As known, the variation of distances between the adjacent coils will affect the transfer efficiency in magnetic coupled resonance system. In order to explore the influence of distance on the optimum transfer efficiency, a
four-coil magnetic coupled resonance system is established. Based on the mutual inductance coupling theory, the relationship between the transfer efficiency and coupling coefficient is obtained. Supported by the simulation analysis and experimental evidences, it is found that the coupling coefficient k₂₃ (between transmitter and receiver coils) is the most significant factor affecting the transfer efficiency. Under the optimal k₂₃, the transfer efficiency will increase with the coupling coefficient k₁₂ (between driver and transmitter coils). Besides, it will firstly increase and then decrease with the coupling coefficient k₃₄ (between receiver and load coils). Moreover, the theoretical calculation formula of k₂₃ is deduced, which can make a contribution to the optimization of parameters and improvement of transfer efficiency for the four-coil magnetic coupled resonance system.