Design and Experimental Performance Evaluation of a Single-Layer Polarization-Insensitive Asymmetric Microwave Metasurface Absorber

This work reports on designing and experimentally evaluating an asymmetric metasurface absorber (MA) for wideband and polarization-insensitive operation in the C- and partial X-bands. As the core building block of the radio frequency (RF) subsystem, a unit-cell has been proposed, comprised of a dual-cut square-ring resonator (SRR) and a square patch placed above an FR4 substrate backed by a copper plate, inherently anisotropic. This arrangement efficiently converts linearly-polarized waves to cross-polarized reflected waves, achieving an 80% conversion efficiency over a wide bandwidth (BW) of 4.72–8.49 GHz. The reflected cross-polarized waves are also inhibited by strategically incorporating just three resistors on the top surface. Therefore, the polarization converter (PC) is transformed into an MA. In contrast to previously known MAs that depended on structural symmetry to maintain stable absorption performances across all polarization angles, this newly proposed asymmetric MA breaks that constraint. It achieves consistent absorption irrespective of variations in the polarization angle of the normal incident waves. The full-wave simulations have resulted in over 80% absorption, closely resembling the initial PC’s reflected BW. This MA has a compact assembly with a size of 0.22λ × 0.22λ and a thickness of 0.07λ. Absorption outputs are experimentally evaluated, resulting in more than 80% absorption, covering almost the entire C-band. This device can be used in different applications, such as radar cross-section assessments and energy-harvesting front-ends

Systematic design of transmission-type polarization converters comprising multilayered anisotropic metasurfaces

A simple but efficient approach for the synthesis of transmission-type wideband polarization converters is presented. The proposed configuration comprises multilayer metasurfaces including resonant particles that are progressively rotated layer by layer. The progressive rotation of the particles allows for a polarization conversion over a large frequency band. The polarizing structure is efficiently designed and optimized through a transmission-line-model approach handling the cascade of anisotropic impedance layers and dielectrics. An optimized eight-layer design based on gradually rotated dipole resonators is presented as a proof of concept. The results obtained through the efficient transmission-line model are compared with full-wave simulations once the structure is optimized, showing satisfactory agreement. A prototype of the wideband polarization converter is fabricated and measured

A Wide-angle Multi-Octave Broadband Waveplate Based on Field Transformation Approach

J.Z. acknowledge the support from the National Nature Science Foundation of China (61571218, 61571216, 61301017, 61371034, 61101011), and the Ph.D. Programs Foundation of Ministry of Education of China (20120091110032, 20110091120052). Y. H. acknowledge the support from the UK EPSRC under the QUEST Programme Grant (EP/I034548/1)

Wideband Cross Polarization Rotation based on Reflective Anisotropic Surfaces

This paper presents a new design to broaden polarization conversion ratio (PCR) bandwidth utilizing reflective surfaces. The proposed design is based on anisotropic surfaces for linearly polarized electromagnetic waves. The combination of a traditional two-corner-cut square patch and a two-layer substrate with defected ground structure contributes to PCR bandwidth expansion and size reduction. The experimental results indicate that PCR fractional bandwidth is higher than 121% in 5.4-22 GHz band for both x- and y-polarized waves and the conversion efficiency is greater than 90%. In addition, the proposed structure is approximately robust under oblique incidences, which verifies the applicability of the structure in a practical environment. The experimental results are in excellent agreement with simulated ones. The reflective surface with wideband PCR can be utilized in various practical applications, such as radiometer, reflector antennas, remote sensors, and imaging sensors

Radiation characteristics enhancement of planar structures using metasurfaces

In this thesis, the focus is on utilizing metasurfaces to improve radiation characteristics of planar structures. The study encompasses various aspects of metasurface applications, including enhancing antenna radiation characteristics and manipulating electromagnetic (EM) waves, such as polarization conversion and anomalous reflection. The thesis introduces the design of a single-port antenna with dual-mode operation, integrating metasurfaces. This antenna serves as the front-end for a next-generation tag, functioning as a position sensor with identification and energy harvesting capabilities. It operates in the lower European Ultra-Wideband (UWB) frequency range for communication/localization and the UHF band for wireless energy reception. The design aims for a low-profile stack-up that remains unaffected by background materials. Researchers worldwide are drawn to metasurfaces due to their EM wave manipulation capabilities. The thesis also demonstrates how a High-Impedance Surface (HIS) can enhance the antenna's versatility through metasurface application, including conformal design using 3D-printing technology, ensuring adaptability for various deformation and tracking/powering scenarios. Additionally, the thesis explores two distinct metasurface applications. One involves designing an angularly stable super-wideband Circular Polarization Converter (CPC) operating from 11 to 35GHz with an impressive relative impedance bandwidth of 104.3%. The CPC shows a stable response even at oblique incidences up to 40 degrees, with a Peak Cross-Polarization Ratio (PCR) exceeding 62% across the entire band. The second application focuses on an Intelligent Reflective Surface (IRS) capable of redirecting incoming waves in unconventional directions. Tunability is achieved through an artificially developed ferroelectric material (HfZrO) and distributed capacitive elements (IDC) to fine-tune impedance and phase responses at the meta-atom level. The IRS demonstrates anomalous reflection for normal incident waves. These innovative applications of metasurfaces offer promising advancements in antenna design, EM wave manipulation, and versatile wireless communication systems

Performance Enhancement of Radiation and Scattering Properties of Circularly Polarized Antennas Using Frequency Selective Surface

At millimetre-wave (MMW) frequencies, losses associated with wireless link and system are critical issues that need to be overcome in designing high-performance wireless systems. To compensate the overall loss in a wireless communication system, a high-gain antenna is required. Circularly polarized (CP) antennas are among preferred choices to design because they offer many advantages due to their good resistance to polarization mismatch, mitigation of multipath effects, and some phasing issues and immunity to Faraday rotation. On the other hand, frequency selective surface (FSS) technology is recently employed to enhance the performance of radiation and scattering properties of antennas used in different sectors such as aerospace, medical, and microwave industry. Therefore, it is appropriate and attractive to propose the use of FSS technology to design practical and efficient CP antennas. CP Fabry-Perot cavity (FPC) antennas based on FSS are investigated in this thesis to fulfil the growing demand for broadband high-gain antennas with low radar cross section (RCS). The thesis investigates both characteristic improvement of CP antennas and RCS reduction issues employing FSS structures. Initially, a high gain CP dielectric resonator (DR) antenna is proposed. Using an FSS superstrate layer, a gain enhancement of 8.5 dB is achieved. A detailed theoretical analysis along with different models are presented and used to optimize the superstrate size and the air gap height between the antenna and superstrate layer. The second research theme focusses on developing an effective approach for mitigating the near-field coupling between four-port CP antennas in a Multiple-Input, Multiple-Output (MIMO) system. This is obtained by incorporating a two-layer transmission-type FSS superstrate based on planar crossed-dipole metal strips. Another technique for suppressing the spatially coupling between DR antennas using a new FSS polarization-rotator wall is studied as well. The coupling reduction is achieved by embedding an FSS wall between two DRAs, which are placed in the H-plane. Utilizing this FSS wall, the TE modes of the antennas become orthogonal, which reduces the spatially coupling between the two DRAs. The third research theme of this thesis is to enhance the purity and bandwidth of CP with the least amount of insertion loss by the use of an LP-to-CP-polarizer which is based on multilayer FSS slab. This polarizer is approximately robust under oblique illuminations. To have a high-gain CP antenna, an 8-element LP array antenna with Chebyshev tapered distribution is designed and integrated with the polarizer. Eventually, in order to enhance the scattering property, the fourth research theme investigates on RCS reduction by the use of two different approaches which are based on FSS. Initially, a wideband FSS metasurface for RCS reduction based on a polarization conversion is proposed. To distribute the scattered EM waves and suppress the maximum bistatic RCS of the metasurface over a broad band of incident angles at both polarizations, the elements are arranged using the binary coding matrix achieved by group search optimization (GSO) algorithm. The reflective two-layer metasurface is designed in such a way to generate reflection phase difference of 180° between two elements “0” and “1” on a broad frequency band. A theoretical analysis is performed on the ratio of the “0” and “1” elements using Least Square Error (LSE) method to find the best ratio value. As the second activity of this research theme, wideband CP antenna with low RCS and high gain properties is presented. The proposed antenna is based on a combination of the FPC and sequential feeding technique

Scattering Fields Control by Metamaterial Device Based on Ultra-Broadband Polarization Converters

We proposed a novel ultra-broadband meta¬material screen with controlling the electromagnetic scat¬tering fields based on the three layers wideband polariza¬tion converter (TLW-PC). The unit cell of TLW-PC was composed of a three layers substrate loaded with double metallic split-rings structure and a metal ground plane. We observed that the polarization converter primarily per¬formed ultra-broadband cross polarization conversion from 5.71 GHz to 14.91 GHz. Furthermore, a metamaterial screen, which contributed to the low scattering charac¬teristics, had been exploited with the orthogonal array based on TLW-PC. The near scattering electronic fields are controlled due to the change of phase and amplitude for incident wave. The metamaterial screen significantly exhibited low scattering characteristics from 5.81 GHz to 15.06 GHz. To demonstrate design, a metamaterial device easily implemented by the common printed circuit board method has been fabricated and measured. Experimental results agreed well with the simulated results

Wideband RCS reduction based on a simple chessboard metasurface

To avoid being detected by radar, it is necessary to reduce stealthy military platforms' radar cross section (RCS). The operation of overlaying the metasurface (MS) on the targets is a good solution. A simple chessboard MS structure that can achieve low RCS over a large bandwidth is proposed. Only one unit cell is used to construct the MS. First, the unit cell working in 0.5 and 1−λ modes is designed to achieve a stable phase difference of 180° for y- and x-polarized waves. Then, the unit cells and rotated ones are used to form a chessboard structure with different distributions. The compared results show that the chessboard MS with 2 × 2 quadrants can facilitate the widest 10 dB RCS reduction band of 111% and the largest RCS reduction. The proposed structure exhibits excellent RCS reduction even when irradiated by y- and x-polarized waves at an oblique incidence of 30°