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The proliferation of smart devices increases the demand for energy-efficient, battery-free technologies essential for sustaining IoT devices in Industry 4.0 and 5G networks, which require zero maintenance and sustainable operation. Integrating radio frequency (RF) energy harvesting with IoT and 5G technologies enables real-time data acquisition, reduces maintenance costs, and enhances productivity, supporting a carbon-free future. This survey reviews the challenges and advancements in RF energy harvesting, focusing on far-field wireless power transfer and powering low-energy devices. It examines miniaturization, circular polarization, fabrication challenges, and efficiency using the metamaterial-inspired antenna, concentrating on improving diode nonlinearity design. This study analyzes key components such as rectifiers, impedance matching networks, and antennas, and evaluates their applications in biomedical and IoT devices. The review concludes with future directions to increase bandwidth, improve power conversion efficiency, and optimize RF energy harvesting system designs. [ABSTRACT FROM AUTHOR]

In this paper, we investigate various graphene monolayer nanomesh structures (diodes) formed only by nanoholes, with a diameter of just 20 nm and etched from the graphene layer in different shapes (such as rhombus, bow tie, rectangle, trapezoid, and triangle), and their electrical properties targeting electromagnetic energy harvesting applications. In this respect, the main parameters characterizing any nonlinear device for energy harvesting are extracted from tens of measurements performed on a single chip containing the fabricated diodes. The best nano-perforated graphene structure is the triangle nanomesh structure, which exhibits remarkable performance in terms of its characteristic parameters, e.g., a 420 Ω differential resistance for optimal impedance matching to an antenna, a high responsivity greater than 103 V/W, and a low noise equivalent power of 847 pW/√Hz at 0 V. [ABSTRACT FROM AUTHOR]

This work presents an analysis and design of the two barrier-quantum well asymmetric spacer tunnel layer (QW-ASPAT) diodes for implantable rectenna circuits application. The RF and DC characteristic of a 10×10 μm² QW-ASPAT devices based on GaAs and In0.53Ga0.47As platform was simulated and extracted by using SILVACO atlas software. The highest extracted curvature coefficient, kv value of the both QW-ASPAT devices at zero bias was about 33V-1 compared with the standard structure GaAs/InGaAs was about 13V-1. The effects of changing in the thickness of the thin AlAs-barrier, the well width, and the spacer layer are fully investigated on the non-linear relationship between current and voltage of these diodes. A CV simulation was carried out, and it was found that the addition of the quantum-well layer between spacers and barrier reduced the junction capacitance of the QW-ASPAT device when compared with standard devices. The cut-off frequency of the proposed QW-GaAs and QW-InGaAs devices are 26GHz and 46GHz respectively. Finally, we conclude that the QW-ASPAT device is the best structure and can be used for microwave rectifiers in the miniaturized integrated rectenna systems. [ABSTRACT FROM AUTHOR]

This study introduces a rectenna, functioning as an RF envelope detector, utilizing a 16 nm bulk MOS transistor (metal-oxide-semiconductor field-effect transistor) for nonlinear detection. A circuit architecture is presented alongside a detailed design methodology and simulations. The detector efficiently demodulates a 2.4 GHz OOK (On/Off Keying) encoded signal, comprising a 32-bit word, within 320 μs. Remarkably, the circuit operates passively, requiring no voltage supply or bias current, and functions effectively with − 53 dBm input power at the antenna. This capability enables the decoding of 32-bit unsigned integer radio packets as a wakeup radio event. The effectiveness of the envelope detector is substantiated through comprehensive simulations. [ABSTRACT FROM AUTHOR]

This work presents the results of the characterization of two 1 × 5 mm2 miniaturized rectennas developed for medical applications. They have been designed for relatively high voltage and high-power applications, given the size of the rectennas. Both rectennas were tested in open-air conditions and surrounded by pork fat and muscle tissues, whose properties are similar to those of the human body. The resonant frequencies of the rectennas were found, and the incident electric field on the rectennas tests was increased. The first chip showed a maximum output voltage of 5.29 V and a maximum output power of 0.056 mW, at 1.446 GHz, under an incident field on the rectenna of 340 V/m, and the second chip, 4.62 V and 4.27 mW, at 1.175 GHz, under 535 V/m. The second rectenna can provide an output power greater than 5 mW. The rectennas presented in this article are beyond the state of the art, as they can deliver about three times more power and voltage than those of similar dimensions reported in the literature. Based on the test results, the efficiency of the rectennas was analyzed at different locations of the human body, considering different thicknesses of tissues with high and low water content. Finally, potential applications are described in which the rectennas could power implantable medical devices or microsurgery tools, for example, pulmonary artery pressure monitors. [ABSTRACT FROM AUTHOR]

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Herein, we report the design, fabrication, and performance of two wireless energy harvesting devices based on highly flexible graphene macroscopic films (FGMFs). We first demonstrate that benefiting from the high conductivity of up to 1 × 106 S m−1 and good resistive stability of FGMFs even under extensive bending, the FGMFs‐based rectifying circuit (GRC) exhibits good flexibility and RF‐to‐DC efficiency of 53% at 2.1 GHz. Moreover, we further expand the application of FGMFs to a flexible wideband monopole rectenna and a 2.45 GHz wearable rectenna for harvesting wireless energy. The wideband rectenna at various bending conditions produces a maximum conversion efficiency of 52%, 46%, and 44% at the 5th Generation (5G) 2.1 GHz, Industrial Long‐Term Evolution (LTE) 2.3 GHz, and Scientific Medical (ISM) 2.45 GHz, respectively. A 2.45 GHz GRC is optimized and integrated with an AMC‐backed wearable antenna. The proposed 2.45 GHz wearable rectenna shows a maximum conversion efficiency of 55.7%. All the results indicate that the highly flexible graphene‐film‐based rectennas have great potential as a wireless power supplier for smart Internet of Things (IoT) applications. This work demonstrates two wireless energy harvesting devices based on highly flexible graphene macroscopic films (FGMFs) for IoT low‐power applications. Benefiting from the high conductivity (1 × 106 S m−1) and good resistive stability of FGMFs, the proposed flexible rectenna produces a peak conversion efficiency of 52% at 2.1 GHz, and the proposed 2.45 GHz wearable rectenna shows a maximum conversion efficiency of 55.7%. [ABSTRACT FROM AUTHOR]

A novel, polarization-independent, wide-angle reception Chand-Bali nano-antenna is proposed. An adjoint-based optimization algorithm is used to create the same resonance at both linear polarizations of the incident radiation. The nano-antenna optimal parameters reveal that two hot spots with a strong field enhancement are created. These hot-spots could be integrated with metal–insulator–metal (MIM) diodes to form a rectenna for infrared (IR) energy harvesting. The metallic resonators allow for selecting several materials to facilitate the fabrication of the nano-antenna and the MIM diode. The Chand-Bali-based IR rectennas are investigated and simulations demonstrate an improvement of more than one order of magnitude in efficiency compared to ones using traditional nano-antennas. [ABSTRACT FROM AUTHOR]

Abstract A novel 24 GHz circularly polarised metasurface rectenna for wireless power transmission is designed in this study. Based on experimental measurements and retro‐simulation, an effective approach is proposed to extract the parasitic parameters of a Schottky diode. A highly efficient millimetre wave rectifier with a measured efficiency of 63% is constructed based on the exact equivalent circuit parameters of a diode. A circularly polarised metasurface antenna is adopted as the receiving antenna, and the gain is enhanced by introducing metal vias around the metasurface. The antenna and the rectifier are connected directly via a microstrip line. Measurements show that the metasurface antenna has a gain of 11.3 dBic and an axial ratio of 2.5 dB at 24 GHz. The measured conversion efficiency of the rectenna reaches 63% at 300 Ω load when the input power is 15.2 dBm. The rectenna has the advantages of low profile, which can be conformal to the electrical equipment.

The Internet of Things is currently one of the fastest‐growing branches in electronics. The development of energy storage systems and the miniaturization of dedicated printed circuit boards significantly influence that growth. However, the need for batteries and traditional printed circuit boards still limits devices' minimum size, weight, and cost, narrowing the application area. Energy harvesters and wireless power transfer systems fabricated with printed electronics can significantly reduce such devices' weight, size, and cost. Printed electronics technology provides scalable tools for many electronics applications, shortening the validation time and enabling new low‐cost or disposable solutions on lightweight and flexible substrates embedded inside 3D printed structures and directly on device housings. Energy harvesting and wireless power transfer systems in electronic devices can provide enough power to minimize battery capacity and size or even eliminate the need for batteries in low‐power applications. This review presents an adaptation of printed electronics technology in the fabrication of radio frequency energy harvesters and wireless power transfer rectennas for IoT applications. Last, perspectives for development towards greater integration with microsystems, transient electronics with ecofriendly materials, adaptation for next‐generation telecommunication systems, and 3D structural electronics solutions are briefly discussed. [ABSTRACT FROM AUTHOR]

This work presents the design, fabrication, and measured results of a fully integrated miniature rectenna using a novel tunnel diode known as the Asymmetrical Spacer Layer Tunnel (ASPAT). The term rectenna is an abbreviation for a rectifying antenna, a device with a rectifier and antenna coexisting as a single design. The ASPAT is the centrepiece of the rectifier used for its strong temperature independence, zero bias, and high dynamic range. The antenna is designed to be impedance matched with the rectifier, eliminating the need for a matching network and saving valuable real estate on the gallium arsenide (GaAs) substrate. The antenna is fully integrated with the rectifier on a single chip, thus enabling antenna miniaturisation due to the high dielectric constant of GaAs and spiral design. This miniaturisation enables the design to be fabricated economically on a GaAs substrate whilst being comparable in size to a 15-gauge needle, thus unlocking applications in medical implants. The design presented here has a total die size of 4 × 1.2 mm2, with a maximum measured output voltage of 0.97 V and a 20 dBm single-tone 2.35 GHz signal transmitted 5 cm away from the rectenna. [ABSTRACT FROM AUTHOR]

Conventional packaging and interconnection methods for two-terminal devices, e.g., diodes often involve expensive and bulky equipment, introduce parasitic effects and have reliability issues. In this study, we propose a built-in packaging method and evaluate its performance compared to probing and wire bonding methods. The built-in packaging approach offers a larger overlap area, improved contact resistance, and direct connection to testing equipment. The experimental results demonstrate a 12% increase in current, an 11% reduction in resistance, and improved performance of the diode. The proposed method is promising for enhancing sensing applications, wireless power transmission, energy harvesting, and solar rectennas. Overall, the built-in packaging method offers a simpler, cheaper, more compact and more reliable packaging solution, paving the way for more efficient and advanced technologies in these domains. [ABSTRACT FROM AUTHOR]

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.

A novel 24 GHz circularly polarised metasurface rectenna for wireless power transmission is designed in this study. Based on experimental measurements and retro‐simulation, an effective approach is proposed to extract the parasitic parameters of a Schottky diode. A highly efficient millimetre wave rectifier with a measured efficiency of 63% is constructed based on the exact equivalent circuit parameters of a diode. A circularly polarised metasurface antenna is adopted as the receiving antenna, and the gain is enhanced by introducing metal vias around the metasurface. The antenna and the rectifier are connected directly via a microstrip line. Measurements show that the metasurface antenna has a gain of 11.3 dBic and an axial ratio of 2.5 dB at 24 GHz. The measured conversion efficiency of the rectenna reaches 63% at 300 Ω load when the input power is 15.2 dBm. The rectenna has the advantages of low profile, which can be conformal to the electrical equipment. [ABSTRACT FROM AUTHOR]

This work describes a dual band compact fully integrated rectenna circuit for implantable medical devices (IMDs). The implantable rectenna circuit consists of tunnel diode 10x10µm2 QW-ASPAT (Quantum Well Asymmetric Spacer Tunnel Layer diode) was used as the RF-DC rectifier due to its temperature insensitivity and nonlinearity compared with conventional SBD diode. SILVACO atlas software is used to design and simulate 100µm2 QW InGaAs ASPAT diode. A miniaturized dual band implantable folded dipole antenna with multiple L-shaped conducting sections is designed using CST microwave suits for operation in the WMTS band is 1.5GHz and ISM band of 5.8GHz. High dielectric constant material Gallium Arsenide (εr=12.94) and folded geometry helps to design compact antennas with a small footprint of 2.84mm3 (1x4.5x0.63) mm3. Four-layer human tissue model was used, where the antenna was implanted in the skin model at depth of 2mm. The 10-dB impedance bandwidth of the proposed compact antenna at 1.5GHz and 5.8GHz are 227MHz (1.4-1.63GHz) with S11 is -22.6dB and 540MHz (5.47-6.02GHz) with S11 is -23.1dB, whereas gains are -36.9dBi, and -24.3dBi, respectively. The output DC voltage and power of the rectenna using two stage voltage doubler rectifier (VDR) are twice that produced by the single stage at input RF power of 10dBm. [ABSTRACT FROM AUTHOR]

Electrically-contacted optical gap antennas are nanoscale interface devices enabling the transduction between photons and electrons. This new generation of device, usually constituted of metal elements (e.g. gold), captures visible to near infrared electromagnetic radiation and rectifies the incident energy in a direct-current (DC) electrical signal. However, light absorption by the metal may lead to additional thermal effects which need to be taken into account to understand the complete photo-response of the devices. The purpose of this communication is to discriminate the contribution of laser-induced thermo-electric effects in the photo-assisted electronic transport. We show case our analysis with the help of electromigrated devices.

Despite an increasing interest in wearable wireless power transmission (WPT), until now, wearable antennas have been unable to simultaneously harvest from near-field resonant and far-field radiative WPT. Here, a dual-port antenna is proposed, integrating an inductive coil with a broadband monopole for near- and far-field wearable WPT. The coil acts simultaneously as a High Frequency (HF) near-field power receiver and an Ultra-High Frequency (UHF) resonator, enabling the miniaturization of the enclosed broadband monopole, both fabricated using all-textile conductors. On-body, the antenna maintains a 10 dB return loss over a measured 135% fractional bandwidth while maintaining compactness ( $0.312\times 0.312\lambda ^{2}$ ). The antenna is substrate-independent and is demonstrated on two textile substrates with different dielectric properties and thicknesses. In far-field mode, the rectenna maintains over 40% efficiency from sub-1 $\mu \text{W}$ /cm2 power densities. In the near-field, a WPT efficiency up to 80% can be achieved. The simulated Specific Absorption Rate (SAR) shows up to 40 and 20 dBm power reception for HF and UHF operation, respectively, without exceeding the 1.7 W/kg limit. The far-field wearable rectenna is demonstrated powering a Bluetooth Low Energy node using a BQ25504 DC-DC converter from a best-in-class low power density of 0.88 and $0.55~\mu \text{W}$ /cm2 on-body and in-space, respectively.

A W band (75 – 110 GHz) communication link using monolithic integrated single-diode circuits is proposed. The circuits are based on GaAs Schottky diodes with cutoff frequencies around 1.5 THz integrated with on chip folded slot antennas and near-field 3D printed dielectric lenses. The total chip area is $1.4\times3.5$ mm2. The receiver module shows a peak isotropic voltage sensitivity of 12260 mV/mW at 94 GHz, with a noise equivalent power of 12 pW/ $\sqrt {Hz} $ . These results include the slot antenna and a 3 mm radius 3D printed lens. The transmitter acts as a free-space W band signal generator using the non-linear properties of the diode for frequency multiplication. Multiplication orders up to $\times 15$ are tested for input powers of 10 mW. The generated power is measured by a horn antenna placed at a distance of 100 mm from the transmitter and a 2D map for the frequency range 90 – 100 GHz and multiplication orders between $\times 2\ldots \times 15$ is presented. The two circuits are used to demonstrate W band communication links using 1 kHz amplitude modulated input signals with carriers at 6 GHz, X band and in the Ku band. Voltages of hundreds of mV are detected for a distance of 150 mm between transmitter and receiver circuits and tens of mV at 600 mm. The proposed approach can be used as a low cost and low complexity alternative for point-to-point high-speed wireless communications.

This paper establishes the practicality of terrestrial microwave power beaming at distances >1 km. To beam microwave power along the surface of the earth, it is necessary to consider and, where possible, exploit the effects of microwave propagation across terrain. Key design considerations include the effect of scattering from surface topography, approaches for characterizing the beam in the presence of multipath, safety, and efficiency. A rapid demonstration at the US Army Research Field in Blossom Point, MD, delivers 1.6 kW of electrical power at a 1046 m standoff from a 5.4-m-diameter X band transmitter. The transmitter is a reflector antenna with a linearly actuated feed horn that can focus the power density at specific standoff distances. Experimental results over cluttered, irregular terrain achieve a 2.3 dB (70%) enhancement in power density at the target site by deliberate exploitation of a ground bounce. A 4 m2 rectenna receiver produces the 1.6 kW of output power at a 73% RF-to-DC conversion efficiency, which exceeds the current state of the art at X band. In addition, a test to destruction of 4 large rectenna arrays demonstrates how overvoltage protection circuits can improve the RF power handling of rectenna arrays by >1 dB. A final experiment demonstrates >1 kW delivered at >1 km distance to a light display. Since these results should readily scale to higher levels of power and performance, recommendations are provided to mature the technology for operational use.

In this paper, we present a harmonic reaction inductive folded dipole antenna (FDA) for a highly efficient rectenna which is close to the fundamental limitation on efficiency. To remove lossy RF circuit components of the rectifier, the proposed antenna topology integrates circuit functionalities for highly efficient rectification, such as impedance transformation to high-impedance, impedance matching, DC blocking, and harmonic reaction. The proposed antenna is connected directly to rectifier diodes. Thus, the rectification efficiency can be improved toward the fundamental limitation restricted by the performance of the rectifier diode. In experimental investigations of the 2.4 GHz band antenna, the antenna radiation efficiency is 99.2%, and the degradation due to integrated circuit functionalities is negligibly small. The 2.4 GHz band bridge rectifier achieves a rectification efficiency of 91.1% at an input power of 30 dBm. This clarifies the effectiveness of the efficiency improvement with the proposed direct connection topology between the proposed FDA and bridge diode.

Radiofrequency energy harvesting (RF-EH) solutions have evolved significantly in recent years due to the ubiquity of electromagnetic waves in any environment. This review presents a comprehensive report on autonomous wireless sensor (WS) design considerations based on RF-EH. The obtainability of RF-EH-WS is driven by development efforts in the areas of RF-EH circuit design, known as rectifying antenna (Rectenna), the minimization of the energy budget of WS (MEB-WS), and finally, power management modules (PMM). The PMM aims to optimize the energy efficiency of the WS. In addition to these three factors, examining the RF power levels harvested related to the rectenna feeding technique (RFT) is essential. Since we did not find any review presenting a holistic view of these design considerations, we strived to provide a detailed picture of recent advances and new enhancements in this review. To address this issue, this review gives an overview of the seminal and contemporary studies in the RF-EH-WS field. The IoT issues are also discussed in terms of their basic requirement to support reduced size or miniaturized smart objects, which are common matters in current applications of WS nodes. Potential open issues that might be considered for future research are also discussed in this article. For a more detailed description of all presented concepts, many significant references are provided for the readers. [ABSTRACT FROM AUTHOR]

This study proposed, for the first time, a newly developed metal–insulator-insulator–metal (MIIM)-based log-spiral rectenna for energy harvesting applications. The designed resonant log-spiral antenna at 28.3 THz forwards the absorbed infrared (IR) electromagnetic (EM) radiations to the installed double insulator-based MIIM rectifying diode. The realized MIIM diode is formed by sandwiching the two insulators among the designed resonant antenna terminals (hot spot region). Antenna terminals are used as the electrodes of the rectifying diode. This novel approach ensures the maximum transfer of captured IR EM radiations by the antenna to the rectifying part with good impedance matching and thus increases the overall conversions efficiency. The study presents an in-depth analysis of the impact of the changing metal types (Au, Al, Ag, and Cu) and insulating materials (Al 2 O 3 , Cu 2 O, Ta 2 O 5 , TiO 2 , and ZnO with10 different formations) on the realized rectenna performance in terms of captured E-field enhancement, I/V , resistivity, and responsivity characteristics. It is found that the overall performance of the proposed double insulator-based rectenna system is better for the asymmetric configuration of Au- Al 2 O 3 - Cu 2 O –Cu and it exhibits 100% enhanced non-linearity as compared to MIM-based configuration. [ABSTRACT FROM AUTHOR]

Remotely driven smart actuator technology by microwave is attractive since it simplifies and reduces the complexity and weight of the remote system. A rectifying antenna (rectenna) array receives and converts microwave power into DC power for actuators, and the power collected from the rectenna array should be accurately allocated and distributed to each actuator. In this research, a new power distribution (PD) logic circuit is studied to control an actuator array effectively. The PD logic circuit was designed and tested to validate it. The preliminary design was tested for a 4 × 4 piezoelectric actuator array with a 16 dual-gate MOSFET array and a computer-controlled 16-channel DAC board. Additionally, power compensation as a remedial approach for a partial power failure of the array was integrated. This PD scheme with a new logic device simplifies the thousands of control cables required for connecting each array element. The performance and limitations of the designed PD circuit are discussed. [ABSTRACT FROM AUTHOR]

In this article, a seven-band omnidirectional rectenna is proposed for the first time to harvest RF energy. The designed rectenna operates in the following practical and up-to-date frequency bands: GSM1800 (1.8 GHz), LTE (2.1 GHz), WLAN/Wi-Fi (2.4 and 5.8 GHz), and 5G bands (2.6, 3.5, and 4.9 GHz). To obtain the multiband rectenna, a novel seven-band rectifier is introduced and composed of three optimized single shunt diode rectifiers in parallel. Overall RF-to- dc conversion efficiencies as high as 44.4% @ 1.84 GHz, 43.9% @ 2.04 GHz, 45.4% @ 2.36 GHz, 43.4% @ 2.54 GHz, 36.1% @ 3.3 GHz, 32.4% @ 4.76 GHz, and 28.3% @ 5.8 GHz are achieved at an input power level of −10 dBm. Moreover, a broadband omnidirectional monopole antenna is further codesigned with elaborate impedance matching to the rectifier. The antenna can harvest RF energy in the bandwidth of 1.67–5.92 GHz ($\text{S}_{\mathbf {11}}$ no more than −10 dB) with stable omnidirectional radiation patterns. By integrating the rectifier and the monopole antenna in a low profile, a prototype of the rectenna was manufactured and tested. The experimental results demonstrate that the proposed rectenna has competitive performance in terms of RF-to- dc conversion efficiency and band coverage, indicating enormous potential for numerous battery-free or low-power-needed applications in the real world. [ABSTRACT FROM AUTHOR]

The dc power collected by radio frequency energy harvesting (RFEH) absorbing surfaces is limited by their physical aperture. Here, a compact, conformable, and ultrathin sub-1 GHz large-area RFEH surface is proposed based on electrically small ($ka=0.58$) “wire-type” rectenna elements, with an effective area exceeding its physical aperture size. Using optimized large-signal complex source tuning, the rectifiers achieve up to 36% measured power conversion efficiency (PCE) at =20 dBm. The proposed 15 cm-diameter six-element array generates a 27.5 $\mu \text{W}$ at 1 V output from a $0.17~\mu \text{W}$ /cm2 incident power density across a 40 $\text{k}\Omega $ load. Owing to its series connection, the voltage output is boosted, and the load resistance dependence is suppressed enabling over 60% of the maximum RF-to-dc PCE to be preserved for loads between 7 and 100 $\text{k}\Omega $ , with an area-normalized figure-of-merit nearly 100% higher than previous arrays. The proposed array is integrated with a commercial DC-DC converter and demonstrated powering a Bluetooth low energy (BLE) wireless sensor node (WSN) from an unprecedented incident power density of 0.25 $\mu \text{W}$ /cm2. A practical demonstration using a commercial 3 W, 915 MHz Powercast source is presented, showing an 11 m operation range with a 1 V output and illustrating the impact of polarization mismatch on the proposed array. [ABSTRACT FROM AUTHOR]

In this communication, the novel low cost hybrid energy harvester combining rectifying antenna with the solar cell for feeding the low power energy systems are reported. The bioinspired leaf shaped monopole antenna is designed to work in the most used communication frequency bands such as GSM-1800, UMTS-2100, WIFI-2.45 and LTE-2.65 GHz for the energy harvesting purposes and microstrip low pass filter is also added on the feeding line for the second harmonic rejection for increasing the efficiency of the harvester. The solar cell is placed on the ground plane of the designed leaf shaped antenna for using volumetric space efficiently and obtain a more compact system. The implemented prototype harvester generate 76 mV, 120 mV and 90 mV when the same type receiver antenna feed from the 25 dBm power from the RF source located at a distance of 1 m which satisfies the far field condition of the antenna without solar cell at the 2.15 GHz, 2.45 GHz and 2.65 GHz frequency bands, respectively. After the illumination of its solar cell component with 165 lux, the total harvested voltages are measured as 150 mV, 170 mV and 190 mV at the same frequency bands, respectively. [ABSTRACT FROM AUTHOR]

This work presents the design, fabrication, and measured results of a fully integrated miniature rectenna using a novel tunnel diode known as the Asymmetrical Spacer Layer Tunnel (ASPAT). The term rectenna is an abbreviation for a rectifying antenna, a device with a rectifier and antenna coexisting as a single design. The ASPAT is the centrepiece of the rectifier used for its strong temperature independence, zero bias, and high dynamic range. The antenna is designed to be impedance matched with the rectifier, eliminating the need for a matching network and saving valuable real estate on the gallium arsenide (GaAs) substrate. The antenna is fully integrated with the rectifier on a single chip, thus enabling antenna miniaturisation due to the high dielectric constant of GaAs and spiral design. This miniaturisation enables the design to be fabricated economically on a GaAs substrate whilst being comparable in size to a 15-gauge needle, thus unlocking applications in medical implants. The design presented here has a total die size of 4 × 1.2 mm2, with a maximum measured output voltage of 0.97 V and a 20 dBm single-tone 2.35 GHz signal transmitted 5 cm away from the rectenna.

In this paper, a dual-band implantable rectenna is proposed for recharging and operating biomedical implantable devices at 0.915 and 2.45 GHz. The rectenna system consists of a compact dual-band antenna based on a meandered-resonator as well as efficient dual-band rectifier circuit. Both components (antenna and rectifier) are integrated inside a capsule device to simulate and experimentally validate the rectenna. The antenna occupies lower volume ( 5 × 5.25 × 0.25 mm 3 ), where compactness is achieved using meandered geometry and a slotted ground plane. It maintains quasi-omnidirectional radiation patterns and peak realized gains of −22.1 dBi (915 MHz) and −19.6 dBi (2.45 GHz); thus, its capability is enhanced to harvest the ambient energy from multiple directions. Moreover, a dual-band rectifier is designed using a dual-branch matching network (an L-matching network and open-circuited stub in each branch) with a radio frequency (RF) to direct current (DC) conversion efficiency of 79.9% for the input power of 1 dBm (lower band: 0.915 GHz) and 72.8% for the input power of 3 dBm (upper band: 2.45 GHz). To validate the concept of the rectenna, the implantable antenna and rectifier are fabricated and attached together inside a capsule device, with the measured results verifying the simulated responses. The proposed rectenna efficiently rectifies two RF signals and effectively superimposes on a single load, thus, providing a distinct advantage compared to single-band rectennas. To the best of the authors' knowledge, this is the first-ever implantable rectenna to perform dual-band RF signal rectification. [ABSTRACT FROM AUTHOR]

The efficiency of a rectenna varies under the influences of receiving polarization tilt angle, operating frequency, input power, and load value in different applications. Yet, no reported rectenna can realize stable efficiency when all these factors vary within a wide range. To solve this problem, an all-polarized rectenna is proposed in this article, composed of a dual-input rectifier and a dual-polarized (DP) antenna. By introducing a novel six-port coupler, the rectifier maintains stable efficiency at wideband, wide input power, and dc load ranges. Moreover, the efficiency of the rectifier keeps stable when the antenna receives power at different polarization tilt angles. Accordingly, a DP antenna with extended bandwidth and 3-dB beamwidth is implemented. Finally, the rectenna is formed. The prototype can maintain beyond 70% of the peak efficiency from 1.7 to 2.5 GHz. At a power density as low as 1 μW/cm2, efficiency can reach 24.4%. Furtherly, the rectenna is demonstrated by forming a battery-less digital clock. The clock can be powered wirelessly by a WiFi router or cellular base station with maximum distances of 1 or 70 m respectively. Besides, the clock can successfully function under significant transmitting-direction deviation or polarization misalignment between the power source and the rectenna. [ABSTRACT FROM AUTHOR]

This communication presents an omnidirectional rectenna array with an enhanced RF power distributing strategy. The proposed rectenna array consists of three bidirectional subarray antennas, a $3\times $ 3 RF power distributing network, and a three-element rectifier array. Utilizing the $3\times $ 3 network to redistribute the varying RF power received by the three antennas, the RF-to-dc power conversion efficiency of the rectenna array can be enhanced under the illumination of electromagnetic (EM) waves with arbitrary incident angles. Measurement results reveal that the proposed rectenna array has an improved omnidirectional-harvesting capability compared with the design without RF power distributing strategy, thus can be readily used for RF energy-harvesting applications. [ABSTRACT FROM AUTHOR]

Radio frequency energy harvesting (RF-EH) is a potential technology via the generation of electromagnetic waves. This advanced technology offers the supply of wireless power that is applicable for battery-free devices, which makes it a prospective alternative energy source for future applications. In addition to the dynamic energy recharging of wireless devices and a wide range of environmentally friendly energy source options, the emergence of the RF-EH technology is advantageous in facilitating various applications that require quality of service. This review highlights the abundant source of RF-EH from the surroundings sources, including nearby mobile phones, Wi-Fi, wireless local area network, broadcast television signal or DTS, and FM/AM radio signals. In contrast, the energy is captured by a receiving antenna and rectified into a working direct current voltage. This review also summarizes the power of RF-EH technology, which would provide a guideline for developing RF-EH units. The energy harvesting circuits depend on cutting-edge electrical technology to achieve significant efficiency, given that they are built to perform with considerably small current and voltage. Hence, the review includes a thorough analysis and discussion of various RF designs and their pros and cons. Finally, the latest applications of RF-EH are presented. [ABSTRACT FROM AUTHOR]

In this paper, a novel dielectric metasurface-inspired multi-beam directional Luneburg lens is proposed as a wireless power transfer medium at 5G mm-wave band. The lens is constructed using dielectric-based unit cells made up of a glide symmetric approach. It is connected with a set of microwave detector integrated multi-port tapered rectangular feeds to convert the received RF energy from different directions to DC power across a combined load. The proposed structure can be a potential candidate to harvest ambient energy from a wide coverage range of around 160° and produce a power conversion efficiency of about 76% for an input power of 14.9 dBm at 24 GHz. [ABSTRACT FROM AUTHOR]

In microwave wireless power receivers, impedance matching networks can maximize power transmission from RF to dc. However, they require circuit components and physical space to implement. To circumvent the problem, we present two compact patch rectennas that do not require matching networks. They comprise a patch antenna and rectifying Schottky diodes which are mounted on the ground plane and connected to the patch antenna through metallic vias. The positions of the vias are chosen in such a way that the input impedance of the antenna and that of the rectifying units are conjugately matched. The two rectenna types are all fabricated and tested: one with one rectifying unit and the other with double rectifying units, respectively. Measurements show that the rectenna with the single rectifying unit has the peak RF-dc conversion efficiency of 77% at 2.45 GHz and the dynamic power range (>60%) is 10 dB. The rectenna with the double rectifying units has the peak conversion efficiency of 74.14% at 2.45 GHz and the dynamic power range (>60%) is 10 dB. Both rectennas are good candidates for integrated microwave wireless power receivers due to the removal of the impedance matching networks and the suppression of harmonic components. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The multibeam antenna can facilitate effective harvesting of RF energy with low power densities, different polarizations, and different incident directions. The use of coupler-based beamforming circuit in the array antenna is a common implementation method. The inevitable coupling between antenna elements is usually an obstacle to compactness due to the separate design of antenna and circuit. In this article, a scheme is proposed to integrate two coupled patches and two hybrid couplers. It exploits the unfavorable coupling in conventional designs and can reduce both the spacing between antenna elements and the redundant circuit of hybrid coupler. A compact four-beam rectenna is then presented with high gains and dual polarizations in enhanced spatial coverage. The complex $J$ inverter, instead of commonly used real one, is introduced to characterize the coupling and reflect its effect on excitation of antennas. With an optimized four-branch rectifier circuit corresponding to the four beams, the rectenna demonstrates a maximum dc power of 54.1– $66.3~\mu \text{W}$ , a maximum RF-to-dc efficiency of 44.3%–52.3%, and a minimum dc power of 24.5– $28.7~\mu \text{W}$ in an azimuth angle range of 90° when the incident RF power is with different linear polarizations and a density of $1~\mu \text{W}$ /cm2. [ABSTRACT FROM AUTHOR]

In this paper, we study multi-user multi-antenna wireless power transfer (WPT) systems, where each antenna at the energy harvesting (EH) nodes is connected to a dedicated non-linear rectifier. We propose optimal transmit strategies which maximize a weighted sum of the average harvested powers at the EH nodes subject to a constraint on the power budget of the transmitter. First, for multiple-input single-output (MISO) WPT systems, we prove that the optimal strategy employs maximum ratio transmission (MRT) beamforming and scalar symbols with arbitrary phases and discrete amplitudes following a probability density function (pdf) with at most two mass points. Then, we prove that for single-input multiple-output (SIMO) WPT systems, the optimal transmit symbol amplitudes are discrete random variables, whose pdf also has no more than two mass points. For general multi-user MIMO WPT, we show that the optimal transmit strategy employs scalar unit-norm symbols with arbitrary phases and at most two beamforming vectors. To determine these vectors, we formulate a non-convex optimization problem and obtain an optimal solution based on monotonic optimization. Since the computational complexity of the optimal solution is high, we propose a low-complexity iterative algorithm to obtain a suboptimal solution, which achieves near-optimal performance. Our simulation results reveal that the proposed transmit strategy for multi-user MIMO WPT systems outperforms baseline schemes based on a linear EH model and a single beamforming vector. For a given transmit power budget, we show that the harvested power saturates when increasing the number of transmit antennas. Finally, we observe that the harvested power region spanned by multiple EH nodes is convex and the power harvested at one EH node can be traded for a higher harvested power at the other nodes. [ABSTRACT FROM AUTHOR]

This paper presents a large, modular X-band rectenna array developed for the planned demonstration of a space-to-Earth power beaming link operating at 10 GHz. The array is composed of 16 tiles assembled into a panel greater than 1 m x 1 m in size. To accommodate tests during the early development and construction of the space-based transmitter, the rectenna cells are designed to maximize performance at low power densities using commercial-off-the-shelf Schottky diodes. To increase efficiency at the expense of some field-of-view, each rectenna cell employs a 4-antenna subarray to feed a single diode. Array performance is validated over a 30-dB range of power densities from 1 mW/m2 to 1 W/m2, reaching a total rectenna efficiency in excess of 39% at 1 W/m2. The rectenna array's field of view, defined by the 3-dB beamwidth of its antenna subarray, is 36° ×36°. The DC power collected by each tile is combined in a reconfigurable manner to control the output voltage level for the protection of electrical loads. These results are expected to directly influence the first demonstrations of space-to-Earth power transfer.

In this work, a dual-polarized wide-angle energy harvester operating at 3.75 GHz is proposed for ambient RF energy harvesting applications. A $4\times 4$ array harvester can efficiently harvest incident RF waves from all polarizations, resulting in high RF-AC efficiency of 94% and final RF-DC efficiency of 34% at 0.02 W/m2 input power density, or −15 dBm input power. This performance is preserved under a wide incident angle up to 65 degrees in TE mode and 75 degrees in TM mode. Owing to its simple power combining configuration, the array can be easily expanded to incorporate more unit cells, increasing the amount of collected power. Not only having the RF-DC conversion function, it is also equipped with a power management circuit to create a usable output voltage level. With this advantage, the harvester is suitable to be employed in real-life applications in which the polarization and angle of incidence are unknown and the incident power is very low.

This paper presents a high-efficiency compact ( $0.016\lambda _{0}^{2}$ ) textile-integrated energy harvesting and storage module for RF power transfer. A flexible 50 $\mu \text{m}$ -thick coplanar waveguide rectenna filament is integrated with a spray-coated supercapacitor to realize an “e-textile” energy supply module. The meandered antenna maintains an $S_{11}< -6$ dB inside and outside the fabric and in human proximity with a 2.3 dBi gain. The rectifier achieves a peak RF-DC efficiency of 80%, across a 4.5 $\text{k}\Omega $ load, and a 1.8 V open-circuit voltage from −7 dBm. The supercapacitor is directly spray-coated on a cotton substrate using carbon and an aqueous electrolyte. When connected to the supercapacitor, the rectifier achieves over an octave half-power bandwidth. The textile-integrated rectenna is demonstrated charging the supercapacitor to 1.5 V (8.4 mJ) in 4 minutes, at 4.2 m from a license-free source, demonstrating a significant improvement over previous rectennas while eliminating power management circuitry. The integrated module has an end-to-end efficiency of 38% at 1.8 m from the transmitter. On-body, the rectenna’s efficiency is 4.8%, inclusive of in-body losses and transient shadowing, harvesting 4 mJ in 32 seconds from 16.6 $\mu \text{W}$ /cm2. It is concluded that e-textile rectennas are the most efficient method for powering wearables from $\mu \text{W}$ /cm2 power densities.

A proposal to use special electron cyclotron devices as effective converters of electromagnetic waves into direct current in modern microwave systems for wireless transmission of electrical energy to the Earth via a microwave channel from solar space power plants located on board geostationary satellites is considered. Such converters are a product of domestic development, they can have a conversion efficiency of more than 80%, they are insensitive to overloads and are several orders of magnitude more economical than the well-known semiconductor rectennas (rectifying antennas). Semiconductor rectennas, assembled from a multitude of individual semiconductor diodes with a Schottky barrier, in the process of nonlinear conversion of microwaves, generate parasitic radiation that forms a powerful electromagnetic background, which seriously interferes with the stable operation of information systems of special and general civil communications. In addition, the cost of semiconductor rectennas is several orders of magnitude higher than that of electron-cyclotron converters with the same input microwave power. Due to the high compactness of the electronic converters, they can also be installed on an intermediate satellite platform in the stratosphere, receiving the energy of the Sun through a laser beam from a geostationary orbit and transmitting it to the Earth with practically no loss through the microwave channel. The possibilities of using electron cyclotron converters in ground-based systems for wireless energy transmission are also promising. Already the first electron cyclotron converters, created at the Torii enterprise according to the project of the Lomonosov Moscow State University, had an efficiency of over 60% at an input microwave power of 10 kW.

A new GaN Nano-Schottky diode rectifier is introduced with capacitance per unit gate width of 381 fF/mm, and series resistance is 0.45 $\Omega \cdot $ mm leading to transition frequency ($f_{\mathrm {T}}$) of 928 GHz. The associated forward current is 1.8 A/mm with breakdown voltage of 30 V. The new device was modeled and an on-wafer voltage doubler rectenna circuit was designed and fabricated on GaN/SiC wafers. A $W$ -band small and large signal characterization was performed to highlight the integrated rectenna performance. An RF-dc efficiency of 61.5% was measured at 95 GHz with associated input power density of 3.61 kW/m2. The latter represents a 15.7% increase in efficiency over the state-of-the-art $W$ -band rectenna circuit. The input power handling has also increased by a factor of 1.5 and 9.5 from reported GaAs and CMOS-based rectenna circuits at 94 GHz. The result highlights key improvements in the rectenna metrics for wireless power beaming demonstrations of the future. [ABSTRACT FROM AUTHOR]

The design of an ambient radio frequency (RF) energy harvesting system is introduced to replace the battery of the mass Internet-of-Things devices. This system can achieve high sensitivity with high efficiency over a wide input power range by optimizing the differential-drive cross-coupled (DDCC) rectifier and the load. The impedance of the dual-band four-port loop antenna array is directly conjugated to match with the rectifier, which reduces insertion loss of the matching network to improve efficiency. At 915 and 945 MHz for ISM and GSM bands, the rectenna can achieve high sensitivity of −30 dBm with 22% and 24% efficiency, high efficiency of 65% and 48% with −20-dBm input power, and a wide input power range from −30 to −18 dBm with 22–65% and 24–48% efficiency. The dual-band design explores the frequency diversity to harvest more RF energy from the environment, while the cross-shaped antenna array utilizes the space diversity to harvest RF energy from different directions with a compact size of $0.15\lambda \times 0.15\lambda $ per antenna element (where $\lambda $ is the free space wavelength at 915 MHz). This new harvesting system can provide output dc power of 21.4 and $2.95~\mu \text{W}$ in real-world outdoor and indoor ambient environments, respectively. [ABSTRACT FROM AUTHOR]

The broad spectrum available at millimeter-wave (mmWave) bands has attracted significant interest for a breadth of applications, with 5G communications being the main commercial drive for mmWave networks. Wireless power transmission and harvesting at mmWave bands have attracted significant attention due to the potential for minimizing the harvesting antenna size, allowing for more compact rectennas. For a fixed antenna size, the received power increases with frequency. Nevertheless, several challenges lie in realizing high efficiency antennas and rectifiers at mmWave bands. This article reviews the recent advances in mmWave rectenna design at a component- and system-level. Low-cost antennas and components for mmWave power harvesting, such as high efficiency scalable rectifiers on polymers and high radiation efficiency antennas on textiles, are reviewed. Both the antenna and rectifier can be realized using low-cost fabrication methods such as additively-manufactured circuits and packages, in addition to digital integrated circuits (ICs) for the rectifiers. Finally, this article provides an overview of future antenna design challenges and research directions for mmWave power harvesting.

Additive manufacturing using direct-write or screen printing represent low-waste methods for fabricating antennas on low-cost flexible substrates. To realize rectennas using low-resolution printing methods, high-impedance antennas with simple printable geometries are required. This article proposes an electrically-small (0.212 × 0.212λ2) folded dipole antenna design with a scalable impedance for directly matching energy harvesting rectifiers. The antenna is demonstrated in a high-efficiency sub-1 GHz rectenna, with varying mesh fill-factors for optical transparency. The proposed solid (non-transparent) and meshed (70%-transparent) rectennas achieve a Power Conversion Efficiency (PCE) of over 70% and 60% from sub-1μW/cm2 power densities, at 940 and 920 MHz, respectively. This represents a 37% improvement in the PCE over state-of-the-art flexible rectennas while maintaining the smallest electrical size and simplest design by not requiring a matching network. The 70%-transparent rectenna's performance is investigated in real-life use-cases showing its suitability for ambient RF energy harvesting with over 500 mV DC output from a phone-call.

The RF to dc conversion efficiency of a rectenna often varies dramatically with the operating frequency, input power, and polarization tilt angle. To stabilize the efficiency, this article presents a polarization-independent rectifier, which employs a novel six-port Lange-based coupler with a grounded isolation point, as well as four subrectifiers. By the adoption of the novel six-port Lange-based coupler, the rectifier not only achieves stable efficiency at varied polarization tilt angles, but also widens operating frequency and input power ranges. In this case, when developing a dual-polarized rectenna with such a rectifier, the rectenna can receive incident energy at any polarization tilt angles with stable efficiency. For verification, the mechanism of the proposed rectifier is analyzed and a prototype is implemented. Measured results show that the rectifier operates efficiently at a frequency band from 1 to 2.7 GHz and an input power range beyond 18.5 dB. Furthermore, when the polarization tilt angle varies from 0° to 90°, the rectifier can work stably. [ABSTRACT FROM AUTHOR]

In this paper, a low-cost rectenna based on an antipodal Vivaldi antenna (AVA) with elliptical slots (AVA-ES) is developed and analyzed. The design of AVA-ES, when compared to a reference AVA, presents significant improvements in directivity and gain (58% at 2.45 GHz for both), evidencing its advantages for using in rectennas. The proposed rectenna prototype presents satisfactory results at 2.45 GHz, such as conversion efficiency for low-power input levels (27% at − 10 dBm) and maximum output voltage (550 mV), that supports its use for radio-frequency energy harvesting (RFEH). [ABSTRACT FROM AUTHOR]

This article presents a textile antenna for dual-band simultaneous wireless information and power transfer (SWIPT). The antenna operates as a 2.4 GHz off-body communications antenna and a sub-1 GHz (785–875 MHz) broad-beam rectenna. Incorporated within the broadside microstrip antenna is a high-impedance rectenna for sub-1 GHz power harvesting. Utilizing antenna-rectifier co-design, the rectenna eliminates the rectifier matching network. The textile antenna is fabricated on a felt substrate and utilizes conductive fabrics for the antenna. At 2.4 GHz, the antenna achieves a realized gain of 7.2 dBi on a body phantom and a minimum radiation efficiency of 63%, with and without the rectifier. The rectenna achieves a best-in-class RF to dc efficiency of 62% from 0.8 $\mu \text{W}$ /cm2, representing over 25% improvement over state-of-the-art textile rectennas and demonstrating that SWIPT does not detrimentally affect the energy harvesting or communications performance. The antenna/rectenna occupies an electrically small area of $0.213\times 0.19\,\,\lambda ^{2}_{0}$. This antenna is the first dual-band, dual-mode antenna demonstrated on textiles for SWIPT applications and the first dual-band matching network-free SWIPT rectenna. [ABSTRACT FROM AUTHOR]

The multibeam multiport antenna is a promising solution for effectively harvesting wireless low-density power in a wide angle range. The traveling-wave series-fed patch array antenna without the requirement of additional beamforming circuits is a good option. However, its application is limited by its large length or low radiation efficiency. In this work, a periodic circuit model consisting of coupled resonators is proposed and analyzed for enhancing the radiation attenuation. A series-coupled patch array based on the model is then designed. It presents a much shorter configuration and could be well applied in a scenario where a tradeoff between size and gain exists. The dc combining method that accumulates currents of rectifying branches corresponding to multiple beams is adopted in the rectifier design. The rectenna exhibits an enhanced spatial or angular coverage with a simple configuration. In the measurement, the output dc power can be more than 0.5– $32~\mu \text{W}$ in an angle range of 90° in the H-plane and reach a maximum value of 1.1– $50.3~\mu \text{W}$ and a maximum efficiency of 19.5%–44.6% when the power density ranges from 0.05 to $1~\mu \text{W}/$ cm2 at 2.45 GHz. [ABSTRACT FROM AUTHOR]

A novel aperture-coupled patch rectenna backed by a substrate integrated waveguide (SIW) cavity operated at 35 GHz for millimeter-wave power transmission (MMPT) with high conversion efficiency is presented in this article. A four-element SIW patch array with a harmonic suppression function based on the barbell-type defect ground structure (DGS) is proposed as the receiving antenna; thus, the low-pass filter in the rectifying circuit can be omitted. The Schottky diode is parallel-mounted in the rectifying circuit, which has a measured rectifying efficiency of 61.2% with a $500~\Omega $ load when the received power is 100 mW. The maximum conversion efficiency of the rectenna element is 60.9% when the received power is 79.4 mW. Additionally, 4–8-and 16-element rectenna arrays are also investigated to obtain high direct current power. When the power density is 9.2 mW/cm2, the conversion efficiencies of the rectenna element, $2\times2$ array, $2\times4$ array and $2\times8$ array are 50.5%, 44.1%, 41.9%, 38.9%, respectively. The measured conversion efficiency of the $2\times8$ rectenna array at 35 GHz is 49.2% when the input power is 1259 mW. The low profile and easy integration of the studied rectenna will make it easy to expand its use to large-scale arrays for high-power MMPT applications. [ABSTRACT FROM AUTHOR]