Second, the declination of the transmission efficiency caused by a position gap of the transmitting
devices is relatively small. These advantages are featured by the characteristic constructions of the
transmitting coils. The transmission efficiency at a resonance frequency is expressed by (1). It
represents the transmission efficiency is determined by an only product of the quality factor of the
transmitting coil Q and the coupling coefficient k. The transmitting coils have a high quality factor Q
owing to the low parasitic resistance of the coils. The high quality factor allows an efficient wireless
power transfer in a middle-range transmission distance even when the coupling coefficient is small [7-
8]