The highest power transfer efficiency (PTE) of the magnetic resonance wireless power transfer (WPT) system with two transmitters (TXs) and a single receiver located at an arbitrary position can be obtained by adjusting the amplitude ratio and the phase difference (PD) of the signals of two adjacent TXs. The coupling coefficient between two coils is calculated, and is used to establish an equivalent circuit model for a WPT system with two TXs. Using the circuit model, the PTE of the WPT system (which is a function of the amplitude ratio and PD) is extracted. The optimum conditions of these values to achieve the highest power efficiency can be analytically determined. The experimental results agree well with the simulated results based on the analysis. The result shows that the suggested optimised transmit scheme can achieve the highest PTE for a magnetic resonance WPT system with two TXs.

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.

The mechanically-enhanced urea-formaldehyde (UF) microcapsules are developed through a multi-step in situ polymerization method. Optical microscope (OM) and field emission scanning electron microscope (FE-SEM) prove that the microcapsules, 147.4 μm in diameter with a shell thickness of 600 nm, are well-formed. From ¹H-nuclear magnetic resonance (¹H-NMR) analysis, we found that dicyclopentadiene (DCPD), a self-healing agent encapsulated by the microcapsules, occupies ca. 40.3 %(v/v) of the internal volume of a single capsule. These microcapsules are mixed with EPDM (ethylene-propylene-diene-monomer) and Grubbs’ catalyst via a solution mixing method, and universal testing machine (UTM) tests show that the composites with mechanically-enhanced microcapsules has ca. 47% higher toughness than the composites with conventionally prepared UF microcapsules, which is attributed to the improved mechanical stability of the microcapsule. When the EPDM/microcapsule rubber composites are notched, Fourier-transform infrared (FT-IR) spectroscopy shows that DCPD leaks from the broken microcapsule to the damaged site and flows to fill the notched valley, and self-heals as it is cured by Grubbs’ catalyst. The self-healing efficiency depends on the capsule concentration in the EPDM matrix. However, the self-healed EPDM/microcapsule rubber composite with over 15 wt% microcapsule shows an almost full recovery of the mechanical strength and 100% healing efficiency.

This paper proposes a sidelobe suppression beamforming scheme using tapered amplitude distribution for microwave power transfer (MPT) with a two-dimensional planar array antenna (PAA). To overcome the undesirable effects of sidelobes, the magnitude of the sidelobes can be lowered by manipulating the amplitude distribution of the PAA. By analyzing the array factor of the PAA, we expressed the amplitude distribution as the product of two one-dimensional vectors. Moreover, we developed a method for obtaining these vectors from an expected sidelobe level (SLL). We used the Chebyshev taper, which has a higher directivity than other distributions given an expected SLL, for the amplitude distribution. To verify the proposed method, we designed and implemented a digital beamforming system with a 4 × 4 transmitter (Tx) array and a receiver (Rx) at 5.8 GHz. Compared with a uniform amplitude distribution, the proposed method reduced the SLL by 7.5 dB but the received RF power by only 0.6 dB. We compared the performance of the proposed MPT system with the results of previous studies.

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.