Impedance matching is a common issue in a wireless power transfer (WPT) system. This paper summarizes impedance matching methods reported in the literature and gives a theoretical analysis on dc-dc converter for impedance matching in a WPT system. A buck-boost converter in discontinuous conduction mode has the characteristic of that the input resistance is independent of the load resistance and the input voltage, which seems to be suitable for impedance matching. Furthermore, the impedance matching ability of buck-boost converter will not depend on the frequency of rectifier, because it implements the solution to the dc output of rectifier. To verify those, we give several application examples of this buck-boost converter applied in rectifiers with different frequencies, such as 2.45, 5.8, and 24 GHz. As a result, those rectifiers with the buck-boost converter achieve a constant RF-dc conversion efficiency over an extremely wide load range. In addition, it is also successfully utilized in a microwave power transmission system for driving a dc motor.

We build a phased array system with four power-variable phase-controlled magnetrons (PCMs) by applying the injection-locking method and phase-locked-loop method. To reduce the cost and ensure the durability of the phased array, a waveguide slot array antenna was designed and used for the output antenna of power-variable PCMs. The slot antenna has an expected angle deflection of 22.5°, a gain of 24.9 dBi, and the half bandwidth of the main lobe was 10°. We demonstrated the properties of microwave beamforming and wireless power transfer based on the magnetron phased array system. In horizontal directions, a beam scanning range of ±3° was obtained by adjusting the output phase of the magnetrons. Furthermore, the received dc power reaches 142 W at a distance of 5 m when the output microwave power of the magnetron phased array is 1304 W.

To reduce the weight of a satellite, an internal wireless system (IWS) for satellites was proposed in a previous study. It is a system that can communicate between the satellite’s subsystems by using wireless communication modules. In this paper, we propose a complete IWS for a satellite using microwave wireless power transmission technology. The design of a 5.8-GHz high efficiency rectifier circuit-based charge pump circuit is also presented. Furthermore, we introduced a class-F load that can process the third harmonic components. The rectifying efficiencies of a normal charge pump rectifier and a charge pump rectifier with the class-F load are theoretically compared. To improve the conversion efficiency of the charge pump rectifier, the applied voltages of diodes are adjusted. Owing to inserting the class-F load and adjusted the applied voltage of diodes in this circuit, the conversion efficiency of the charge pump circuit at 30 mW increases to about 78% at 30 mW in the simulation, and to about 71% conversion efficiency in the experiment. In addition, the output voltage is higher than 5 V at an optimal load 1300 $\Omega $ .

This paper describes the voltage doubler (VD) behavior of a 2.45-GHz VD-type rectenna. An important feature of the developed VD-type rectenna is the absence of the charge pump capacitor that is present in conventional VDs. Based on the circuit simulation and measurement results, the developed VD-type rectenna shows drastic improvement to the radio frequency-to-direct current (RF-dc) conversion efficiency in comparison with a half-wave rectifier (HWR)-type rectenna at output loads exceeding 100 Ω. From the simulation, both the VD- and HWR-type rectennas show VD behavior, even without the charge pump capacitor. This behavior was found to be attributable to the electric charge stored in the junction and package capacitances of the Schottky barrier diode in the rectennas. Additionally, it was found that the improvement of the RF-dc conversion efficiency is caused by the voltage conservation in the series diode. This improvement is dependent on the microwave period and the electric discharge time constant. When the microwave period is comparable with the electric discharge time constant, the VD-type rectenna conserves a dc voltage even when the input microwave voltage is negative. These phenomena were observed by analyzing the voltage and current waveforms of the rectenna equivalent circuit obtained via circuit simulation.

This article proposes the application of microwave power transfer (MPT) with the drone. As a feasibility study, MPT experiments were conducted to a receiver 1.2 away from a transmitter at 2.45 GHz. We aimed to enhance the rectification efficiency of the MPT by array antenna beamformings, named flat-topped beams. We conducted power transfer experiments between the transmitting antenna and the receiving antenna, where a port-to-port efficiency of 31.4% was confirmed on the flat-topped beams. Using the measured illuminance distributions of flat-topped beams on a rectenna plane, we provided a detailed comparison of the rectification efficiencies between conventional uniform beams and the designed flat-topped beams. The obtained rectification efficiency of the series/parallel rectenna array was 68.5%/65.4% in the flat-topped beams and 60.8%/61.1% in the uniform beams. The results of the experiments revealed that the flat-topped beams almost exhibited no connection loss of the rectenna array. Based on the above results, there is a possibility to achieve a high overall efficiency of approximately 60% with the flat-topped beam MPT.