Efficient, Compact, Wireless Battery Design

Wireless batteries or rectennas - rectifying antennas - are conceived for converting wireless RF power into DC power. Although power conversion efficiencies exceeding 80% have been reported for high (20 dBm) rectenna input power levels, wireless batteries will be most beneficial at large distances from sources that will radiate at power levels limited by national and international regulations. Therefore, the challenge is in maximizing the power conversion efficiency of wireless batteries for low input power levels, say 0dBm and below. By directly conjugate matching a rectifying circuit to a microstrip patch antenna, the need for a matching network between the two no longer exists. Thus the efficiency of the wireless battery will improve. Moreover, this matching technique automatically suppresses the reradiation of harmonics by the microstrip patch antenna since the harmonics will be mismatched. Thus, the impedance matching and filtering network encountered in traditional wireless battery designs has become obsolete. With the aid of analytical models developed for antenna and rectifier, single-layer, internally matched and filtered PCB rectennas have been designed for low input power levels. An efficiency of 52% for OdBm input power has been realized at 2.45 GHz for a wireless battery realized on FR4, showing an improvement -next to the size and complexity reduction -of more than 10% over a traditional rectenna design. A series connection of these wireless batteries is shown to be able to power a standard household wall clock.