Abstract：Relay coil is an effective method to improve the performance of wireless power transmission. But

the selection of relay coil position is usually based on experience. To elucidate the mechanism of the impact of relay

coils on system performance，the port impedance differences between typical two-coil and relay-type coupling

mechanisms were compared. The dynamic sweep analysis was used to summarize the influence of relative position

on mutual inductance. Experiments was conducted. The impact of relative spatial position of relay coils on system

performance was verified，as well as the correctness of theoretical and simulation analysis. The research on the

position characteristics of relay coils has some reference significance for the optimization design of relay-type

wireless power transmission.

      Key words：wireless power transmission；relay coils；mutual inductance；position