While an antenna impedance may be adjustable by design variations (as
illustrated in Section 4.5), the input impedance of an RFID IC cannot be altered without
using external circuit components. This is an undesirable result. The following section
investigates the practicability of matching to an RFID IC??™s input impedance at UHF
frequencies allocated for RFID around the world.
4.3.3 Matching to an RFID Chip Impedance
Before embarking on the design of tag antennas, it is useful to consider the impedance to
which a tag antenna must be matched. A useful tool for evaluating the practicability of
FIGURE 4.11
An equivalent circuit for a small magnetic ?¬?eld-sensitive antenna.
+
_
Vs
Rr
L
Rl
FIGURE 4.12
An equivalent circuit for a small electric ?¬?eld-sensitive antenna.
C
+
_
Vs
Rr Rl
72 RFID Handbook: Applications, Technology, Security, and Privacy
such a match can be derived from the Bode and Fano theorem (Fano, 1950). The theorem
can be used to investigate the existence of any theoretical limitations to matching to an
RFID IC??™s chip impedance (Ranasinghe et al., 2006). Figure 4.13 shows a circuit with a real
source impedance, a lossless matching network, and an input impedance of an RFID IC
represented by a parallel RC load. According to Bode and Fano, the fundamental limitation
on impedance matching takes the form given in the following equation (Fano, 1950):
?°1
0
ln
1
G j j
dv
p
RC
: (4:1)
In Equation 4.
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