In recent years, development of technology for wirelessly transmitting electric power (electric energy) without using power supply cords and the like has become popular. Among methods for wirelessly transmitting electric power, the technique called a magnetic resonance method is gaining particular attention. The magnetic resonance method was proposed by a research group at the Massachusetts Institute of Technology in 2007. The related technology is disclosed in Patent Document 1 (JP2009-501510A), for example.
In a wireless electric power transmission system of the magnetic resonance method, a resonance frequency of a power transmission antenna is equal to a resonance frequency of a power receiving antenna; the antennas with a high Q-value (100 or more) are used. Therefore, from the power transmission antenna to the power receiving antenna, energy can be efficiently transmitted. One of major features is that the power transmission distance can be from several tens of centimeters to several meters.
Studies have been under way on how to apply the above-described magnetic resonance-type wireless electric power transmission system to a process of charging vehicle-mounted batteries, such as those of an electric vehicle (EV) or a hybrid electric vehicle (HEV). The use of such a wireless system for the above vehicles would eliminate the need to handle power supply connectors, power supply lines, and the like for the supply of electric power to the vehicles.
For example, what is disclosed in Patent Document 2 (JP2010-68657A) is one antenna that is mounted in a bottom surface portion of a moving object such as an electric vehicle, with another antenna provided on the ground to wirelessly transmit electric power to charge a battery of the electric vehicle with the transmitted electric power.
Several proposals have been made on a specific configuration of an antenna used in the magnetic resonance-type wireless electric power transmission system. For example, as for the configuration of communication coils that are provided in a power feeding circuit and a power receiving circuit in a wireless electric power transmission device that wirelessly transmits electric power from the power feeding circuit to the power receiving circuit, what is disclosed in Patent Document 3 (JP2010-73976A) is the configuration of the communication coils of the wireless electric power transmission device characterized by including: a printed board made of a material with a relative permittivity of more than 1; a primary coil that is provided on a first layer of the printed board and is formed as a conductive pattern making at least one loop; and a resonance coil that is provided on a second layer of the printed board and is formed as a spiral conductive pattern.    Patent Document 1: JP2009-501510A    Patent Document 2: JP2010-68657A    Patent Document 3: JP2010-73976A