23 to achieve a match to the RFID chip
impedance. The tuning element, LSmatch can be physically incorporated into the antenna
design. Such a series inductor may be conceptualized as indicated in Figure 4.24. An
outline of the antenna design structure is given in Figure 4.25. The copper strip width,
lw, the size of the gap between the wings, g, and the amount of copper removed, ic, can then
be adjusted to ?¬?nd a suitable inductor value to form a match between the bow-tie
impedance and the RFID IC impedance.
Taking a bow-tie antenna 80 mm in height, the properties of the antenna can be
calculated as outlined in Table 4.7 where the center frequency of operation is taken as
915 MHz. However, adding the inductive strip will modify the antenna model developed
earlier since the added strip will affect the self-capacitance and the inductance of the
model. Instead of using an empirical method, simulation results from Ansoft HFSS (?¬?nite
element method based simulation package) can be used to understand the effect of adding
TABLE 4.6
Empirical Values for Evaluating Bow-Tie Antenna Circuit
Model Parameters
Flare Angle KBR (V) KBL KBC
5 7.5 0.2888 0.5275
10 8.5 0.2823 0.5875
30 11.0 0.2605 0.8175
40 11.5 0.2470 0.9875
50 12.0 0.2349 1.1525
60 12.5 0.2250 1.2500
90 15.0 0.2128 1.9000
Equivalent circuit of
the bow-tie antenna
Equivalent
circuit of
RFID IC
LSmatch
Lossless
matching
network
+
_
Vs
RBr
LB CB
R C
FIGURE 4.
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