Effective Midrange Wireless Power Transfer with Compensated Radiation Loss

In conventional inductive wireless power devices, the energy is transferred via only reactive near fields, which is equivalent to nonradiative Forster energy transfer in optics. Radiation from transmitting and receiving coils is usually considered as a parasitic effect that reduces the power-transfer efficiency. As long as the distance between the two antennas is small compared to the antenna size, conventional wireless power-transfer devices offer rather high power-transfer efficiency, of the order of 80%-90%. However, for larger distances, the transfer efficiency dramatically drops, making such devices impractical. In this paper, we develop a dynamic theory of wireless power transfer between two small loop antennas, clarify the role of far-field radiation, and find a possibility to realize efficient wireless power transfer at large distances utilizing the regime of radiation suppression due to optimized mutual dynamic interactions between the transmitting and receiving antennas. The analytical results are validated by simulations and measurements, and they open a possibility to greatly expand the range of distances of compact wireless power-transfer devices. The developed theory can also be applied to coupling between antennas of different types and to energy transfer between nano-objects.