A stacked-coil technology is demonstrated that realizes size-adjustable coil systems to compensate the notorious problem of lateral misalignment in nonradiative wireless power transfer (WPT) systems. The proposed system has a simple structure that consists of multiple layers of coils with various sizes. Depending on the misalignment, the coil pair that provides the best transfer efficiency is switched on. Thus, not only the transfer null is removed, but also a very high transfer efficiency is maintained even with extreme misalignment. Experimental results at 6.78 MHz show the effectiveness of the stacked coil for both short- and mid-range wireless power transfer systems. The transfer efficiency maintains abovementioned 80% up to 87% and 65% lateral misalignment when the separation between Tx and Rx coils are 10% and 50%, respectively, relative to the largest dimension of the coil. The three-layer short-range system successfully removes the transfer null to maintain a minimum transfer efficiency of 39.7% up to 126% misalignment. The two-layer mid-range system maintains transfer efficiency abovementioned 50% up to 103% misalignment. Furthermore, the performance is virtually the same even in an asymmetric WPT system, i.e., when the technique is applied to only one of the two coils. A detailed design procedure is also provided.

A design method for a frequency-tunable planar bandpass filter is demonstrated. With the method, all reasonators can be designed with identical capacitances, allowing the filter to be tuned with identical varactors using a single bias control. This allows not only low-cost fabrication, but also a simple structure and design procedure. Moreover, the filter is based on miniaturized transmission lines. Therefore, a wide tuning range is achieved with a small size and outstanding out-of-band response. Most importantly, the method facilitates the design of a filter to vary the fractional bandwidth in a predetermined way as the center frequency is tuned. Thus, a tunable filter can be designed not only to maintain the fractional bandwidth, but also to expand or to reduce the bandwidth as the frequency is tuned. The experimental results of a compact third-order bandpass filter based on CPW lines with a 0.11λg×0.8λg size show a very wide frequency tuning range of 62.8% with a 3-dB fraction bandwidth that remains within 28.65±1.55% and an outstanding spurious suppression of under –40 dB up to as high as 3f0.

This work develops a reconfigurable coil array that compensates for the misalignment between coils in magnetic resonance coupled wireless power transfer (WPT) systems. By tuning the magnitude of the currents induced in the coil array with the variable capacitors that load them, the effect of reconfiguring the size and/or shape of the coil can be achieved. Therefore, the near field can be steered in a certain direction to overcome the notorious misalignment problem in WPT systems. For experimental demonstration, a $2\times 1$ array is designed and fabricated for a $60\times 60$ mm ² coil. For a vertical distance between Tx and Rx coils of 10 mm, which is 16.7% of the largest coil dimension, the transfer efficiency maintains more than 40% and up to 141.6% misalignment. This is more than an 81% increase over the conventional coil. For a vertical distance of 30 mm or 50%, the minimum transfer efficiency is 37.1% up to 100% misalignment, which is almost 5.5 times that of the conventional counterpart. The results suggest that the demonstrated technology successfully overcomes the notorious misalignment problem in both short-range and midrange WPT systems and even in asymmetric systems where the reconfigurable coil array technique is applied only on the Rx.

Herein, a passive low-profile moisture sensor design based on radio frequency identification (RFID) technology is proposed. The sensor consists of an LC resonant loop, and the sensing mechanism is based on the fringing electric field generated by the capacitor in the circuit. A standard planar inductor and a two-layer interdigital capacitor (IDC) with a significantly higher fringing capacitance compared to that of a conventional parallel plate capacitor (PPC) are used, resulting in improved frequency offset and sensitivity of the sensor. Furthermore, a sensor tag was designed to operate at an 8.2 MHz electronic article surveillance (EAS) frequency range and the corresponding simulation results were experimentally verified. The IDC- and PPC-based capacitor designs were comprehensively compared. The proposed IDC sensor exhibits enhanced sensitivity of 10% in terms of frequency offset that is maintained over time, increased detection distance of 5%, and more than 20% increase in the quality factor compared to sensors based on PPC. The sensor’s performance as a urine detector was experimentally qualified. Additionally, it was shown experimentally that the proposed sensor shows a faster response to moisture. Both simulation and experimental data are presented and elucidated herein.

In this paper, self-matching rectifiers for enhanced input dynamic range are demonstrated. The method is based on an artificial transmission line, which consists of a stepped-impedance transmission line loaded with shunt varactors. The output DC voltage of the rectifier is fed back to the varactors to control the effective electrical length and characteristic impedance of the line. Because this bias voltage changes as the power input to the rectifier changes, a matching network that maintains impedance matching regardless of the input power can be developed without additional circuits, thus substantially improving the input dynamic range. The experimental results for prototype rectifiers show a dynamic range as wide as 25 dB with only a single Schottky diode, an improvement of 11 dB over the rectifier with a conventional matching scheme, in which the conversion efficiency remains above 50%. The effectiveness of the proposed method is also verified with the experimental results for rectennas based on the demonstrated rectifiers.

Dual-band branch-line couplers with arbitrary power-split ratios are presented. The use of crossed lines at the center of the dual-band coupler enables it to independently provide different power-split ratios to the two bands. Additionally, open stubs are utilized to enhance the stopband responses. The complete design procedure with example design curves is provided. For experimental verification, three dual-band couplers with power-split ratio combinations of +3 dB ($S_{21}:S_{31}=2:1$) and −3 dB ($S_{21}:S_{31}=1:2$), −3 dB and +3 dB, and 0 dB ($S_{21}:S_{31}=1:1$) and +13 dB ($S_{21}:S_{31}=20:1$) at 1 GHz and 2.5 GHz were designed and fabricated. The measured results are in excellent agreement with the ideal and full-wave simulated results. The measured difference of −13.3 dB between the power-split ratios of the two bands is the largest reported for a dual-band branch-line coupler.