This paper presents the design of a capacitive coupler for underwater wireless power transfer (U-WPT) focusing on kQ product. Power transfer efficiency hinges on the coupling coefficient k between the couplers and Q-factor of water calculated from the complex permittivity. High efficiency can be achieved by handling k and the Q-factor effectively. First, the pivotal elements on k are derived from the equivalent circuit of the coupler. Next, the frequency characteristic of the Q-factor in tap water is calculated from the measured results. Then, the design parameters in which kQ product has the maximal values are determined. Finally, it is demonstrated that the efficiency of U-WPT with the capacitive coupling designed by our method achieves approximately 80%.

This paper formulates a class-E synchronous RF rectifier from a new viewpoint. The key point is to introduce a matrix and convolute the DC terms into RF matrices. The explicit expression of input impedance is demonstrated in plane geometry. We find out their input impedance exhibits a geodesic arc in hyperbolic geometry under ZVS operation, where the theoretical RF-DC conversion efficiency results in 100%. We verify the developed theory both numerically (circuit simulation) and experimentally (6.78MHz, 100W). We confirm that the input impedance becomes a geodesic arc for a wide range of DC load resistance. The presented theory is quite elegant since it is based on a matrix-based formulation and plane-geometrical expression.

The misalignment of a coupler is a significant issue for capacitive wireless power transfer (WPT). This paper presents a capacitive WPT system specifically designed for underwater drones operating in flowing freshwater environments. The primary design features include a capacitive coupler with an opposite relative position between feeding and receiving points on the coupler electrode, two phase compensation circuits, and a load-independent inverter. A stable and energy-efficient power transmission is achieved by maintaining a 90° phase difference on the coupler electrode in dielectrics with a large unloaded quality factor (Q factor), such as in freshwater. Although a 622-mm coupler electrode is required at 13.56MHz, the phase compensation circuits can reduce to 250mm as one example, which is mountable to small underwater drones. Furthermore, the electricity waste is automatically reduced using the constant-current (CC) output inverter in the event of misalignment where efficiency drops occur. Finally, their functions are simulated and demonstrated at various receiver positions and transfer distances in tap water.

This paper presents the design of a capacitive coupler for underwater wireless power transfer focused on the landing direction of a drone. The main design feature is the relative position of power feeding/receiving points on the coupler electrodes, which depends on the landing direction of the drone. First, the maximum power transfer efficiencies of coupled lines with different feeding positions are derived in a uniform dielectric environment, such as that realized underwater. As a result, these are formulated by the coupling coefficient of the capacitive coupler, the unloaded qualify factor of dielectrics, and hyperbolic functions with complex propagation constants. The hyperbolic functions vary depending on the relative positions and whether these are identical or opposite couplers, and the efficiencies of each coupler depend on the type of water, such as seawater and tap water. The design method was demonstrated and achieved the highest efficiencies of 95.2%, 91.5%, and 85.3% in tap water at transfer distances of 20, 50, and 100 mm, respectively.

Couplers in a film-type capacitive wireless power charging (CWC) system for an implantable medical device were designed and analyzed in this work. Due to the high conductivity of the human body, two paths contribute to the power transmission, namely a high-frequency current and an electric field. This was confirmed by an equivalent circuit of the system. During analysis of the system, we used pig skin with subcutaneous fat, which has a high affinity with the human body, to search for a highly efficient electrode shape. Subsequently, we fabricated the designed coupler and measured ηmax. An ηmax of 56.6% was obtained for a half-circular coupler with a radius of 20 mm and a distance of 10 mm between adjacent couplers. This study will contribute to the realization of implantable devices that can be recharged during breaks or while sleeping at home and is expected to significantly reduce the burden on patients.

The load-independent zero-voltage switching class-E inverter has garnered considerable interest as an essential component in wireless power transfer systems. This inverter achieves high efficiency across a broad spectrum of load conditions by incorporating a load adjustment circuit (LAC) subsequent to the resonant filter. Nevertheless, the presence of the LAC influences the output impedance of the inverter, thereby inducing a divergence between the targeted and observed output power, even in ideal lossless simulations. Consequently, iterative adjustments to component values are required via an LC element implementation. We introduce a novel design methodology that incorporates an external quality factor on the side of the resonant filter, inclusive of the LAC. Thus, the optimized circuit achieves the intended output power without necessitating alterations in component values.

This paper focuses on underwater wireless power transfer with electric coupling. First, the maximum available efficiency is derived by using the S-parameters of the parallel plate coupler. The frequency which represents the maximal value of the efficiency is revealed. Further, the elevation in the efficiency in association with a reduction of the electrode size is found. It is clarified that the elevation depends on the characteristic of the water dielectric loss. From these results, the optimal electrode size that obtains the maximal value of the efficiency is provided. Finally, we fabricate the couplers by utilizing the optimal frequency and the electrode size. The efficiency of 75.8% under water is achieved.

In this paper, a resonator based on composite right/left handed transmission line concept is discussed. This resonator excites --1st order resonance mode. We start with half-wave resonators consisting of two unit cells of a composite right/left handed transmission line. From the simulated field distributions, the center of these half-wave resonators can be short-circuited to obtain a quarter-wave resonator in the --1st mode. Susceptance slope parameters are calculated for the resonator. Then this resonator is applied for a 2-pole filter made by LTCC, which can be designed with standard filter design theory owing to the slope parameter. The dimension of the experimental filter implemented by LTCC is 2.5 mm by 1.35 mm by 0.52 mm. The insertion loss is 1.80 dB at the 2.4 GHz band. Good agreement between measured and computed results is obtained.

This paper presents the feasibility of a capacitive coupler utilizing an electric double layer for wireless power transfer under seawater. Since seawater is an electrolyte solution, an electric double layer (EDL) is formed on the electrode surface of the coupler in direct current. If the EDL can be utilized in radio frequency, it is possible that high power transfer efficiency can be achieved under seawater because a high Q-factor can be obtained. To clarify this, the following steps need taking; First, measure the frequency characteristics of the complex permittivity in seawater and elucidate the behaviors of the EDL from the results. Second, clarify that EDL leads to an improvement in the Q-factor of seawater. It will be shown in this paper that capacitive coupling by EDL occurs using two kinds of the coupler models. Third, design a coupler with high efficiency as measured by the Q-factor and relative permittivity of EDL. Last, demonstrate that the designed coupler under seawater can achieve over 85% efficiency at a transfer distance of 5 mm and feasibility of the coupler with EDL.

In this paper, we applied cavity resonator wireless power transfer (CR WPT) to an enclosed space with scatterers and revealed that high transfer efficiency at line-of-sight (LOS) and non-line-of-sight (NLOS) position in the power transmitter can be achieved by this method. In addition, we propose a method for limiting the wireless power transfer space utilizing metal mesh and show its effectiveness by experiment. First, we confirm that the constructed experimental model is working as a cavity resonator by theoretical formula and electromagnetic field analysis. Next, we calculate the maximum power transfer efficiency using a model including a plurality of scatterers by installing a power receiver at LOS and NLOS positions in the power transmitter, and it was confirmed that transfer efficiency of 30% or more could be expected even at the NLOS position. Then, we measured the frequency characteristics of a model in which one surface of the outer wall was replaced with a metal mesh, and it was clarified that the characteristics hardly changed in the power transfer frequency band. Finally, we confirmed that simultaneous communication can be performed with driving of the battery-less sensor by CR WPT, and clarify effectiveness of the proposed method.

This study proposes a design method for a rectifier circuit that can be rapidly charged by focusing on the design-load value of the circuit and the load fluctuation of a storage capacitor. The design-load value is suitable for rapidly charging the capacitor. It can be obtained at the lowest reflection condition and estimated according to the circuit design. This is a conventional method for designing the rectifier circuit using the optimum load. First, we designed rectifier circuits for the following three cases. The first circuit design uses a load set to 10 kΩ. The second design uses a load of 30 kΩ that is larger than the optimum load. The third design utilizes a load of 3 kΩ. Then, we measure the charging time to design the capacitor on each circuit. Consequently, the results show that the charge time could be shortened by employing the design-load value lower than that used in the conventional design. Finally, we discuss herein whether this design method can be applied regardless of the rectifier circuit topology.

We present a design approach and analysis of a multimode stripline resonator (MSR). Furthermore, a bandpass filter (BPF) using a single MSR is presented. MSR has three fundamental modes, incorporating two transmission resonance modes and one quasi-lumped component (LC) resonance mode. The resonant frequencies and unloaded Q factors of those modes are theoretically derived by transmission modes and LC modes. By our equations, it is also explained that the resonant frequencies can be shown to be easily handled by an increase and decrease in the number of via holes. These frequencies calculated by our equations are in good agreement with those of 3-D simulations and measurements. Finally, design approach of a narrow bandpass filter using our resonator is introduced. Good agreement between measured and computed result is obtained.