The complex permittivity of water is a crucial factor in the development of wireless underwater power transfer. Since the measurement method using the coupled transmission lines that we propose calculates permittivity from the terminal voltages, the method enables the measurement under high power RF. In this paper, the approach for calculating the complex permittivity of the medium surrounding the coupled lines from the measured terminal complex voltages is presented.
Each terminal complex voltage is calculated by a circuit simulator using the measured results. Then, odd-mode impedance is extracted from those complex voltages. Since the odd-mode impedance gives the complex permittivity in odd mode, the complex permittivity of medium is derived from the equivalent capacitance network in odd-mode excitation. The proposed approach is verified by using the measured and simulated result.