This brief proposes a compact high-efficiency broadband microwave rectifier for energy harvesting. The proposed rectifier is designed to operate over a broad bandwidth spanning from 2 to 3 GHz and an input power dynamic range from 0 dBm to 10 dBm. In the proposed structure, a compact broadband low-loss matching network and a dc-pass filter with a broad stopband were designed, leading to a high conversion efficiency in a wide frequency range with compactness. For the matching network, low loss and compact size were achieved by strategically choosing and designing three transmission line segments. Moreover, the proposed dc-pass filter is compact and can effectively block broadband RF signal and its in-band high order harmonics. Theoretical analysis and simulations of the proposed structure were carried out. A prototype was fabricated, which demonstrates a broadband bandwidth of 41.5% (2.0-3.05 GHz) where the RF-dc power conversion efficiency is higher than 70% at an input power of 10 dBm. The measured efficiency remains over 45% within 1.9-3.05 GHz when the input power decreases to 0 dBm. The maximum measured efficiency is 75.8% at an input power of 14 dBm.

Microstrip line (MLIN)-based glucose sensors have been used for noninvasive glucose monitoring. However, the current MLIN-based solutions do not provide enough sensitivity. The reason for this is that they rely on the fringing field of an MLIN that leads to a shallow penetration depth and thus low sensitivity. In this paper, we propose a sensitive MLIN-based glucose sensor using the main field. The idea is illustrated using the following three steps. Step 1 is sensing the glucose level by using the main field, which uses the material under test (MUT) as the substrate of an MLIN terminated with a load; Step 2 is identifying the contribution of different parameters and different components of parameters to sensitivity, which leads to Step 3, in which different parameters and/or different components of parameters are crosschecked for sensing. Two frequency bands, 100 to 500 MHz and 1.4 to 1.9 GHz, were investigated. In Step 1, the main field configuration was compared with its fringing field counterpart. It is shown that the proposed MLIN configuration presents an average sensitivity of 1.8×10 ^-3 dB/(mg/dL) in terms of the reflection coefficient that is more than 10 times higher than that of its counterpart. In Step 2, different components of the reflection coefficient and the input impedance of the proposed configuration were examined, and their diverse contributions to the sensitivity were shown, which allows crosschecking to improve sensing accuracy. This was successfully shown in Step 3, in which algorithms for crosschecking were proposed, and the sensing accuracy was examined. In terms of the optimal sensing frequency band, 1.4 to 1.9 GHz shows higher sensitivity than 100 to 500 MHz, where the molecules may interact the most with the waves. This implies that the interaction between a wave and the sensing structure with the MUT contributes more to sensitivity than the direct interaction between the wave and the MUT. The radiofrequency safety of the proposed configuration was examined by checking the specific absorption rate of the MUT with the proposed structure. This paper offers a new avenue for the development of an MLIN-based noninvasive glucose sensor for continuous monitoring with high sensitivity in the near future. It can be used both as a single sensor and in a multiple-sensor glucose monitoring system.

Hyperbolic (or indefinite) materials have attracted significant attention due to their unique capabilities for engineering electromagnetic space and controlling light propagation. A current challenge is to find a hyperbolic material with wide working frequency window, low energy loss, and easy controllability. Here, we propose that naturally existing electride materials could serve as high-performance hyperbolic medium. Taking the electride ${\mathrm{Ca}}_2\mathrm{N}$ as a concrete example and using first-principles calculations, we show that the material is hyperbolic over a wide frequency window from short-wavelength infrared to near infrared (from about $3.3\mu \mathrm{m}$ to 880 nm). More importantly, it is almost lossless in the window. We clarify the physical origin of these remarkable properties and show its all-angle negative refraction effect. Moreover, we find that the optical properties can be effectively tuned by strain. With moderate strain, the material can even be switched between elliptic and hyperbolic for a particular frequency. Our result points out a new route toward high-performance natural hyperbolic materials, and it offers realistic materials and novel methods to achieve controllable hyperbolic dispersion with great potential for applications.

In this letter, a microwave rectifier with a wide input power range is presented by proposing a self-tuning impedance matching (STIM). The STIM consists of a varactor reversely biased by the output voltage of the rectifier to compensate the variation of the input impedance caused by the input power, so that impedance matching is obtained over a wider input power range. For validation, the proposed rectifier at 2.4 GHz was designed and implemented. The experimental results show that the RF-dc power conversion efficiency maintains over 50% and 70% within the input power ranges of 2.5-25.5 dBm and 11-24.5 dBm, respectively. Compared with the counterpart without the STIM, an improvement of 7.2 dB in terms of the input power range when the reflection coefficient is less than -10 dB was successfully demonstrated.

In this paper, we present the design and optimization of a ring-pair permanent magnet array for head imaging in a low-field portable magnetic resonance imaging (MRI) system. The proposed array generates a longitudinal main static magnetic field (B₀ field) with an average field strength of 169.7 mT and a homogeneity of 24 786 ppm in a field of view with a diameter of 200 mm and a thickness of 50 mm. It is a significant increase in field homogeneity by 79.7% compared to a traditional ring-pair structure of the same dimension and mass while still maintaining a similar field strength. The optimization was implemented by applying a genetic algorithm and by proposing an efficient current model for the forward calculation of the magnetic field. The effectiveness of the optimization is validated by realistic simulations using COMSOL Multiphysics. Compared to a Halbach array, where the field is transversal and the existing coil designs cannot be applied directly, the proposed array generates stronger fields. Its magnetic field is longitudinal, which allows the direct application of the advancements in RF coil designs in a conventional MRI system to the imaging system using the proposed magnet array.

In this paper, a high-efficiency rectifier with one octave bandwidth is proposed by proposing a diode array with a frequency selective topology. With the frequency selective topology, the low-and high-band rectifiers are combined together without extra matching circuit at the input leading to a high-efficiency broadband design. For validation, a prototype rectifier was fabricated and characterized. The measured results show that the RF-dc power conversion efficiency (PCE) maintains above 70% from 1.75 to 3.55 GHz at an input power of 10 dBm. The measured PCE remains more than 50% within 1.4-3.7 GHz and the |S₁₁| is lower than -10 dB from 1.7 to 3.6 GHz.