As early as in the nineteenth century, American physicist and engineer Nikola Tesla had designed and experimented a long range wireless power transfer system — the Wardenclyffe Tower [1]. Unfortunately, the project was not completed because of insufficient funding. Subsequently, the possibilities of microwave power transmission were explored by William C. Brown in 1961 [2]. Since the new century, the technology of wireless power transfer has received increasing attentions from all over the world because of its intrinsic safety and convenience characteristics.