This article proposes a quad-band multiport harvester that can scavenge ambient radio frequency (RF) energy of available frequency bands (i.e. GSM-900, GSM-1800, 3G and Wi-Fi) efficiently. This design's novelty is the use of frequency-dependent multiple antenna ports that enables the harvester to fully exploit all available frequency bands, spatial diversity, and polarization for maximizing the harvested RF energy at a lower power density level in an ambient environment. Indistinctly, the proposed antenna consists of 8-port which maintain 0.33λ×0.33λ (i.e. wavelength λ calculated at 0.9 GHz in free space) average area per port and has achieved more than 35% relative bandwidth (BW) with a maximum gain of 6.5 dBi to cover the selected frequency bands. Besides, the proposed quad-band rectifier with a multi-stub impedance matching network is feasible for RF harvesting over the available ambient frequency bands. The proposed rectenna's measurement results show that the generated average dc output voltage is increased up to 0.750 V at a low RF input power density of -27 dBm for a load resistance of 2.11 kΩ. The overall dc rectification efficiency of the 8-port pixel harvester is also assessed and depicted to be 66.52% when the resultant RF input power density level is -27 dBm. Measurement in an ambient outdoor and controlled indoor environment is also conducted, demonstrating that the new 8-port pixel RF harvester can produce a dc output voltage of 0.797 V which is greater than some of the reference RF harvesters.

RF power is broadly available in both urban and semi-urban areas and thus exhibits as a promising candidate for ambient energy scavenging sources. In this research, a high-efficiency quad-band rectenna is designed for ambient RF wireless energy scavenging over the frequency range from 0.8 to 2.5 GHz. Firstly, the detailed characteristics (i.e., available frequency bands and associated power density levels) of the ambient RF power are studied and analyzed. The data (i.e., RF survey results) are then applied to aid the design of a new quad-band RF harvester. A newly designed impedance matching network (IMN) with an additional L-network in a third-branch of dual-port rectifier circuit is familiarized to increase the performance and RF-to-DC conversion efficiency of the harvester with comparatively very low input RF power density levels. A dual-polarized multi-frequency bow-tie antenna is designed, which has a wide bandwidth (BW) and is miniature in size. The dual cross planer structure internal triangular shape and co-axial feeding are used to decrease the size and enhance the antenna performance. Consequently, the suggested RF harvester is designed to cover all available frequency bands, including part of most mobile phone and wireless local area network (WLAN) bands in Malaysia, while the optimum resistance value for maximum dc rectification efficiency (up to 48%) is from 1 to 10 kΩ. The measurement result in the ambient environment (i.e., both indoor and outdoor) depicts that the new harvester is able to harvest dc voltage of 124.3 and 191.0 mV, respectively, which can be used for low power sensors and wireless applications.

This article addresses the design and implementation of a novel quad-band electromagnetic (EM) and solar energy scavenging system, ensuring energy harvesting from ambient RF environment with excellent “cold start” power level. The proposed scavenger consists of a single port quad-band rectangular slot antenna, power film solar cell, a quad-band RF- to-DC converter, a microcell power management module, and a battery. The harmonic balance of EM solver is used to design and maximize the RF- to - DC rectification efficiency with the combination of the antenna and the solar cell. The power film solar cell is placed in the middle of the antenna with positive and negative edges connected to the top and bottom layer of the antenna so that total harvested energy passes through the rectifier and forms an ambient hybrid energy harvesting system. One significant benefit of this method is the utilization of the antenna free space for the effective area of the power film. Another important contribution is the employment of multiband antennas for increasing the total ambient RF scavenged energy. Besides, a cost-effective and flexible FR4 substrate and a micropower film solar cell are used to make it conformal and cheaper. The prototype of hybrid harvester demonstrated that with 360 lux ambient light intensity, at the solar cell can generate 0.109 V energy while the harvester can attain an extra 5% - 48% energy with ambient RF input level variation from −15 to −20 dBm. The rectifier circuit achieves 74.5% RF-to-DC rectification efficiency for the value of load resistance 2.7 $\text{k}\Omega $ . These performances depict that the proposed multiband ambient hybrid RF-solar power scavenger can raise the scavenged power level and offer energy multiplicity.

For low input radio frequency (RF) power from −35 to 5 dBm, a novel quad-band RF energy harvester (RFEH) with an improved impedance matching network (IMN) is proposed to overcome the poor conversion efficiency and limited RF power range of the ambient environment. In this research, an RF spectral survey was performed in the semi-urban region of Malaysia, and using these results, a multi-frequency highly sensitive RF energy harvester was designed to harvest energy from available frequency bands within the 0.8 GHz to 2.6 GHz frequency range. Firstly, a new IMN is implemented to improve the rectifying circuit’s efficiency in ambient conditions. Secondly, a self-complementary log-periodic higher bandwidth antenna is proposed. Finally, the design and manufacture of the proposed RF harvester’s prototype are carried out and tested to realize its output in the desired frequency bands. For an accumulative −15 dBm input RF power that is uniformly universal across the four radio frequency bands, the harvester’s calculated dc rectification efficiency is about 35 percent and reaches 52 percent at −20 dBm. Measurement in an ambient RF setting shows that the proposed harvester is able to harvest dc energy at −20 dBm up to 0.678 V.

Over the last few decades, far-field RF-based wireless power beaming (WPB) has been a topic of interest for the microwave and the antenna engineering communities alike, and there have been quite a few articles published in the IEEE Microwave Magazine related to this topic in the last 20 years, which is evidence of the high interest of the community. This interest is likely due to the dire need for such a system for applications, ranging from biomedicine to drone charging [1] , [2] , [3] , [4] , [5] . While these prior published articles have been a great resource for new researchers in this area, who can gather tons of resources and information by reading these articles, we (the authors of this article) realized a comprehensive review is still lacking, where the challenges and opportunities regarding WPB to dynamically and rapidly moving targets [e.g., unmanned aerial vehicles (UAVs)] using multiple, distributed transmitters (TXs) are summarized and discussed in detail. We envision that the next generation of wireless power transfer (WPT) networks will be scalable, safe, and efficient, enabling a robust RF energy web that can be deployed using mobile autonomous platforms, such as unmanned aerial systems, as depicted in Figure 1 .

This paper presents the design of fully metallic 3D Vivaldi antenna that can be used for wireless power transmission applications. The 3D antenna consists of 1) a tapered profile, 2) a rectangular cavity, and 3) a horizontal slot cut that is used as a transition between the cavity and the tapered profile. The proposed antenna design is fabricated using two distinct approaches, the first of which is a 3D metal additive manufacturing (AM scheme) with a sequential material layer addition technique. The second version is based on the CNC milling (CNCM) technique implemented by selectively removing material in a controlled way. The measured gain of the AM and CNCA-based 3D Vivaldi antenna is 4.95 dBi, and 5.70 dBi, respectively. The measured bandwidth (BW) of the AM-based 3D antenna is 4.70 GHz (fractional BW (FBW) of $52.86\%$ ), whereas the CNCM-based 3D antenna is 4.95 GHz (FBW of $56.73\%$ ). Measurement outcomes indicate that the CNCM version of the 3D Vivaldi antenna is $\sim ~1.2x$ more effective than the AM version in terms of realized gain and can be used for metal-based antenna system power telemetry due to its high gain and wide operational bandwidth capability.