A novel highly efficient radio-frequency (RF) energy harvester (rectenna) array with multifrequency coverages (1.7–1.8 GHz and 2.1–2.7 GHz) as well as an omnidirectional 3-D radiation pattern is presented. The harvester array is very compact and of low-profile, which strategically combines 12 single Vivaldi slot rectenna elements. More importantly, the proposed rectenna array is significantly simplified through a codesigned effective impedance tuning and array combination method to reduce the overall circuit complexity whilst maintaining high energy conversion efficiency (up to 67%) over the desired frequency band under different input powers (−25–5 dBm) and circuit loads (0.2–8 kΩ). Having utilized the proposed wireless energy harvester array, a complete system-level demonstration of ambient RF energy-powered, a self-sustainable IoT sensor node is demonstrated for reliably operating in a typical indoor environment. Such a demonstration has never been reported before. In addition, the proposed rectenna array has novel features in terms of simplified Vivaldi rectenna matching, compact array combination and optimal beam shaping, which has proven to be an effective energy harvester in most domestic environments; therefore, having significant implications in powering small electronics for real-world Internet-of-Things (IoT) and industrial Internet-of-Things (IIoT) applications.

A multi-polarization rectenna, consisting of a twoport antenna fed by an orthogonal hybrid coupler and a medium power rectifying circuit, is proposed. The orthogonal hybrid coupler has realized good isolation (S12 or S21 < -15 dB) between the two ports, which can be respectively connected to rectifying circuit and/or communication device for simultaneous wireless information and power transfer (SWIPT). Due to the symmetry of the coupler, the proposed rectenna can work at multipolarization modes, i.e. the circular polarization for the singleport operation and the linear polarization for the dual-port operation. Besides, a medium-power rectifying circuit is proposed, where its maximum radio-frequency (RF)-to-direct current (DC) power conversion efficiencies is 53.6% (simulated) and 51.1% (experimental) at the input power of 0 dBm and 0.5 dBm, respectively. By using the coupler, the conversion efficiency and impedance matching of the rectenna can be significantly improved. In addition, the power division between the two ports is considered at different modes. Compared to other rectennas, the proposed multi-functional rectenna has merits of multiple polarizations and multi-mode, which can therefore be used for the incoming waves in arbitrary polarization for SWIPT and cope with the uncertainty in realworld communication environments.

An ultra-wideband (UWB) rectenna (fractional bandwidth >100%) using a novel wideband complementary matching stub is proposed for microwave power transmission and energy harvesting. A simple resonant structure, i.e., LC series-parallel resonant circuit, is embedded to the L-shaped complementary matching stub. Due to the unique frequency response of the LC resonant circuit, the proposed matching stub can exhibit “open” and “short” circuits as a function of frequency, thereby acting as a complementary matching circuit covering a relatively wide frequency range. Having utilized the proposed matching stub, the nonlinear input impedance of the rectifier can be tuned to conjugately match the antenna impedance throughout the frequency band of interest. Simulated and measured results show that the proposed rectenna has good matching performance ( S ₁₁ <; -10 dB) and high RF-dc conversion efficiency (>50%) over a relatively wide frequency range from 0.9 to 3 GHz (for GSM, Wi-Fi, and WLAN bands). The maximum conversion efficiency of 73.4% is realized at 3-dBm input power. It is evident that the proposed resonant structure-based matching scheme is a promising and effective solution to facilitate the UWB rectenna design with stably high efficiency over a very wide frequency band.

Traditionally, the signal power is divided equally between dual functions in wireless information and power transmission (WIPT), which, intuitively, is not an optimal solution since the power sensitivities for communication and charging node pose different constraints for signals. To address this challenge, a two-channel rectenna with an asymmetrical coupler feeding network (ACFN) has been proposed for WIPT under the power-splitting and time-sharing (PS and TS) schemes. The proposed two-port rectenna consists of a receiving antenna integrated with ACFN where one output port is connected to a rectifying circuit, whereas the other is used for information recovery. Different from the conventional PS scheme with equal division, the proposed rectenna can adjust ratios of power at two outputs. That is, routing high power to the radio frequency (RF)-dc port, and transferring low power but of a sufficient signal-to-noise-ratio (SNR) to the port being used for signal demodulation/decoding. As a result, the simulated/measured maximum RF-dc conversion efficiency of the rectenna can reach up to 73.9%/70.4% for quadrature phase shift keying (QPSK) modulated signals and 63.9%/60.7% for continuous wave (CW) signals. In addition, good isolation between the two output ports ensures low interference against information decoding.

A compact rectenna with a wide input power range is proposed for wireless power transfer (WPT). Two different diodes, having distinct operating powers, i.e. high-input-power diode and low-input-power diode, are introduced to the rectenna design for extending the operating input power range. A mid-inductor is placed in the rectifying circuit to isolate the radio frequency (RF) signal from DC power, thereby delivering the rectified DC power to the load. Also, with the aid of mid-inductor, the fundamental RF signal and harmonic signals could flow back to the two rectifying diodes for achieving higher conversion efficiency via the means of harmonic signal re-rectification. Owing to the mid-inductor, the sub-rectifier circuit can be avoided, and a compact structure can therefore be realized. Over 50% measured conversion efficiency ${\eta_{c}}$ can be achieved in a very wide input power range of 35 dB (-10 to 25 dBm). The proposed rectenna can be widely used in WPT systems with unpredictable and variable RF energy levels.

In recent years, microwave wireless power transmission (MWPT) has emerged as a promising technology for supplying energy by receiving radiative power without wires and converting it into DC power. A high transmission efficiency is crucial for improving the performance of MWPT systems. To address the challenge of propagation loss, beam synthesis using transmitting antennas has gained significant attention, resulting in several synthesis methods becoming available, including whisper beams, flat-top beams, non-diffractive beams, supergain/superdirective beams, focused beams and adaptive beamforming technique such as time reversal methods. This comprehensive review covers these advanced beam synthesis techniques for MWPT transmitters and provides a detailed comparison of different synthesized beams. The article includes an in-depth discussion on current designs and existing technological challenges, as well as suggestions for future research directions. Although beam synthesis can substantially improve the transmission efficiency, the overall power transfer efficiency of the entire MWPT system still requires improvement as the performance, including impedance matching, is currently only considered at a component level and not at a system level. Furthermore, when using advanced beam synthesis techniques, the engineering and implementation challenges of high power (≥kW) and long-distance (≥km) MWPT become significant issues due to the high cost and large size involved. The review concludes that existing technologies for synthesized beams still require significant long-term efforts to meet the realistic engineering requirements for achieving highly efficient MWPT systems. The joint utilization of beam synthesis techniques and comprehensive system matching/optimization is identified as a research direction with the potential to realize highly efficient MWPT systems, offering foreseeable impacts in both terrestrial and space-based MWPT applications. This review serves as a solid foundation for the design of transmitter antennas in long-distance, high-power MWPT systems, and furthermore provides novel insights for future designs of highly efficient MWPT systems.