The low coupling coefficient is the main reason that results in difficulties to design a high performance mid-range wireless power transfer (WPT) system which uses magnetic resonant coupling. The advantages of improving the coupling coefficient are quantitatively discussed in this paper. It is highlighted that improving coupling coefficient is the only method to reduce the sensitivity to frequencies.

Furthermore, this paper proposes a method that improves the coupling coefficient by optimizing the structure of the coils. The optimization of a pair of spiral coils is presented. The method has much lower computation cost than finite element analysis (FEA) and its accuracy is sufficient for design procedures.