Radiation Suppression Regime in Wireless Power Transfer between Two Loop Antennas

In this paper, we identify and discuss a regime of radiation suppression in wireless power transfer (WPT) between two loop antennas in free space. We show that at distances much larger than the antenna sizes there is a possibility to realize high power transfer efficiency due to partial compensation of the radiation resistances of both loops by optimizing the load impedance and the working frequency. This regime of WPT is analyzed for two relevant arrangements of these antennas: the coaxial one, when WPT is solely due to near fields, and the coplanar one, when both power transfer mechanisms are combined and the regions of mainly far-field and mainly near-field power transfer can be distinguished. We analyze and visualize the radiation suppression regime for both arrangements. Finally, the theoretical results are validated by simulations and experiments with two loop antennas.