This paper presents a general efficiency enhancement method for compact broadband rectifiers. First, a simple LCR model is employed to characterize the frequency-dependent parasitic effects of Schottky barrier diodes (SBDs) in broadband. Subsequently, based on the LCR model, a compact rectifier circuit configuration is proposed to minimize impedance mismatch in broadband, thereby significantly improving the rectification efficiency. To validate the proposed approach, two representative rectifier prototypes are designed, fabricated, and measured. The measured results show that Rectifier I achieves a power conversion efficiency (PCE) exceeding 70% across 0.5–2.5 GHz under 20 dBm input power with a${}\,1000{\Omega }$load, while Rectifier II attains a PCE above 64% from 1.1 to 5.4 GHz under 10 dBm input power with the same load. Compared with state-of-the-art designs, the proposed rectifiers exhibit superior PCE performance while achieving a fractional bandwidth (FBW) exceeding 100%. Notably, the proposed efficiency enhancement technique is suitable for most broadband rectifier designs, such as diverse rectifier topologies, SBD types, input power levels, and load conditions.

This paper reports a feasibility study including experimental implementation of a battery-less wireless sensor whose power is supplied by a microwave power transmitter. In particular, both microwave power transmissions (MPTs) and wireless communications using wireless local area networks (WLANs) are performed on the same frequency band, 2.4 GHz, to achieve high spectral efficiency. In this case, time scheduling of MPT and wireless communication is necessary to avoid interference. In addition, for stable operation, the duty ratio, which is defined as the ratio of the active duration to the whole period, is set depending on the charge remaining in the capacitor of the sensor. In the initial experimental setup, a single battery-less wireless sensor is successfully operated, and then the feasible transmission range of the designed time scheduling system is discussed.

In wireless sensor networks, individual nodes must be batteryless to make them maintenance-free, which is particularly important for networks that have an enormous number of nodes. When these wireless batteryless nodes are microwave-powered devices, they would necessitate scheduling of power and data transmission. In addition, to ensure network sustainability, a constant amount of energy should be stored in the capacitors of the batteryless nodes. In the experiment conducted in this study, we confirm the necessity of scheduling. In addition, sleep periods are scheduled to ensure that a constant amount of energy remains in the capacitor.

Beam control and/or beamforming technologies involving phased arrays can be used to enable highly efficient microwave power transmission (MPT) systems. In Japan, several field MPT experiments using a phased array have been conducted over the years. In 1992, a joint collaborative group successfully conducted a fuel-free flight experiment using phased-array technology, which was referred to as the MIcrowave Lifted Airplane eXperiment. In 2008, Kobe University (Kyoto, Japan) and a U.S. research group successfully conducted an MPT experiment using a phased array that resulted in power transmission over a distance of 150 km. Furthermore, Kyoto University proposed and demonstrated a magnetron-based phased array and conducted MPT from an airship to the ground in spring 2009. In FY2010, Kyoto University developed new highly efficient phased arrays for MPT and solar power satellites. This study provides an overview of past and present Japanese MPT experiments using phased-array technologies.