This article proposed a multiple beamforming scheme using a multi-tone for microwave power transfer (MPT). To simultaneously transmit power to multiple targets, it is necessary to form a multibeam. It can be formed by generating multi-tone signals with frequency offset in each channel of an array antenna. For $K$ targets, $K$ -tone signals are generated, and the directions of the formed $K$ beams are changed by adjusting the weights of the signals. Therefore, we introduced a multi-tone modulation scheme based on a matrix calculation to express the modulated multi-tone signal for an $M \times N$ planar array antenna (PAA). Moreover, for comparison with conventional beamforming using a continuous-wave (CW) signal, the multi-tone signal was analyzed in the time domain according to the directions of the multibeam. For verification, a $4\times $ 4 digital beamforming (DBF) system operating at 5.8 GHz capable of generating multi-tone signals in each channel was designed. Based on the relative received power, beam patterns were measured for two, three, and four beams. When 39 dBm of RF power was radiated from a transmitting antenna located 1 m from a receiver, an average RF power of 14.7 dBm was received for four beams. We confirmed that the DBF system using the proposed scheme can concurrently transmit power for up to four receivers.

This paper proposes a 5.8 GHz highly efficient rectifier design using harmonic termination for wireless power transfer. The diode used to convert the received RF power to DC is a non-linear device, and a harmonic component is generated, which causes performance degradation. Therefore, in this paper, we designed a band stop filter for harmonic termination and proposed the N-stage harmonic terminated voltage multiplier (N-stage HTVM). The number of stages N can be designed differently to operate with high efficiency at various input powers for the proposed rectifier. In the proposed rectifier circuit, mathematical analysis of output DC voltage, power loss of the diode, and the power conversion efficiency (PCE) were evaluated through voltage/current waveform analysis of the diode. The design method of the filter for terminating harmonics is presented. Furthermore, the change of PCE according to the increase in the number of stages was analyzed using the equivalent model of the proposed circuit and verified through measurement. The maximum PCE of one-stage HTVM was 68% when 18 dBm of input power was applied. The DC output voltage was measured to 11.6 V. When the RF input power was 25 dBm and the load was 1500 $\Omega$, the maximum PCE of the two-stage HTVM was 71% and the maximum DC output voltage was measured as 15.8 V. The measured performance of three-stage HTVM had a PCE of 67% and DC output voltage of 19.8 V when the input power was 30 dBm.

In this study, we propose a digital beamforming system for microwave power transfer with a real-time beam-scanning algorithm for tracking a moving receiver. To perform scanning, each channel in the transmitter generates and converts a baseband signal for the allocated beam direction based on the pilot slots in a frame structure. An initial timer reset process and a preamble are included in the algorithm to maintain time synchronization among units of the scanning period in a wireless environment. The average received power is simulated, and the optimal frame configuration is derived by considering the proportion of an energy transmission slot in the frame. For experimental verification, we design a 4 × 4 digital beamforming system operating at 5.8 GHz and a receiver. The scanning performance is experimentally validated with various numbers of scanning beams and placements of the receiver. Moreover, when radiating 16 W of RF power, an RF power of 100 mW or more is received in all sections for the scenario of the receiver moving in a 1 m distance using 1 second period beam scanning. The proposed microwave power transfer system with a real-time beam scanning algorithm is verified to track the moving receiver in a wireless environment and perform optimal microwave power transfer.

한국전자파학회논문지 (The Journal of Korean Institute of Electromagnetic Engineering and Science)