transmitted by the antenna, PT, in Figure 1.1, is governed by the Federal
Communications Commission (FCC). This power is spread over a three-dimensional area
as determined by the antenna radiation pattern as shown in Figure 1.2. The frequency used,
915 MHz in EPCglobal Gen2, has a wavelength, l, which is calculated from the relationship:
FIGURE 1.2
A radiation pattern.
6 RFID Handbook: Applications, Technology, Security, and Privacy
c ?? f l, (1:1)
where c is the speed of light. Thus, l of the Friis equation is 0.3278?? m. The gain of the
transmitting antenna, GR(uT,fT), means that the energy provided to the antenna is focused
to some extent thus causing the energy density to be greater in some area than would have
been if it had been radiated in all directions equally (an isotropic antenna). The angular
orientations uT, fT make it clear that the tag orientation will receive power that will be
varied over all the angles of orientation with respect to transmitting energy (power). In the
pattern of Figure 1.2, an RFID tag placed at the orientation (0,0,0.7) would receive the most
energy from the transmitter. This location is part of the terminology of the most favorable
location which is used by most vendors as the distance at which the tag can be read.
The next term of the Friis equation considered is the gain of the tag antenna, GT(uT,fT),
where a pattern is likewise to be produced.
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