  Many applications benefit from the ability of an RFID tag to operate both on
and off a conducting ground plane. This paper presents an electrically small
loop antenna at 433 MHz that passively maintains its free-space tune and match
when located a certain distance away from a large conducting ground plane. The
design achieves this using a single radiation mechanism (that of a loop) in
both environments without the use of a ground plane or EBG/AMC structure. An
equivalent circuit model is developed that explains the dual-environment
behavior and shows that the geometry balances inductive and capacitive
parasitics introduced by the ground plane such that the free-space loop
reactance, and thus resonant frequency, does not change. A design equation for
balancing the inductive and capacitive parasitic effects is derived. Finally,
experimental data showing the design eliminates ground plane detuning in
practice is presented. The design is suitable for active, "hard" RFID tag
applications.
