Analysis of wireless power transfer using spherical modes

To investigate the wireless power transfer, we need to analyze the antenna coupling. We derive a Z-parameter between two antennas using spherical modes and the addition theorem. We present formulas that calculate the maximum power transfer efficiency and the optimum load impedance for the antennas generating arbitrary modes. To simplify the formula, we assume that the antennas are canonical minimum scattering antennas. We find from the formula presented in this paper that the power transfer efficiency increases as the mode number and the radiation efficiency increase.