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.

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.

In this study, a coil array technology is proposed to compensate for 2-D misalignment between coils, regardless of its direction, in a wireless power transfer (WPT) system at 6.78 MHz. The primary coil is printed on one side of the substrate, while the assistant coil is printed on the other side in a 2 $\times$ 2 array configuration. Depending on the degree of misalignment, the coil pair with the best alignment can be switched on. Thus, high efficiency can be maintained without increasing the size of the coils. Experimental results for a 47 $\times$ 47 mm $^2$ -sized coil system demonstrate null-free operation that maintains transfer efficiency of more than 80% with diagonal misalignment of up to 98% relative to the largest coil dimension. Regardless of the direction of the misalignment, its efficiency is 80% or higher with up to 72% misalignment, and 50.5% or higher with extremely high misalignment (up to 120%). The results indicate that the proposed coil array technology successfully addresses the notorious misalignment issue in a near-field WPT system, regardless of the direction of the misalignment, without increasing the coil size. Furthermore, the results for an asymmetric system in which the coil array technology is adopted only on the Tx side reveal the feasibility of the proposed method for practical applications.

This study applies a multilayer coil technology that can compensate for a decrease in transfer efficiency due to a lateral misalignment in a practical 100 kHz-band wireless power transfer system and validates its effect on the efficiency of compensation. The effectiveness is investigated using coils fabricated with Litz wires. Three-turn rectangular assistant coils 22.4 × 45.3 mm² in size were stacked on a five-turn circular primary coil with a diameter of 45.3 mm in a 2 × 1 array. Transfer efficiency between two such coils was measured by producing lateral misalignment, while maintaining the vertical distance between the Tx and Rx coils at 7 mm. The experimental results showed that the transfer efficiency was compensated by approximately 46.1%P maximum in a misalignment state of 30 mm, which corresponded to 67% of the maximum size of the coil, compared to the transfer efficiency of the structure, in which the multilayer coil was not applied. Furthermore, transfer efficiency was compensated by 37.6%P, even in an asymmetric system in which the multilayer structure was applied only to the Tx coil, thereby confirming an excellent multilayer coil technology effect on compensation for lateral misalignment in practical cases.