However, the derived expressions are suitable only for electrically small antennas obeying
the strict limit given by the following equation:
hB <<
l
3
, (4:9)
where hB is the height of the antenna as indicated in Figure 4.17.
TABLE 4.5
Expressions for Evaluating Bow-Tie Antenna Circuit Model Parameters
Description Expression
Capacitance (CB) in farads KBC?«0hB
Inductance (LB) in henrys KBLm0hB
Radiation resistance (RBr) in ohms KBR (bhB)2
FIGURE 4.20
Field con?¬?guration for calculating the effective area of a bowtie
antenna.
Far-Field Tag Antenna Design Methodology 83
4.5.3 Bow-Tie Antenna Design
While it is possible to ?¬?nd a bow-tie antenna with an adequate radiation resistance to
match to an RFID chip impedance of 18.95 V (refer to Figure 4.21), it is not possible to ?¬?nd
a bow tie with a ?¬‚are angle and a height that will provide a conjugate match to the RFID
chip??™s reactance as an examination of Figure 4.22 reveals that all bow-tie antennas of
<90 mm in height appear to be capacitive. Hence any resulting bow-tie antenna, while
possibly having the correct matching real impedance, will not be inductive to form a
conjugate match to the chip impedance (Table 4.5).
The following sections will consider a different design of a bow-tie antenna. The design
illustrates the design process for an RFID tag antenna and shows how a useful antenna can
0 0.
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