Wireless power transfer is essential for the spread of
Electric Vehicle(EV) usage as it provides a safe and convenient
way to charge the EVs. Recently, a highly efficient mid-range
wireless power transfer technology using electromagnetic
resonance coupling, WiTricity, was proposed. Studies show that
the resonant frequencies of the two antennas change according
to the air gap in between the antennas. To achieve maximum
efficiency using this system, the resonance frequencies of the
antennas and the frequency of the system has to be matched.
However, when this technology is applied in the MHz range
(which allows small sized antennas), the usable frequency is
bounded by the Industrial, Scientific, and Medical(ISM) band.
Hence a method to fix the resonance frequency within the ISM
band is required. In this paper, the possibility of using
impedance matching (IM) networks to adjust the resonance
frequency of a pair of antennas at a certain distance to
13.56MHz is studied. We studied the electrical characteristics of
the antenna with equivalent circuits, electromagnetic analysis
and experiments. The equivalent circuits are used as reference
to calculate the parameters of the IM circuits. The simulations
and experiments shows that the IM circuits can change the
resonance frequency to 13.56MHz for different air gaps, thus
improving the power transfer efficiency.