A surface resonator is used to improve the efficiency and working-frequency stability of three- and four-coil wireless power transmission (WPT) systems in both simulations and experiments. A surface resonator comprises chip capacitors and a structured printed circuit board. Compared to the simple three-coil WPT system, the transmission coefficient is increased by ∼10% for mid-range transmission in a three-coil WPT system with a surface resonator, and the quality factor is enhanced. For a four-coil WPT system with a surface resonator, the quality factor is improved without drastic reduction in transmittance. Moreover, by introducing a surface resonator, working-frequency splitting is suppressed strongly for short-range transmission in three- and four-coil WPT systems. The present findings will help in designing WPT systems for practical applications.
