Study of Characteristic Impedance of Near-Field Antenna in Coupled State as Transmission Line

There are two ways of wireless power transfer by near-field resonance. One is CWPT (Capacitive Wireless Power Transfer) using electric field coupling between electrodes, and the other is IWPT (Inductive Wireless Power Transfer) using magnetic field coupling between coils. In both cases, the impedance of the antenna changes significantly depending on the distance between the antennas. As a result, it is known that the peak of the transmission efficiency splits into two peaks when the distance is very close and tightly coupled, and the transmission efficiency at the resonant frequency decreases. In this paper, a method to replace the coupling state between the antennas with an equivalent transmission line model is introduced, and explain the transmission efficiency by the change of characteristic impedance between the antennas. The maximum theoretical distance-dependent transmission efficiency is obtained by this model.