An equivalent circuit of wireless power transmission is discussed. We demonstrate that an input impedance of the wireless power transmission is expressed by a simple equivalent circuit. The equivalent circuit spells out how the resonant frequency is determined. The parameters in the equivalent circuit is found by using genetic algorithm so that the input impedance calculated by method of moment (MoM) agreed with one calculated by the equivalent circuit. The equivalent circuit expression will provide analytical approach of consideration on power transmission efficiency.

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A procedure to estimate S parameters and farfield radiation power from structural parameters of coupledresonant wireless power transfer in kHz range is discussed. Helical antenna is modeled by series connection of loop antennas. All combinations of loop antennas are taken into account for calculation of self and mutual inductance. Stray capacitance between neighboring loops and ohmic loss are also regarded. Numerical demonstration shows that the proposed procedure has calculation accuracy as same as method of moment (MoM) calculation, when wire length is much smaller than the wavelength. Calculation time for the proposed procedure was 0.5 seconds whereas MoM requires 200minutes.