We studied the frequency characteristics and the power
transfer efficiency of the antennas using equivalent circuits,
electromagnetic analysis, simulations and experiments. The
resonance frequency of the antennas changes as the air gap
changes. Since the maximum power transfer efficiency occurs
at the resonance frequency, the resonance frequency must
match the frequency of the power source. When this is
applied in the MHz range (which allows smaller size
antennas), the usable frequency range is bounded by the ISM
band. Therefore, a system which uses an impedance matching
network to match the resonant frequency of the antennas to a
power source fixed at 13.56MHz was proposed.
The tuning parameters of the impedance matching circuits
were estimated using the equivalent circuits. The effects were
analyzed with equivalent circuits, electromagnetic analysis,
simulations and experiments. The experiment and simulation
results show that the resonance frequency of the system can
be changed using impedance matching circuits. They also
show that the equivalent circuit model can be used to estimate
the frequency characteristics.