Therefore, before postulating a theoretical ??????best??™??™ antenna, it is important to
consider the load to which an RFID label antenna must provide power. The resulting load
impedance presented to a tag antenna is considered later.
A UHF RFID label IC with an antenna terminal and a rectifying circuit that is intended
to produce a recti?¬?ed voltage used for powering the label circuits can be modeled as
FIGURE 4.6
An electromagnetic antenna.
Far-Field Tag Antenna Design Methodology 69
indicated in Figure 4.7. Here, Xl represents the reactance of the diode capacitance, XB is the
reactance of the reservoir capacitor that also serves as an RF bypass, and Rl represents
the loss in bringing reactive power into and out of the diode junction capacitance. It is clear
from Figure 4.7 that the input impedance of an RFID chip is largely dictated by the junction
capacitance of the recti?¬?cation diode. The recti?¬?ers on modern UHF RFID ICs are fabricated
using Schottky diodes with a junction capacitance value in the range of a few
picofarads or less. Due to the sensitivity of the junction capacitance to the biasing voltage,
the input impedance of an RFID chip is a complex function of both the operating frequency
and the input power to the chip from the antenna. Thus in general, the chip impedance, Zc
is measured at the threshold of operation so that the antenna impedance is a conjugate
match at the lowest power level at which the chip will operate successfully.
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