Design and development of a multi-functional bi-anisotropic metasurface with ultra-wide out of band transmission

This paper presents a multi-functional bi-anisotropic metasurface having ultra-wide out of band transmission characteristics. The proposed metasurface is comprised of 90° rotated T-shaped configuration yielding greater than or equal to 50% out-of-band transmission from above L- to X-band. Moreover, this metasurface achieves a maximum of 99% out-of-band transmission at lower frequency bands (i.e., L-band). The simultaneous absorptive and controlled reflection functionalities are achieved at 15.028 to 15.164 GHz along with polarization-insensitive and angular stable properties. The proposed metasurface yields state-of-the-art features compared to already published papers and has broader scope for Fabry Perot cavity, Radar cross-section (RCS) reduction, electromagnetic compatibility and interference (EMC/I) shielding, selective multi-frequency bolometers, ultrathin wave trapping filters, sensors and beam-splitters in the microwave domain

A multifunctional ultrathin flexible bianisotropic metasurface with miniaturized cell size

In this paper, a flexible bianisotropic metasurface possessing omega-type coupling is presented. The designed metasurface behaves differently when excited from either forward (port 1) or back (port 2) sides. It provides an absorption of 99.46% at 15.1 Gigahertz (GHz), when illuminated from port 1, whereas, on simultaneous illumination from port 2, it behaves like a partially reflective surface (PRS). Furthermore, the presented metasurface not only acts as an in-band absorptive surface (port 1) and partially reflective surface (port 2), but it also provides 97% out-of-band transmission at 7.8 GHz. The response of the presented metasurface remains the same for both transverse Electric (TE) and transverse magnetic (TM) polarized wave or any arbitrary linearly polarized wave. Additionally, the response of the metasurface is angularly stable for any oblique incidence up to 45º. The proposed ultrathin flexible metasurface with absorption, partial reflection and out-of-band transmission properties can be used in the Fabry Perrot cavity antenna for gain enhancement with radar cross-section (RCS) reduction both for passband and stop-band filtering, and conformal antenna applications

Circularly polarized and reconfigurable frequency selective surface based transmit array antenna for x-band applications

Transmitarray (TA) antennas have attracted much attention in recent years due to their number of applications. These include the 5G/6G mobile networks and satellite communication systems for the microwave frequency range. The various satellite applications require high-gain antennas with polarization agility. Also, the state-ofthe- art smart communication systems require reconfigurable antennas allowing the frequency and beam switching according to the application requirements. In this research, three different TA antennas have been studied and designed for X-band applications which are high gain and wideband TA antenna, circularly polarized TA antenna, frequency and beam reconfigurable TA antenna. For the first design, two Frequency-Selective Surface (FSS) unit cells which include Double Square Ring with Center Patch (DSR-CP) and Split Ring Resonator (SRR), have been applied to increase the antenna gain and bandwidth. The optimized unit cell structure shows that a fourlayer configuration could provide maximum phase range with low insertion losses. The complete DSR-CP TA consisting of 121 elements has produced an impedance bandwidth of 33.3% with a peak gain value of 20.4 dBi and 1-dB gain for bandwidth of 10%. SRR-based TA achieved the impedance bandwidth of 35% with a peak gain value of 21.9 dBi and 11.6% 1-dB gain bandwidth. A circularly polarized TA using a Meander Line Polarizer (MLP) superstrate has been studied and presented. The MLP unit cell was simulated and optimized at 12 GHz, having 900 phase difference between the two orthogonal E-field components, Ex and Ey. The final prototype measurement results show that a low axial ratio of 1.89 and 20.17 dBi gain at 11.2 GHz has been obtained. Finally, the last part of the research focused on the frequency and beam reconfigurable TA antenna. A U-shape superstrate layer has been added to introduce frequency selectivity in front of the horn antenna that acts as a bandpass filter. Then, by varying the strip length of the U-shape unit cell, the antenna frequency can be reconfigured from 8.5 GHz to 11.2 GHz. On the other hand, a new active TA unit cell equipped with four switchable strips using Positive Intrinsic Negative (PIN) diodes has been employed to achieve beam reconfigurable TA antenna. Thus, the antenna beam can be tilted by controlling the PIN diodes ON and OFF switching states. Results show that a full-beam switching range of 43.20 has been obtained. In conclusion, this research has successfully presented three new TA antenna designs, which are highly potential for the X-band applications

Cross-Polarization Control in FSSs by Means of an Equivalent Circuit Approach

This work was supported in part by Spanish Research and Development National Program under Project TIN2016-75097-P, Project RTI2018-102002-A-I00, Project TEC2017-84724-P, Project PID2020-112545RB-C54, and Project EQC2018-004988-P; in part by the Predoctoral under Grant FPU18/01965; and in part by Junta de Andalucia under Project P18-RT-4830 and Project B-TIC-402-UGR18.This paper presents an ef cient equivalent circuit approach (ECA), based on a Floquet modal expansion, for the study of the co- and cross-polarization in frequency selective surfaces (FSS) formed by periodic arrays of patches/apertures in either single or stacked con gurations. The ECA makes it possible the derivation of analytical expressions for the generalized scattering parameters associated with the proposed circuit networks. Furthermore, the proposed circuit approach is an ef cient surrogate model that can be combined with optimization techniques and arti cial intelligence algorithms for the ef cient design of FSS structures, saving efforts in the computation compared to time-consuming full-wave simulators and tedious synthesis (simulation-assisted) techniques. Due to the simplicity of the topology of the involved networks, the ECA can also be advantageously used to gain physical insight. The proposed approach is applied and validated in different FSS con gurations where the cross-pol component plays a fundamental role in the design, as in circular polarizers, polarization rotators, and reflectarray cells.Spanish Research and Development National Program TIN2016-75097-P RTI2018-102002-A-I00 TEC2017-84724-P PID2020-112545RB-C54 EQC2018-004988-P FPU18/01965Junta de Andalucia P18-RT-4830 B-TIC-402-UGR1

Synthesis Technique of Thickness-Customizable Multilayered Frequency Selective Surface for Plasma-Based Electromagnetic Structures

This dissertation provides a synthesis technique for the design of thickness-customizable high-order (N ≥ 2) bandpass frequency selective surface (FSS) and its application in realizing versatile multi-layered FSS and absorbers. Admittance inverters layers are used to synthesize the transfer response of the filter given desired characteristics such as filter type, center frequency, and bandwidth. These inverter layers are essentially electromagnetic coupling interlayers that can be adjusted to customize the thickness of multilayered FSS without degrading the desired filter performance. A generalized equivalent circuit model is used to provide physical insights of the proposed design. This synthesis technique is adopted to deliver a versatile implementation capability of high-order FSS filters using various dielectric spacers with arbitrary thicknesses. Such technique enables the realization of spatial filters with variable size, while maintaining the desired filter response. To highlight the significance of the proposed synthesis technique, its concept is applied to two practical problems including the design of compact switchable FSS and adaptive/tunable microwave absorbers as it may allow simpler integration of active components that require specific physical dimensions. In the first practical problem, the feasibility of deploying plasma switchable compact spatial filter in harsh electromagnetic radiation environments is investigated. The proposed FSS integrates contained plasma (plasma-shells) as active tuning elements. These ceramic, gas-encapsulating shells are ideal for high-power microwave and electromagnetic pulse protection because they are rugged, hermetic, operable at extreme temperatures, and insensitive to ionizing radiation. A 2D periodic second-order switchable spatial filter is implemented. It is composed of electrically small Jerusalem cross structures embedded with discrete plasma shells strategically located to effectively switch the transfer function of the filter. This technique is used to realize compact low profile second order band pass spatial filter operating at S-band. It also has the ability to switch its transfer function within 20 to 100 ns while enabling in-band shielding protection for aerospace or terrestrial electromagnetic systems subjected to high power microwave energy (HPME) and high electromagnetic pulse (HEMP) in harsh space environment. Experimental results are shown to be in good agreement with simulation results. The second practical problem is addressed by deploying a large-scale adaptable compressed Jaumann absorber for harsh and dynamic electromagnetic environments. The multilayered conductor-backed absorbers are realized by integrating ceramic gas-encapsulating shells and a closely coupled resonant layer that also serves as a biasing electrode to sustain the plasma. These active frequency selective absorbers are analyzed using a transmission line approach to provide the working principle and its frequency tuning capability. By varying the voltage of the sustainer, the plasma can be modeled as a lossy, variable, frequency-power-dependent inductor, providing a dynamic tuning response of the absorption spectral band. To study the power handling capability of the tunable absorber, dielectric and air breakdowns within the device are numerically emulated using electromagnetic simulation by quantifying the maximum field enhancement factor (MFEF). Furthermore, a comprehensive thermal analysis using a simulation method that couples electromagnetics and heat transfer is performed for the absorber under high power continuous microwave excitations. The maximum power level handling capability of the microwave absorber has been numerically predicted and validated experimentally

Inverse Design of Three-Dimensional Frequency Selective Structures and Metamaterials using Multi-Objective Lazy Ant Colony Optimization

With the rise of big data and the “internet of things,” wireless signals permeate today’s environment more than ever before. As the demand for information and security continues to expand, the need for filtering a crowded signal space will become increasingly important. Although existing devices can achieve this with additional components, such as in-line filters and low noise amplifiers, these approaches introduce additional bulk, cost and complexity. An alternative, low-cost solution to filtering these signals can be achieved through the use of Frequency Selective Surfaces (FSSs), which are commonly used in antennas, polarizers, radomes, and intelligent architecture. FSSs typically consist of a doubly-periodic array of unit cells, which acts as a spatial electromagnetic filter that selectively rejects or transmits electromagnetic waves, based on the unit cell’s geometry and material properties. Unlike traditional analog filters, spatial filters must also account for the polarization and incidence angle of signals; thus, an ideal FSS maintains a given frequency response for all polarizations and incidence angles. Traditional FSS designs have ranged from planar structures with canonical shapes to miniaturized and multi-layer designs using fractals and other space-filling geometries. More recently, FSS research has expanded into three-dimensional (3D) designs, which have demonstrated enhanced fields of view over traditional planar and multi-layer designs. To date, nearly all FSSs still suffer from significant shifts in resonant frequencies or onset of grating lobes at incidence angles beyond 60 degrees in one or more polarizations. Additionally, while recent advances in additive manufacturing techniques have made fully 3D FSS designs increasingly popular, design tools to exploit these fabrication methods to develop FSSs with ultra-wide Fields of View (FOV) do not currently exist. In this dissertation, a Multi-Objective Lazy Ant Colony Optimization (MOLACO) scheme will be introduced and applied to the problem of 3D FSS design for extreme FOVs. The versatility of this algorithm will further be demonstrated through application to the design of meander line antennas, optical antennas, and phase-gradient metasurfaces

Submillimeter, Millimeter and Microwave Frequency Selective Surfaces, Design and Development

This dissertation presents new approaches to design and development of submillimeter, millimeter, and microwave frequency-selective surfaces (FSSs) having extensive applications in wireless communications and radar systems. The theory of the surfaces is introduced in Chapter 3 where a new approach to miniaturise the size of an FSS array element is presented by interconnecting array elements in one direction in a two-layer FSS structure. The top layer acts as an enhanced inductor while the bottom layer acts as a capacitor. The interconnection between adjacent array elements changes the equivalent circuit and produces a strong cross-layer capacitance, which lowers the resonant frequency significantly. The dimensions of the miniaturised FSS element are much smaller than the wavelength at the resonant frequency (periodicity << λ). Chapter 4 introduces a new methodology to design the FSS by maximizing the value of the capacitance between adjacent layers. The proposed structure offers three distinctive advantages: Firstly, the strong cross-layer capacitance makes the FSS element very compact. Secondly, for the proposed structure, the lower the profile, the stronger the cross-layer capacitance, and the lower the resonant frequency. This is unique to the proposed structure since the resonant frequency is usually higher for a lower profile than for traditional structures. Thirdly and most importantly, any external dielectric material attached to the FSS will not significantly affect the performance of the FSS due to this strong cross-layer capacitance. Chapter 5 introduces novel methodologies to design dual band spatial filters by using FSS periodic arrays composed of a bandpass and a bandstop element. The fabrication of the dual band filters is significantly simplified by using a single metal layer on a dielectric substrate. Chapter 6 introduces a new schematic to design a miniaturised high order bandpass FSSs (N ≥ 1), where N is the order of the FSS filter, with high performance with a flat in-band frequency response and fast roll-off is introduced. Two miniaturised resonant surfaces coupled by a non-resonant inductive layer are used to build the proposed FSSs. An FSS operating at around 3.8 GHz is designed to verify the method. The element size is smaller than 0.076λ×0.076λ for the proposed structure. This is significantly smaller than the element size of second-order FSSs designed using conventional approaches. The overall thickness is less than λ/24. The method could be particularly useful for the design of FSSs at lower frequencies with longer wavelengths. Thus, a novel approach for designing extreme low profile high-order bandpass frequency selective surfaces is introduced in this chapter. The structure is built in such way to obtain bandpass response by the coupling between the third harmonic responses of the resonators instead of the fundamentals. By parametric study of the proposed structure, one can make the coupling between the third harmonics weak with a thinner substrate, and then a flat in-band response can be achieved. The overall thickness can be reduced to λ/75. Chapter 7 demonstrates FSSs with sharp transition edges and almost flat bandpass for submillimetre wave and terahertz applications. The proposed structure exhibits a low insertion loss in the desired band. The structure is realised by combining bandstop and bandpass FSS structures on the same plane. By cascading more than one layer of surfaces, separated by dielectric slabs, the response with the desired flat passband characteristics can be achieved. The structure is polarisation independent and exhibits low insertion loss at the passband around 170 GHz. Finally, Chapter 8 demonstrates an extremely small-size high impedance surface (HIS) array element. A trade-off between a miniaturised element size and a lowered thickness of the grounded substrate is made to design an extremely low profile HIS. Additionally, we propose a way to modify existing classical RFID tag designs to enable them to operate well when they are attached to dielectric materials. Compared with using an HIS, the antenna bandwidth after being loaded with the proposed FSS is increased by approximately 100%

Tilted beam fabry-perot antenna with enhanced gain and broadband low backscattering

Communication with low radar signature platforms requires antennas with low backscatter, to uphold the low observability attribute of the platforms. In this work, we present the design for a Fabry–Perot (F-P) cavity antenna with low monostatic radar cross section (RCS) and enhanced gain. In addition, peak radiation is tilted inthe elevation plane. This is achieved by incorporating phase gradient metasurface (PGM) with absorptive frequency selective surface (FSS). The periodic surface of metallic square loops with lumped resistors forms the absorptive surface, placed on top of a partially reflecting surface (PRS) with an intervening air gap. The double-sided PRS consists of uniform metallic patches etched in a periodic fashion on its upper side. The bottom surface consists of variable-sized metallic patches, to realize phase gradient. The superstrate assembly is placed at about half free space wavelength above the patch antenna resonating at 6.6 GHz. The antenna’s ground plane and PRS together construct the F-P cavity. A peak gain of 11.5 dBi is achieved at 13◦ tilt of the elevation plane. Wideband RCS reduction is achieved, spanning 5.6–16 GHz, for x-and y-polarizations of normally incident plane wave. The average RCS reduction is 13 dB. Simulation results with experimental verifications are presented

Reconfigurable Antennas

In this new book, we present a collection of the advanced developments in reconfigurable antennas and metasurfaces. It begins with a review of reconfigurability technologies, and proceeds to the presentation of a series of reconfigurable antennas, UWB MIMO antennas and reconfigurable arrays. Then, reconfigurable metasurfaces are introduced and the latest advances are presented and discussed