Microwave power transfer system using retrodirective array for mid-range applications

Abstract

Recently, interest in the wireless power transfer technology has been rapidly increasing, the microwave power transfer technology is emerging for applications of the transmission distance much farther than magnetic resonance techniques only effective to the target of a very short distance. Most of previous microwave power transfer systems are studied primarily for very long distance applications such as solar power satellites. In recent years, however, as the demand for wireless charging for mobile devices and IoT sensors has rapidly increased, microwave power transfer systems for mid-range applications in indoor environments have been developed. Recent applications such as mobile devices and IoT sensors in the indoor environment require just a distance of several meters, not requiring wireless charging of a too long distance. Unfortunately, Friis transmission equation based method which has been widely used in a variety of conventional microwave systems such as communication and radar, cannot be applied to the microwave power transfer system with mid-range applications. Another conventional method, Goubau formula used in recently developed MPT systems is also not valid for mid-range targets. Moreover, using commercial EM solvers for accurate analysis consumes too much computation time and computer resources. Therefore, in this dissertation, a new novel method of analyzing microwave power transfer systems called ‘element based ray method’ is proposed. The proposed method is verified to be much simpler and faster to analyze with similar accuracy to commercial EM simulators.

On the other hand, in order to concentrate wireless power on mobile devices in indoor environment, the power transmitter is designed to have a retrodirective structure. Previous retrodirective arrays have been used to automatically generate beams on moving targets, while they can also perform near-field focused beamforming for mid-range applications. When the retrodirective transmitter radiates power, the phenomenon that radio power is concentrated in the receiver position is analyzed by using the proposed analysis method. And a new accurate mathematical expression of the microwave focal width, which is an indicator of how much power is focused is proposed. The optimal numbers of transmitting and receiving antennas are mathematically proposed using the analyzed results. In addition, the analysis of the power combining methodologies at rectenna array is carried out, and DC parallel power combining technique is confirmed to be appropriate for MPT systems.

Furthermore, the ground reflection effects are analyzed to take into account the multipath environment of the indoor. It is confirmed that microwave power transfer systems should be designed to have a certain height on the ground so that the power transfer efficiency is not seriously reduced by the ground reflection effect. To experimentally verify the proposed theories, a microwave power transfer system operating at 2.45 GHz frequency is implemented. The transmitter is designed to perform retrodirective operation, and the receiver is designed to have a beacon generator and rectenna array structure. Experimental results from the implemented MPT system show good agreement with the proposed theories.