Area extension of a wireless battery charging system using multiple power repeater coil antennas

Multiple power repeater coil antennas for coverage area extension of a wireless battery charging system are presented in this paper. The low-profile repeater coil is simply constructed using a copper wire and a capacitor. The transmitter resonant coil (Tx), the repeater resonant coils (Rp) and the receiving resonant coil (Rx) are arranged in a planar configuration. Experiment results with two Rp coils are presented to demonstrate the charging range extension. The power transfer efficiency is investigated by the analysis of the scattering parameters of the system as a two-port network with a reference impedance of 50 Ohm. The operating frequency is chosen at 6.78 MHz according to the A4WP wireless battery charging standard. As the power source, a 10 Watt class-E RF generator circuit was used. The load coil which absorbs the energy via magnetic coupling from the Rx resonant coil is connected to a converter circuit which provides 5V constant DC voltage for battery charging of portable devices. The optimum impedance matching is considered for all coupling stages by optimizing the distance between the resonant coils. It is found that the charging area in the horizontal plane can be extended with the proposed multiple Rp coil antennas.