The multipole nature of the extraneous field of a magnetic field wireless power transfer system

Because efficiency is paramount, magnetic field wireless power transfer (MF-WPT) systems are tuned in a manner similar to some tuned linear transformers, such as IF transformers, in which the primary and secondary circuits are each tuned to the same resonant frequency (synchronous tuning) and then the load resistance adjusted for critical coupling. This causes the primary and secondary currents to be 90° out of phase with one another. The phase difference gives rise to a unique extraneous electromagnetic field consisting primarily of contributions from a magnetic dipole and a linear magnetic quadrupole. At great distance, the dipole moment contribution dominates. However, in the immediate vicinity of the system, the contribution from the quadrupole moment can dominate and thus there exists a certain distance from the system (dependent on direction) at which the dipole and quadrupole contributions are of the same order. We determine this distance for typical MF-WPT designs at ground level and find that the contribution of the dipole moment dominates the magnetic field at distances frequently specified in EMC standards, e.g. 3 and 10 meters. This conclusion has implications for characterization procedures which involve the the measurement of only the magnetic dipole moment.