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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.

In this article, a comprehensive review specifically on the applications of rectennas for different wireless systems has been presented. Rectenna systems are used to convert radio-frequency (RF) energy into dc signals. Attracting ever-increasing interest among researchers, the rectenna has occupied a unique position in RF and microwave engineering systems. During the last four decades, the research on rectenna design has been focused on improving the power conversion efficiency (PCE) and compatibility while reducing the design complexity as well. This review article summarizes the overall rectenna research carried out for different applications, viz., wireless power transmission (WPT), wireless sensor networks (WSNs), solar power transmission (SPT), medical implants, and wireless energy-harvesting (WEH) systems. Such a review article has rarely been proposed in the open literature to the best of the authors’ knowledge. The main objectives of the article are to 1) accommodate the key design requirements of the rectenna, 2) study the potential application of rectennas and their possible configurations to achieve desired performance characteristics, and 3) to present a comparative analysis of the performance parameters of the different reported works. [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]

This work describes a novel fully integrated rectenna circuit using tunnelling-based devices for implanted medical devices. An ASPAT (Asymmetric Spacer Layer Tunnel Diode) was used as the active rectifier due to its high non-linearity and temperature insensitivity features. A miniaturized geometry rectenna ($1 \times 5$ mm2) with improved matching characteristics was demonstrated, by integrating a Cockcroft-Walton rectifier with an L-shaped planar folded antenna structure operating at ISM frequency bands. The circuit performance was experimentally explored at various separation distance between transmitter and receiver units. For a 5cm transmission set-up, the rectenna with a single-stage rectifier delivered 0.8V output at 20dBm transmit power. An extended doubler configuration exhibited enhanced performance when multiple stages are used, is predicted to reach 0.24mW output power at 23dBm transmit power and yielding ~1.6V output voltage with an efficiency of 0.12%. These findings can assist in compensating for the degraded antenna gain attributed to the extremely small effective-radiating area of $0.04\lambda $. Furthermore, the ability of controlling the antenna input impedance helps in circumventing the requirement for a matching circuitry thereby offering further reduction in chip size. [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]

In this letter, a compact dual-band rectenna is modeled and developed for low radio frequency (RF) power harvesting. The proposed theoretical model can provide a comprehensive analysis of the rectenna, including the power conversion efficiency (PCE) of the diode, matching efficiency of the rectifier, and total PCE of the rectenna. In the rectenna design, a very compact omnidirectional dielectric resonator antenna (DRA) is proposed with a broad bandwidth exceeding 40%. Meanwhile, a dual-band rectifier is designed based on a single branch in the low-power range with bandwidths covering the two fifth-generation (5G) frequency bands in China (2.515–2.675 and 3.4–3.6 GHz). It was found that the theoretical model can show a high accuracy with the calculation error within 5% in analyzing the dual-band rectifier. The DRA and the dual-band rectifier are integrated to form a compact dual-band rectenna. It shows a higher PCE than those of the existing multiband designs working at similar low-power levels. It is hoped to be implemented in 5G-enabled Internet of Things (IoT) applications for powering wireless sensor nodes. [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]

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 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.

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. [ABSTRACT FROM AUTHOR]

A dual-band UHF/industrial, scientific and medical (ISM) compact rectenna with wideband UHF matching (855–935 MHz) covering all, EU, U.S., and Chinese UHF radio frequency identification (RFID), standards is presented. The proposed design consists of a compact single-port, dual-band antenna, a three-port dual-branch matching network, and two voltage doubler rectifiers implemented with Schottky diodes and terminated with a common 10 kΩ load. For each band, the measured RF-to-dc efficiency is around 60% for a −5 dBm unmodulated signal. The wideband behavior of the rectenna supports harvesting of multitone signals. The use of a six-tone signal increases the maximum RF-to-dc efficiency of the dual-band rectifier, from approximately 60% for a single-tone UHF signal (890 MHz) to 80% for a six-tone signal for a received RF power of −5 dBm in both cases. A signal generator is used to generate a six-tone signal of equally weighted and equally spaced (10 MHz separation) monochromatic tones, with a total bandwidth of 50 MHz. The measurement results demonstrate that the dc voltages rectified from the two separate RF signals are effectively superimposed on the load, providing a clear advantage compared to single-band rectennas. The single-layer, compact design with a size of 80 × 45 mm2, with a high RF-to-dc efficiency, is an excellent candidate for wirelessly powered RFID tags. [ABSTRACT FROM AUTHOR]

An electrically small, single-substrate Huygens dipole rectenna with exceptional physical and radiation performance characteristics is reported. A highly efficient rectifier circuit is seamlessly integrated with an ultrathin, electrically small, Huygens dipole antenna (HDA) on a single piece of Rogers 5880 substrate. It consists of two metamaterial-inspired near-field resonant parasitic (NFRP) elements, an Egyptian axe dipole (EAD) and a capacitively loaded loop (CLL) that are etched on the top and bottom metallization layers of the substrate, respectively. A printed receiving dipole is amalgamated tightly with the rectifier on the CLL layer. This ultracompact rectenna system has a large electromagnetic wave capture capability and achieves nearly complete conversion of the incident energy into dc power. The HDA prototype has a realized gain of 4.6 dBi and a half power beamwidth (HPBW) greater than 130°. The entire rectenna is electrically small with ka =0.98, is low cost and easy to fabricate, and has a measured 88% ac-to-dc conversion efficiency. The developed rectenna system is the ideal candidate for ultracompact far-field wireless power transfer (WPT) applications. [ABSTRACT FROM AUTHOR]

In this paper, we study simultaneous wireless information and power transfer (SWIPT) systems employing practical non-linear energy harvester (EH) circuits. Since the voltage across the reactive elements of realistic EH circuits cannot drop or rise instantaneously, EHs have memory which we model with a Markov decision process (MDP). Moreover, since an analytical model that accurately models all non-linear effects and the unavoidable impedance mismatch of EHs is not tractable, we propose a learning based model for the EH circuit. We optimize the input signal distribution for maximization of the harvested power under a constraint on the minimum mutual information between transmitter (TX) and information receiver (IR). We distinguish the cases where the MDP state is known and not known at TX and IR. When the MDP state is known, the formulated optimization problem for the harvested power is convex. In contrast, if TX and IR do not know the MDP state, the resulting optimization problem is non-convex and solved via alternating optimization, which is shown to yield a limit point of the problem. Our simulation results reveal that the rate-power region of the considered SWIPT system depends on the symbol duration, the EH input power level, the EH impedance mismatch, and the type of EH circuit. In particular, a shorter symbol duration enables higher bit rates at the expense of a significant decrease in the average harvested power. Furthermore, whereas half-wave rectifiers outperform full-wave rectifiers in the low and medium input power regimes, full-wave rectifiers are preferable if the input power at the EH is high. [ABSTRACT FROM AUTHOR]

Radio-frequency (RF) energy harvesting (RFEH) and radiative wireless power transfer (WPT) have attracted significant interest as methods of enabling battery-free sustainable wireless networks. Rectifying antennas (rectennas) are the cornerstone of WPT and RFEH systems and critically affect the amount of dc power delivered to the load. The antenna element of the rectenna directly impacts the radiation-to-ac harvesting efficiency, which can vary the harvested power by orders of magnitude. In this article, antenna designs employed in WPT and ambient RFEH applications are reviewed. Reported rectennas are categorized based on two main criteria: the antenna-rectifier impedance bandwidth and the antenna's radiation properties. For each criterion, the figure of merit (FoM) is identified for different applications and reviewed comparatively. [ABSTRACT FROM AUTHOR]

This article presents a new compact lightweight radio frequency (RF) energy harvesting system. The system relies on a dual-tapered transmission line-based matching network that stretches the rectification capability of an integrated Schottky diode. This topology is demonstrated on a 2.4 GHz rigid harvesting system with a resulting power conversion efficiency that reaches up to 58% over 0 dBm input RF power. Its performance is compared with a reference rectifier that relies on a typical open circuit shunt-stub matching network. The rectifier along with a miniaturized monopole antenna is then tailored for a flexible substrate with a resulting efficiency around 50% at 0 dBm input power. The rectifier exhibits an almost flat efficiency over the 2.3–2.5 GHz frequency span despite wide load variations. The system is characterized in multiple bent configurations featuring high and stable performance. It is demonstrated that for different bent states, the proposed flexible harvester does not display large variations in harvested power. Thus, the presented rectenna demonstrates the remarkable combination of compactness, flexibility, and stability. Equipped with these features, such rectifier can be plugged into a variety of sensors, even on wearable surfaces, which makes it ideal for Internet of Things (IoT) applications. [ABSTRACT FROM AUTHOR]

In this paper, we study information-theoretic limits for simultaneous wireless information and power transfer (SWIPT) systems employing practical nonlinear radio frequency (RF) energy harvesting (EH) receivers (Rxs). In particular, we consider a SWIPT system with one transmitter that broadcasts a common signal to an information decoding (ID) Rx and multiple EH Rxs. Owing to the nonlinearity of the EH Rxs’ circuitry, the efficiency of wireless power transfer depends on the waveform of the transmitted signal. We aim to answer the following fundamental question: What is the optimal input distribution of the transmit signal waveform that maximizes the information transfer rate at the ID Rx conditioned on individual minimum required direct-current (DC) powers to be harvested at the EH Rxs? Specifically, we study the conditional capacity problem of a SWIPT system impaired by additive white Gaussian noise subject to average-power (AP) and peak-power (PP) constraints at the transmitter and nonlinear EH constraints at the EH Rxs. To this end, we develop a novel nonlinear EH model that captures the saturation of the harvested DC power by taking into account not only the forward current of the rectifying diode but also the reverse breakdown current. Then, we derive a novel semi-closed-form expression for the harvested DC power, which simplifies to closed form for low input RF powers. The derived analytical expressions are shown to closely match circuit simulation results. We solve the conditional capacity problem for real- and complex-valued signalling and prove that the optimal input distribution that maximizes the rate-energy (R-E) region is unique and discrete with a finite number of mass points. Furthermore, we show that, for the considered nonlinear EH model and a given AP constraint, the boundary of the R-E region saturates for high PP constraints due to the saturation of the harvested DC power for high input RF powers. In addition, we devise a suboptimal input distribution whose R-E tradeoff performance is close to optimal. All theoretical findings are verified by numerical evaluations. [ABSTRACT FROM AUTHOR]

An arm-implantable rectenna, supported by a compact planar inverted F-antenna (PIFA) and a rectifier, is proposed for wireless data telemetry and power transfer in the Medical Device Radiocommunications Service (401–406 MHz) and industrial, scientific, and medical (ISM) (902.8–928 MHz) bands. The rectifier is integrated into the system by using the antenna’s ground plane delivering, thus, a robust solution for the implanted devices. Each development stage is theoretically analyzed first and then experimentally tested. The antenna size is $16 \times 14 \times 1.27$ mm3. The slit/slot loading techniques applied onto the radiator offer the antenna size reduction and the dual-band operation. PIFA’s resonance stability is shown and its radiation and safety responses are addressed. Further, an arm-attached matching layer (ML) is proposed to enhance the wireless power link. Then, an analysis of rectifier circuits is conducted in order to obtain optimum conversion efficiency at low input powers. By following an attentive design process, the final system (rectenna) is built and tested in whole; the measurements verify our approach. [ABSTRACT FROM AUTHOR]

Ambient electromagnetic and RF energy is an ubiquitous energy resource that is found everywhere but difficult to harvest because of the time-varying orientation of incoming radiations and the low efficiency of RF rectifiers resulting from a low input power density operation. In response to these two challenges that come with the design of rectennas for ambient energy harvesting, this paper discusses a figure of merit to evaluate the rectenna performance that combines the rectification efficiency, the radiation efficiency, and the spherical coverage of the antenna. To illustrate this proposed figure of merit, a rectenna with a strengthen RF-to-dc efficiency and also a full spatial coverage is designed using beam-forming networks. The resulting system is demonstrated for a wireless sensor mounted on a pole. To maintain a reasonable size of the prototype, a miniaturization technique of a $4 \times 4$ Butler matrix (BM) is investigated, which allows for the reduction of the footprint of the BM by 2 compared to its classical microstrip design counterpart at 2.4 GHz. The overall rectification efficiency and dc power patterns are reported for the elevation plane of one multidirectional rectenna at 2.4 GHz for an incident power density of $0.45\,\,\mu \text {W}\cdot \text {cm}^{-2}$. It is shown that the system can triple the dc output power compared to a three-dipole counterpart occupying the same area. Furthermore, while the maximum harvesting capability at $1 \,\,\mu \text {W}\cdot \text {cm}-^{2}$ does not exceed $28\pi $ %.steradian in the literature, it reaches $43.6\pi $ %.steradian with the proposed demonstrator. [ABSTRACT FROM AUTHOR]

An impedance matching network inserted between an antenna and a rectifier is essential for rectennas, but few studies have characterized the insertion loss of the matching network if nonideal matching elements are considered. In general, the matching elements, including lumped capacitors, lumped inductors, and transmission lines, are approximated as lossless; if the overall conversion efficiency decreases, the underlying reason seldom attributes to the loss of the matching network. In this paper, we derive the insertion loss of the matching network comprising actual lumped elements and transmission lines. The loss is quantified using the operating power gain of a two-port network, which varies with different levels of input power. When the input power ranges between −30 and 0 dBm, more than 95%, 90%, and 75% of power dissipates at 900 MHz, 1.8 GHz, and 2.4 GHz, respectively, due to the use of actual matching elements. The numerical results are verified by experimental data, and all the analyses demonstrate that the loss of the matching network becomes very serious as the input power is low. These observations are explained using the contour plot of operating power gain. Accordingly, this study provides a theoretical development for rectenna designers. [ABSTRACT FROM AUTHOR]

A compact circularly polarized (CP) rectenna based on wide-slot antenna is designed at 5.8 GHz. The feedback wide-slot antenna is used for its broad band, easy-to-integrate circuitry, simple structure, and low sidelobes. The designed receiving antenna has a bandwidth of 31.8%. In order to minimize the profile of the rectenna, the rectifier is integrated on the back side of the substrate with a matching circuit and a dc-pass filter. It features stable conversion efficiency on three azimuthal planes. The maximum efficiency of 66.2% and an output dc voltage of 6.33 V have been measured with a 900 Ω load at the power density of 11.4 mW/cm2. A 5 × 5 rectenna array is built for large physical aperture and increased output power. The maximum conversion efficiency of 62% is observed with the array. Taking the advantage of immunity to misalignment of the CP antenna, the proposed rectenna and array are suitable to be used in mobile applications for wireless power transfer. [ABSTRACT FROM AUTHOR]

Wireless power transfer (WPT) and wireless energy harvesting (WEH) using rectennas are becoming an emerging technology. This paper presents a novel design method for a rectenna suitable for a wide range of selectable operating frequency band, input power level, and load impedance. Most importantly, it is realized without using complex impedance matching networks which shows significant advantages over existing rectennas in terms of the structure and cost. A rectenna example has been designed, made, and tested using this novel method. The proposed rectenna has a compact size of $90 \times 90 \times 1.58$ mm3 and operates at four different frequency bands that are selectable from 1.1 to 2.7 GHz. Over 60% (up to 85%) energy conversion efficiency is achieved for the input power from 0 to 15 dBm and load impedance between 700 and $4500~\Omega $. The rectenna shows excellent performance for the target applications (WPT and WEH) with a much-simplified structure and reduced cost. [ABSTRACT FROM AUTHOR]

A triple band differential rectenna for RF energy harvesting applications is proposed in this paper. The rectenna is designed to operate in frequency bands of universal mobile telecommunication service (2.1 GHz), lower WLAN/Wi-Fi (2.4–2.48 GHz), and WiMAX (3.3–3.8 GHz). For designing the proposed rectenna, first a differentially fed multiband slot antenna that works as the front-end receiving unit is designed, fabricated, and tested to check its performance. It is observed that a peak antenna gain of 7, 5.5, and 9.2 dBi is achieved at 2, 2.5, and 3.5 GHz, respectively. In the next step, a triple band differential rectifier is designed using the Villard voltage doubler where interdigital capacitors (IDCs) in lieu of lumped components are used. The full rectifier circuit comprising of the rectifying unit and impedance matching circuit is fabricated and tested to check its performance in the desired bands. The peak RF-dc conversion efficiency of 68% is obtained using the three-tone measurement. In the final stage, both antenna and the rectifier circuit are integrated through SMA connecter in order to implement the proposed rectenna. Measurement of the proposed rectenna shows an approximate maximum efficiency of 53% at 2 GHz, 31% at 2.5 GHz, and 15.56% at 3.5 GHz. [ABSTRACT FROM AUTHOR]

A compact circularly polarized rectenna with harmonic suppression for wireless power transmission at 5.8 GHz is proposed in this letter. The substrate integrated waveguide structure is adopted for enhanced gain, unidirectional patterns, and surface wave suppression. A rectifier, which consists of a matching network, a diode, and a dc-pass filter, is integrated with the antenna at the back side. The proposed rectenna is fabricated and measured. It features 6.9 dB gain, 0.67 dB axial ratio, and 30 dB return loss. The conversion efficiency is 65.87% with a 700 Ω load. A rectenna array is also built for an increased aperture. It shows 61.49% conversion efficiency with 650 Ω load in measurement. Both the rectenna and array are satisfactory choices for wireless power transfer usage. [ABSTRACT FROM AUTHOR]

The output power level and the power conversion efficiency (PCE) rate of the energy-harvesting systems are vital factors in realizing effective millimeter-wave wireless power transfer solutions that can power battery-less and charge coil-free smart everyday objects. Two 60-GHz energy harvesters that use a tuned complementary cross-coupled oscillator-like rectifying circuitry in 40-nm digital CMOS process are presented. They harvest energy at power rates and peak PCE levels in excess of 1 mW and 30%. These figures are significantly higher than those of the prior art. In one design, two rectifier stages are in a cascode configuration to produce higher dc output voltage levels at the low-current regime. Novel theoretical analyses of the unique rectifying circuitry of the single-stage and two-stage cascode harvesters are presented, which formulate all of the key specifications. [ABSTRACT FROM AUTHOR]

High-efficiency rectennas for radio frequency (RF) energy harvesting have been studied for decades, but most of the literature straightforwardly applies the rectenna aiming at dedicated RF sources to this situation, even though the level of input power is significantly different. Since previous studies address antenna design collecting more ambient RF power, the improvement of power conversion efficiency (PCE) has emerged in a scattered way, because the theoretical limit of PCE has not yet been characterized, and the optimal rectenna structure approaching such maximum PCE is still uninvestigated. In this paper, we characterize the performance limit of rectennas with input power ranging from −20 to 0 dBm, proposing optimal rectenna design demonstrating the maximum PCE. The maximum achievable PCE is cast into a mathematical programming problem. Solving this optimization model clarifies the effect of design factors, including operational frequencies, rectifier topologies, and parameterization. To achieve the maximum PCE, our investigation shows that the optimal rectenna structure should not only optimize those design factors but also eliminate the matching circuit between an antenna and a rectifier for ultralow-power scenarios. The resultant PCE at 2.45 GHz is 61.4% and 31.8% at −5 and −15 dBm, respectively, closely approaching the theoretical bound. [ABSTRACT FROM PUBLISHER]

Impedance matching networks for nonlinear devices such as amplifiers and rectifiers are normally very challenging to design, particularly for broadband and multiband devices. A novel design concept for a broadband high-efficiency rectenna without using matching networks is presented in this paper for the first time. An off-center-fed dipole antenna with relatively high input impedance over a wide frequency band is proposed. The antenna impedance can be tuned to the desired value and directly provides a complex conjugate match to the impedance of a rectifier. The received RF power by the antenna can be delivered to the rectifier efficiently without using impedance matching networks; thus, the proposed rectenna is of a simple structure, low cost, and compact size. In addition, the rectenna can work well under different operating conditions and using different types of rectifying diodes. A rectenna has been designed and made based on this concept. The measured results show that the rectenna is of high power conversion efficiency (more than 60%) in two wide bands, which are 0.9–1.1 and 1.8–2.5 GHz, for mobile, Wi-Fi, and ISM bands. Moreover, by using different diodes, the rectenna can maintain its wide bandwidth and high efficiency over a wide range of input power levels (from 0 to 23 dBm) and load values (from 200 to 2000 Ω). It is, therefore, suitable for high-efficiency wireless power transfer or energy harvesting applications. The proposed rectenna is general and simple in structure without the need for a matching network hence is of great significance for many applications. [ABSTRACT FROM AUTHOR]

This communication presents a novel hybrid resistance compression technique (HRCT) to improve the performance of broadband rectennas. The HRCT can offer resistance compression characteristic by reducing the nonlinear effects of the rectifier and provide improved impedance matching performance over a wide frequency range. Two ultrawideband rectennas (with/without using the HRCT) have been designed, made, and tested. The proposed rectennas have a very wide bandwidth from 450 to 900 MHz (covers the entire DTV, LTE700, and GSM900 bands in the U.K.) with relatively high conversion efficiency (up to 77%). The HRCT can maintain the high efficiency of the rectenna in a wide load impedance range (from 5 to 80 \textk\Omega ) . The measured harvested dc power of the rectenna from an outdoor ambient environment is about 66.7 \mu \textW with an overall conversion efficiency of 42.2%, which is therefore suitable for wireless energy harvesting to power low power electronic devices with different load impedance. [ABSTRACT FROM PUBLISHER]

A novel six-band dual circular polarization (CP) rectenna for ambient radio frequency (RF) energy harvesting is presented. Due to the nonlinearity and complex input impedance of the rectifying circuit, the design of a multiband and/or broadband rectenna is always challenging and its performance can be easily affected by variation in the input power level and load. Therefore, an improved impedance matching technique is introduced, which is aimed to improve the performance of the rectifier with a varying condition. A broadband dual CP receiving antenna is proposed, which has a very wide bandwidth (from 550 to 2.5 GHz) and a compact size. An annular ring structure and a novel feeding technique are employed in order to reduce the size and improve the antenna performance. As a result, the proposed rectenna is the first design that covers six frequency bands, including part of the digital TV and most cellular mobile and WLAN bands in the U.K., while the optimal load range for a constant conversion efficiency is from 10 to 75 \textk\Omega . The measured results show that the maximum harvested dc power of the rectenna in typical outdoor and indoor environments are 26 and 8 \mu \textW , respectively; it can therefore be applied to a range of low-power wireless applications. [ABSTRACT FROM AUTHOR]

This work presents an efficient and high-sensitivity radio frequency (RF) energy harvesting supply. The harvester consists of a single-series circuit with one double diode on a low-cost, lossy FR-4 substrate, despite the fact that losses decrease RF harvesting efficiency. The design targeted minimum reflection coefficient and maximum rectification efficiency, taking into account not only the impedance matching network, but also the rectifier microstrip trace dimensions and the load. The simulated and measured rectenna efficiency was 28.4% for -\20-dBm power input. In order to increase sensitivity, i.e., ability to harvest energy and operate at low power density, rectennas were connected in series configuration (voltage summing), forming rectenna arrays. The proposed RF harvesting system ability was tested at various input power levels, various sizes of rectenna arrays, with or without a commercial boost converter, allowing operation at RF power density as low as 0.0139 \mu \W/cm^2. It is emphasized that the boost converter, whenever used, was self-started, without any additional external energy. The system was tested in supplying a scatter radio sensor, showing experimentally the effect of input power density on the operational cold start duration and duty cycle of the sensor. [ABSTRACT FROM PUBLISHER]

This letter presents a new rectenna with all-polarization-receiving capability for wireless power transmission applications. By utilizing a dual linearly-polarized antenna, the incident wave of arbitrary polarization can be totally collected at its two ports. To be connected with the antenna as a rectenna, a dual-input rectifier is designed so that the RF power supplied by the two ports of the antenna can be efficiently rectified. With a proper polarization of the incident wave, the proposed rectenna exhibits a maximum efficiency of 78% under the input power density of 295.3 μW/cm2. Moreover, under the same power density, the rectenna's efficiency can always stay higher than 61% regardless of the incident wave's polarization. [ABSTRACT FROM PUBLISHER]

This letter presents an enhanced rectenna with a differentially-driven rectifier for wireless power transmission applications. By utilizing differential feeding, a patch antenna with increased gain is proposed. To be directly connected with the differentially-fed patch antenna as a rectenna, a differentially-driven rectifier is designed so that the use of balun can be omitted. The rectenna shows a maximum efficiency of 73.9% under the input power density of 207 μW/cm2. Furthermore, compared with a reference rectenna of the same aperture size, the proposed rectenna performs higher efficiency and yields larger output DC power, which is more than 1.5 times as high as that of the reference rectenna. [ABSTRACT FROM PUBLISHER]

A subterahertz 303-GHz rectenna was developed and then assessed using a 300-kW-class gyrotron. Measurement of the antenna gain was conducted using finline, which is a waveguide-to-microstrip line (MSL) transducer: gain of 8.32 dBi was obtained. The input power was estimated by the application of the beam profile measured using an IR camera and the beam output power. Results show that the maximum dc output power and rectenna power density (dc output power per unit area) were 17.1 mW and 3.43 kW/ $\text{m}^{2}$ , respectively. The diode breakpoint was observed. The RF-dc conversion efficiency was obtained as 2.17% with 342-mW input power and 130- $\Omega $ dc load. This letter is the cutting edge of high-frequency operation in a microwave MSL-type rectenna. [ABSTRACT FROM AUTHOR]

This paper presents a novel broadband rectenna for ambient wireless energy harvesting over the frequency band from 1.8 to 2.5 GHz. First of all, the characteristics of the ambient radio-frequency energy are studied. The results are then used to aid the design of a new rectenna. A novel two-branch impedance matching circuit is introduced to enhance the performance and efficiency of the rectenna at a relatively low ambient input power level. A novel broadband dual-polarized cross-dipole antenna is proposed which has embedded harmonic rejection property and can reject the second and third harmonics to further improve the rectenna efficiency. The measured power sensitivity of this design is down to - \bf 35\;\bf dBm and the conversion efficiency reaches 55% when the input power to the rectifier is - \bf 10\;\bf dBm. It is demonstrated that the output power from the proposed rectenna is higher than the other published designs with a similar antenna size under the same ambient condition. The proposed broadband rectenna could be used to power many low-power electronic devices and sensors and found a range of potential applications. [ABSTRACT FROM PUBLISHER]

In this paper, rectennas formed from nanodipole antennas terminated by plasmonic metal–insulator–metal (MIM) travelling wave transmission line rectifiers are developed for ambient thermal energy harvesting at 30 THz. The transmission lines are formed from two strips coupled either vertically or laterally. A systematic design approach is presented, that shows how different components can be integrated with each other with maximum radiation receiving nantenna efficiency, maximum coupling efficiency between nantenna and rectifier, and maximum MIM diode rectifier efficiency. The tunneling current of the rectifier is calculated using the transfer matrix method and the nonequilibrium Green's function. A detailed parametric study of the coupled strips plasmonic transmission lines is presented and thoroughly discussed. The overall efficiencies of the proposed travelling wave rectennas are fully expressed and compared. [ABSTRACT FROM PUBLISHER]

A novel transparent ultra-wideband antenna for photovoltaic solar-panel integration and RF energy harvesting is proposed in this paper. Since the approval by the Federal Communications Committee (FCC) in 2002, much research has been undertaken on UWB technology, especially for wireless communications. However, in the last decade, UWB has also been proposed as a power harvester. In this paper, a transparent cone-top-tapered slot antenna covering the frequency range from 2.2 to 12.1 GHz is designed and fabricated to provide UWB communications whilst integrated onto solar panels as well as harvest electromagnetic waves from free space and convert them into electrical energy. The antenna when sandwiched between an a-Si solar panel and glass is able to demonstrate a quasi omni-directional pattern that is characteristic of a UWB. The antenna when connected to a 2.55-GHz rectifier is able to produce 18-mV dc in free space and 4.4-mV dc on glass for an input power of 10 dBm at a distance of 5 cm. Although the antenna presented in this paper is a UWB antenna, only an operating range of 2.49 to 2.58 GHz for power scavenging is possible due to the limitation of the narrowband rectifier used for the study. [ABSTRACT FROM PUBLISHER]

The second annual Student Wireless Energy Harvesting (WEH) Design Competition was held during the 2013 IEEE Microwave Theory and Techniques Society (MTT_S) International Microwave Symposium (IMS2013) in Seattle, Washington, United States. This year, the competition parameters were modified from those of last year [1], and a new figure of merit (FoM) was established. The overall goal of the competition was to demonstrate low-mass hardware that can efficiently receive and rectify extremely low-incident power densities at two frequencies, with a fixed dc load. As the radio-frequency (RF) environment gets more saturated with spurious power, designs from this competition will become a feasible way to energize ultralow-powered or low-duty-cycle hard-to-reach sensors. Concepts such as Internet-of-Things, in which small ubiquitous devices and sensors will log data and send it to the cloud, could benefit from wireless energy harvesters. These sensors will not have convenient ways to stay powered unless power harvesting circuits are used for the sensor hardware. [ABSTRACT FROM PUBLISHER]

In the late 1960s, a new concept was proposed for an infrared absorbing device called a “rectenna” that, combining an antenna and a nanoscale metal–insulator–metal diode rectifier, collects electromagnetic radiation in the terahertz regime, with applications as detectors and energy harvesters. Previous theories hold that the diode rectifies the induced terahertz currents. Our results, however, demonstrate that the Seebeck thermal effect is the actual dominant rectifying mechanism. This new realization that the underlying mechanism is thermal-based, rather than tunneling-based, can open the way to important new developments in the field, since the fabrication process of rectennas based on the Seebeck effect is far simpler than existing processes that require delicate tunnel junctions. We demonstrate for the first time the fabrication of a rectenna array using an efficient parallel transfer printing process featuring nearly one million elements. [ABSTRACT FROM PUBLISHER]