Retro-reflective Beamforming Microwave Power Transmission System Based On Heterodyne Architecture

High efficient point-to-point microwave power transmission(MPT)can be achieved by beamforming using directional antennas or arrays with large aperture.In practical applications,however,it is desirable that the power beam is able to autonomously steer its direction when the power receiver changes in position.The beam steering technique based on retro-reflective phased arrays has been considered to be an effective approach of tracking and delivering power to mobile devices.The approach is based on the principle of phase conjugation,i.e.,the phased array is fed with the conjugate phase of the pilot signal broadcasted by the power receiver,so that the power beam can automatically focus on the location of the power receiver.In order for this approach to operate effectively,it is critical to ensure that(1)precise phase-conjugation of the pilot signal can be obtained efficiently,feeding phases of the array can be accurately controlled accordingly,and(2)enough isolation is provided between the pilot signal and power carrier so that the phase information of the pilot signal is not interfered by the high-power microwave beam.To address these demands,this work proposes a retro-reflective MPT system and component design based on heterodyne circuit architecture.The specific work is as follows.(1)Design of the retro-reflective MPT system based on frequency division,and the phase conjugation scheme based on frequency conversion via hardware mixing.The frequency of the power carrier is selected be twice of that of the pilot signal to provide high isolation,based on which the heterodyne receiving and phase conjugation circuit system for the pilot signal is designed.Received phases of the pilot signal,feeding phases of power beam as well as the optical distances of different channels are measured and compared,which verifies the feasibility of the proposed retro-reflective scheme.(2)Design and implementation of the circuit components of the proposed beamforming system,including:(i)a 4-element microwave power transmission array,each containing an 8 by 8 fixed feeding subarray at 5.8GHz and a pilot signal receiving patch antenna at 2.9GHz;(ii)pilot signal receiving module,containing isolating,filtering and amplification circuits;(iii)conjugate phase generation circuits based on double-balanced mixer,and the corresponding phase conjugation testing system;(iv)power generation and feeding module,containing frequency doubling and amplification of the conjugate-phase pilot signal.(3)Experimental demonstration of the proposed retro-reflective MPT scheme and measurements of the beam focusing performance.An experimental system of MPT to mobile receiver at a distance of 4 m is constructed to demonstrate the performance of the proposed retro-reflective beam focusing system.Measurement data are provided to verify the effectiveness of the proposed scheme in tracking and delivering power to mobile receivers.