With the recent advancement and progress in the field of wireless power transfer (WPT), there is an ever increasing demand for high power transfer efficiency (PTE) of the WPT systems and improved transfer distance for the end-users. However, some existing WPT systems have limited PTE and transfer distance as they take the inductive coupling approach, where the PTE dramatically decreases as the distance between (Tx) and receiver (Rx) coils increases. Alternatively, magnetic resonance coupling (MRC) is used as a mid-range WPT approach, for which the insertion of metamaterials (MTMs) between Tx and Rx coils is exploited to improve efficiency. MTMs are artificially engineered materials that show uncommon electromagnetic properties, such as evanescent wave amplification and negative refractive characteristics, which could be utilized for the enhancement of PTE. In this article, a comprehensive review on recent progresses in the MTM-based WPT systems is reported, where previously reported MTM-based WPT systems are compared in terms of various parameters such as configurations, operating frequencies, dimensions and PTE. Also, the PTEs of these systems were plotted as a function of the normalized transfer distance. This review is expected to provide an insight for understanding the trends of the MTM-based WPT systems and serve as a reference for researchers who work on WPT systems and their applications.

A patch loaded with a complementary split-ring resonator (CSRR) is fabricated on a flexible substrate and folded in a cylindrical shape, forming a self-packaged folded patch antenna with a quasi-omnidirectional radiation pattern. The space inside the cylindrical cavity is electromagnetically shielded by the ground plane of the patch, and therefore electronic circuits can be accommodated in it with little electromagnetic interference (EMI) from the antenna or other external electronics. The CSRR contributes to size reduction. As a test vehicle, a 2.4-GHz ISM-band folded patch antenna is designed, fabricated, and characterized for a wireless capsule endoscope application, where the implemented antenna has a patch length of 10.5 mm (0.11λ ) and a folded cylinder diameter of 10 mm. A 74% size reduction is achieved after CSRR loading. The antenna located at the outermost surface not only functions as an electromagnetic radiator and an EMI shield, but also serve as a mechanical packaging structure.

This letter presents a tunable metamaterial (MTM) slab for high-efficiency wireless power transfer (WPT) in misaligned conditions, which operates at 6.78 MHz, an AirFuel Alliance Standard frequency. The tunable MTM slab is located between transmitter (Tx) and receiver (Rx) coils serving as a tunable lens to help focus the magnetic field toward the place where the Rx coil is located. This tunable slab consists of two types of unit cells which can be independently tuned to have proper negative refractive indices. The experimental results show that the tunable MTM slab effectively focuses magnetic fields, thereby improving the power transfer efficiency (PTE) of WPT even in a misaligned condition.

We report a self-biased, low-profile circulator operating in the Ka-band fabricated by an entirely low-temperature (70 °C) screen-printing approach on printed circuit board (PCB). The magnetic material for the circulator is a barium hexaferrite BaFe12O19 (BaM)/polydimethylsiloxane (PDMS) nanocomposite that exhibits a zero-bias ferromagnetic resonance frequency (fFMR) at 46.6 GHz. Using this material, a microstrip-based circulator operating at 35 GHz is designed, fabricated, and characterized. A combination of mechanical milling, screen printing, photolithography, and electroplating is used for fabricating the circulator, in which the circulator disk (diameter of 2 mm and thickness of 250 μm) is completely embedded in the PCB, realizing a packaging compatible low-profile architecture with the total device area of 33 mm². The measured isolation (IS) and insertion loss (IL) of the fabricated circulator at 35 GHz is 3.9 and 8.7 dB, respectively. When the additional magnetic bias is applied to the circulator using external permanent magnets, the IS and IL performance is improved to be 7.4 and 8.4 dB, respectively. The impact of the loss factors associated with dielectric loss and surface roughness on the device performance is analyzed using High Frequency Structural Simulator (HFSS).

This paper presents copper/cobalt (Cu/Co) multilayer-based low RF loss conductors and their usage in high signal integrity coplanar waveguide (CPW) transmission lines (TLs) and high Q-factor inductors for the applications of millimeter wave bands including a 28-GHz 5G communication band. Low conductor loss, reduced signal dispersion, and low noise figure of the fabricated devices are featured. CPW (TLs) employing 10 pairs of Cu (150 nm)/Co (25 nm) superlattice conductors, the so-called metaconductors, have been designed and fabricated. Their RF characterization has been performed. Experimental results show a broadband RF resistance reduction of up to 50% (max.) in 7-32 GHz (3.5 Octaves), a phase delay improvement of 25.5%, and a thermal noise voltage reduction of 33% compared to reference Cu-based CPW TLs. In comparison with the devices from the other literature, the presented TLs show the best signal integrity performance in Ku, K, and Ka bands. In addition, a high Q-factor planar inductor on a glass substrate has been designed, fabricated, and characterized as an example of high-performance RF passive components, showing record high Q-factors of 54 at 28 GHz and 63.5 (max.) at 29 GHz.

This paper presents a metamaterial (MTM)-integrated high-gain rectenna for RF sensing and energy harvesting applications that operates at 2.45 GHz, an industry, science, medicine (ISM) band. The novel MTM superstrate approach with a three-layered integration method is firstly introduced for rectenna applications. The integrated rectenna consists of three layers, where the first layer is an MTM superstrate consisting of four-by-four MTM unit cell arrays, the second layer a patch antenna, and the third layer a rectifier circuit. By integrating the MTM superstrate on top of the patch antenna, the gain of the antenna is enhanced, owing to its beam focusing capability of the MTM superstrate. This induces the increase of the captured RF power at the rectifier input, resulting in high-output DC power and high entire end-to-end efficiency. A parametric analysis is performed in order to optimize the near-zero property of the MTM unit cell. In addition, the effects of the number of MTM unit cells on the performance of the integrated rectenna are studied. A prototype MTM-integrated rectenna, which is designed on an RO5880 substrate, is fabricated and characterized. The measured gain of the MTM-integrated rectenna is 11.87 dB. It shows a gain improvement of 6.12 dB compared to a counterpart patch antenna without an MTM superstrate and a maximum RF–DC conversion efficiency of 78.9% at an input RF power of 9 dBm. This results in the improvement of the RF–DC efficiency from 39.2% to 78.9% and the increase of the output DC power from 0.7 mW to 6.27 mW (a factor of 8.96 improvements). The demonstrated MTM-integrated rectenna has shown outstanding performance compared to other previously reported work. We emphasize that the demonstrated MTM-integrated rectenna has a low design complexity compared with other work, as the MTM superstrate layer is integrated on top of the simple patch antenna and rectifier circuit. In addition, the number of MTM units can be determined depending on applications. It is highly envisioned that the demonstrated MTM-integrated rectenna will provide new possibilities for practical energy harvesting applications with improved antenna gain and efficiency in various IoT environments.