Metamaterial is a kind of material/structure that is artificially designed and has exceptional electromagnetic properties and/or other physical properties, not found in nature. A class of electromagnetic metamaterial with only one or a few layers of periodic or aperiodic arranged cell structures in the direction of electromagnetic waves propagation can be referred to as a metasurface. Metasurface can be considered as a two-dimensional representation of metamaterial and can realize the controlling of the amplitude, phase, and polarization state/direction of the incident electromagnetic wave. According to the novel electromagnetic characteristics of metasurface and its big advantages, a series of new planar devices and systems based on metasurface can be developed. The goal of this review article is firstly to provide introductions for basic metasurface, its significance properties, and application principles. Meanwhile, the main research progresses of regular metasurfaces and the newly developed reconfigurable metasurfaces are analyzed, focusing on the types of amplitude modulation, phase modulation, polarization modulation, and multidimensional modulation. Finally, the research significances of metasurface development trend and important engineering practical applications are analyzed in the end.

This letter presents a new broadband circularly polarized (CP) planar monopole antenna with a simple structure and compact size. Broadband CP operation is achieved by vertically extending one side of the ground plane, while a cross-shaped monopole structure and the presence of a slot in the ground plane assure a broad impedance bandwidth. The antenna is successfully simulated, and a prototype is fabricated. Measurement results indicate that the 3 dB axial-ratio bandwidth is about 42.6% (2.6 GHz, 4.8-7.4 GHz), fully covered by a -10 dB impedance bandwidth of 55.5% (3.35 GHz, 4.36-7.71 GHz). Furthermore, the proposed antenna can be used together with a metallic reflector to obtain directive radiation patterns with enhanced broadside gain.

A high-efficiency inverse class-F microwave rectifier for wireless power transmission is described in this letter. The proposed inverse class-F harmonic termination network is realized by connecting a λ/12 open-ended transmission line in parallel with the diode cathode, and a λ/8 short-ended transmission line in series with the diode anode. This structure yields a compact layout, and measurement results have demonstrated a peak conversion efficiency of 80.4% for an input power of 13 dBm at 2.35 GHz, and an efficiency larger than 60% within the bandwidth 2.25-3 GHz and for a wide range of output loads.

Lifetime is one of the most critical indexes of the Wireless Sensor Network (WSN). In this paper, we propose a clustering protocol based on the meta-heuristic approach (CPMA). CPMA takes the network lifetime as the primary consideration and consists of two parts. The first part focuses on the online cluster head selection and network communication coordination. The selection is based on the Harmony Search (HS) Algorithm, which aims to reduce the total energy dissipation and smooth the energy distribution throughout the network. Currently, most clustering protocols cannot automatically tune the corresponding protocol parameters according to the diversity of different WSNs. To solve such issue, the second part of CPMA uses the Artificial Bee Colony (ABC) algorithm to optimize its crucial parameters. The optimization is offline and will be executed only once before the network is working. We make a detailed comparison of CPMA with classical clustering protocols. The results show that CPMA can better prolong the network lifetime and improve network throughput under almost all the network conditions. Furthermore, our simulation also exhibits that CPMA has good adaptability and performs well under different network lifetime definitions. All the results prove that CPMA has the advantages of being suitable and efficient for a wide number of WSN applications.

In this paper, a two-dimensional (2D) Airy beam is experimentally generated at microwave frequency by using a simple and thin metasurface composed of a single-layer of square C-shaped complementary split ring resonators (C-SRR). It is shown that the cross-polarization transmission coefficient, including the magnitude and phase, of a single C-SRR unit can be controlled by modifying the single C-SRR unit rotation angle, and this property is herein used to reshape the impinging field in accordance with the desired 2D Airy beam profile. The designed metasurface is then fabricated to measure the most significant features of the generated microwave Airy beam, including quasinon- diffraction, self-bending, and self-healing properties, achieving good results in agreement with those predicted by theory. The generated microwave 2D Airy beam with its properties can be widely applied for high-efficiency wireless power transfer, wireless covert communication, and directional communication.

In this article, a Schottky diode-based microwave rectifier that employs coupled transmission lines (CTLs) is proposed. CTL is herein used to enhance the voltage amplitude across the Schottky diode with the consequent increase in the rectifier efficiency, especially for low input powers. It is also shown that multiple cascaded CTLs can further enhance the rectifier conversion efficiency, even if this benefit is partially annihilated by the increased insertion loss. Several compact prototypes with single and dual CTLs for 2.45 GHz have been fabricated and measured for different input power levels. Particularly, a single CTL rectifier exhibits RF-to-dc conversion efficiency of 67.7% for an input power of 0 dBm, leading to an improvement of about 3% with respect to other referred rectifiers (with a dc load of 4.47 kΩ), while dual CTL rectifier has an outstanding efficiency of 75.3% for an input power of only 5.5 dBm (with a dc load of 1.76 kΩ). Passive voltage boost by means of CTLs can be applied to other rectifier circuits as also demonstrated.