13.4.1 RFID Tag with a Simple Rectangular Patch Antenna
Since patch antennas require a ground plane to operate, they will less likely be affected
when attached to metallic surfaces.* As will be shown in the following, we have experimented
with an RFID tag consisting of a simple basic rectangular patch antenna that can be
used for metallic object identi?¬?cation.
Shown in Figure 13.2 is the top view of the RFID tag. The tag is designed to operate in
the Australian UHF RFID band that spans 920??“926 MHz. A double-sided copper clad FR4
board material with substrate thickness of h??1.6 mm and relative dielectric permittivity
?«r??4.4 is used. The dimensions of the rectangular patch are determined ?¬?rst by theoretical
calculations using the methods in Balanis (2005) and later veri?¬?ed (with some slight
adjustments) by simulations using the Ansoft HFSS simulation program. The patch has a
length Lpatch??76 mm and width Wpatch??99 mm. Both the substrate and ground plane
have the same size and are 6h longer on each side compared with the rectangular patch.
The tag design also consists of a simple impedance matching method, whereby the
antenna is fed using an inset feed method and the impedance is transformed using a
microstrip line to have an overall antenna impedance that is the complex conjugate of the
tag chip impedance. The chip used has an impedance of 20j141 V at 923 MHz.
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