A self-tuning antenna that automatically adjusts its input impedance to
compensate for externally induced impedance variations is provided. A
variable impedance is adjusted by a control circuit to reconfigure the
input impedance of than antenna to compensate for different environmental
situations and different transponder mismatch situations. A
negative-feedback signal is employed to determine or infer impedance
mismatches and reconfigure the antenna input impedance (e.g., capacitance
and/or resistance) until a desired equilibrium of the antenna input
impedance is reached. A reference measurement (e.g., VSWR measurement) is
automatically performed by an antenna tuning circuit that adjusts the
antenna's impedance matching circuit to compensate for object
interference. The antenna's impedance matching circuit includes a
variable capacitor circuit having a plurality of individually controlled
parallel plate capacitors that can be added or removed from the variable
capacitor circuit, as necessary.