Recently, a highly efficient midrange wireless transfer
technology using electromagnetic resonance coupling has been
proposed and has received much attention due to its practical
range and efficiency. The resonance frequency of the resonators
changes as the gap between the resonators changes. However,
when this technology is applied in the megahertz range, the usable
frequency is bounded by the industrial, scientific, and medical
(ISM) band. Therefore, to achieve maximum power transmission
efficiency, the resonance frequency has to be fixed within the
ISM band. In this paper, an automated impedance matching (IM)
system is proposed to increase the efficiency by matching the
resonance frequency of the resonator pair to that of the power
source. The simulations and experiments verify that the IM circuits
can change the resonance frequency to 13.56 MHz (in the
ISM band) for different air gaps, improving the power transfer
efficiency. Experiments also verified that automated IM can be
easily achieved just by observing and minimizing the reflected
wave at the transmitting side of the system.