Inductive power transfer systems considered as electrically-small antennas

Inductive wireless power transfer (IPT) systems differ from other magnetic near-field systems such as RFID systems. Both IPT and RFID systems behave as loosely-coupled transformers. The RFID system behaves as a simple magnetic dipole because the primary fields are dominant. In an IPT system the primary and secondary Ampere-turns are similar in magnitude. When an IPT system is tuned for maximum efficiency, the primary and secondary currents are approximately 90° out of phase. An electromagnetic model for an IPT system tuned for maximum efficiency then consists of two closely-spaced magnetic dipoles which are in phase quadrature resulting in an admixture of dipole and quadrupole moments. The IPT system can be thought of as an electrically-small antenna. We characterize an IPT system as an antenna including the near and far fields. Although the electrical size of such an antenna is very small, the currents in the antenna are extremely large due to large vehicular power requirements. From the calculation presented here, it would appear that the radiated power from a typical IPT system is quite small, although the near magnetic field is intense and extends over appreciable distance. We note that rectification on the secondary side introduces harmonic content into the secondary current and the strong frequency dependence of the radiated power favors the radiation of these harmonics.