Comparison between Hybrid- and TM-polarized Bessel-Beam Launchers for Wireless Power Transfer in the Radiative Near-field at Millimeter Waves

Bessel beams can be generated at millimeter waves by means of compact devices by exciting a TM leaky mode in a resonant Fabry-Perot cavity through a simple coaxial feeder. Recently, a loop-antenna feeder has been proposed to excite a TE leaky mode. In this work, we investigate the radiative near-field coupling of two launchers of the same kind at around 30 GHz, using two different feeding systems. Interestingly, the full-wave field analysis proposed here reveals that coaxial cables allow for having a pure TM Bessel beam, whereas loop antennas excite a hybrid mode. Analytical procedures for a simple design workflow are presented assuming ideal feeding systems. Numerical validations and a rigorous evaluation of the link budget is instead proposed for a more realistic case. Finally, a rectifier is connected to the receiver Bessel-beam launcher and the estimation of the RF-to-DC efficiency is provided for several TX - RX distances. These results demonstrate the feasibility of this technology for mm-wave WPT.