Frequency Selective Surface Element Research Articles

Miniaturized frequency‐selective surface with reduced number of metallic vias for electromagnetic shielding

SummaryA miniaturized frequency‐selective surface (FSS) for radio frequency (RF) filtering applications is reported in this paper. The proposed FSS is designed to operate with band stop property at 1.57 GHz. The FSS element is developed using highly convoluted lines running on either side of the substrate interconnected using via lines. This specific arrangement increases the electrical length of the resonator without increasing the physical size. Furthermore, the stability towards polarization variation and independence to different angles of incidence are other major concerns in the design of FSS. The FSS unit cell has rotational symmetry thereby providing polarization‐independent operation. The angular independence is tested for various incidence angles, and the results are presented. The performance of the FSS is validated in real time, and the results are presented. The maximum realized transmission loss is 27 dB at 1.57 GHz, and the attenuation bandwidth with reference 10 dB shielding level is estimated to be 400 MHz centered around 1.57 GHz. The convolution technique along with shorting vias has offered this extreme miniaturization ranging from 7% to 95.7% with reference to most of the FSS reported for L‐band services such as radio, telecommunications, military, and aircraft surveillance.

Miniaturised element frequency selective surface design for high power microwave applications

AbstractA third‐order miniaturised element frequency selective surface (MEFSS) for high‐power microwave (HPM) systems is proposed. The MEFSS design is characterised by the excellent capability to handle high‐power waves and the stable frequency response at large incidence angles. Through combining the particle swarm optimisation algorithm and the equivalent circuit method to improve the efficiency, the design has low resonant temperature while maintaining a low maximum field enhancement factor. A theoretical guide for the selection of substrate parameters in high‐power systems is provided. Instead of being as small as possible, the loss tangent should be properly selected to reach a balance between metal loss and dielectric loss. To the author's best knowledge, it is the first time in‐band low temperature and out‐of‐band high temperature suppression at various incidence angles are realised. The results show that the frequency response remains stable for incident angles up to 60°, and the maximum temperature is reduced from 148°C to 44°C for incident power density of 10 kW/m2. A MEFSS prototype was fabricated, and the transmission coefficients were measured. The full‐wave simulation and measurement results agree well. It has significance to the engineering progress of FSS as radomes for HPM antenna systems.

Miniaturization of a Fully Metallic Bandpass Frequency Selective Surface for Millimeter-Wave Band Applications

This article presents a miniaturized all-metallic bandpass frequency selective surface (FSS) for millimeter-wave (mmWave) applications. The designed FSS is realized as a slot-type element that can be easily incorporated into all-metallic device architectures. The miniaturization is achieved by a convoluted or meander slot structure, i.e., a miniaturization technique borrowed from conventional substrate-based FSS design. It is demonstrated that the proposed FSS element provides a stable and wide bandpass performance from 24.9 to 31.4 GHz for both the transverse electric (TE) and the transverse magnetic (TM) polarizations at the broadside direction ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\theta = 0^{\circ }$</tex-math></inline-formula> ). Adequate performance is maintained at oblique incidence angles up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\theta = \pm \,45^{\circ }$</tex-math></inline-formula> . A 25 × 25-elements FSS array prototype has been manufactured using the cost-effective chemical etching technique and experimentally characterized utilizing a free-space measurement setup. The results demonstrate a remarkably good correlation between simulations and measurements. Hence, the proposed miniaturized FSS represents an excellent potential to realize an all-metallic FSS with low insertion loss, low profile, and wideband performance at low fabrication costs for mmWave applications.

Data-Driven Surrogate-Assisted Optimization of Metamaterial-Based Filtenna Using Deep Learning

In this work, a computationally efficient method based on data-driven surrogate models is proposed for the design optimization procedure of a Frequency Selective Surface (FSS)-based filtering antenna (Filtenna). A Filtenna acts as a module that simultaneously pre-filters unwanted signals, and enhances the desired signals at the operating frequency. However, due to a typically large number of design variables of FSS unit elements, and their complex interrelations affecting the scattering response, FSS optimization is a challenging task. Herein, a deep-learning-based algorithm, Modified-Multi-Layer-Perceptron (M2LP), is developed to render an accurate behavioral model of the unit cell. Subsequently, the M2LP model is applied to optimize FSS elements being parts of the Filtenna under design. The exemplary device operates at 5 GHz to 7 GHz band. The numerical results demonstrate that the presented approach allows for an almost 90% reduction of the computational cost of the optimization process as compared to direct EM-driven design. At the same time, physical measurements of the fabricated Filtenna prototype corroborate the relevance of the proposed methodology. One of the important advantages of our technique is that the unit cell model can be re-used to design FSS and Filtenna operating various operating bands without incurring any extra computational expenses.

Design and Characterization of the Fully Metallic Gap Waveguide-Based Frequency Selective Radome for Millimeter Wave Fixed Beam Array Antenna

This article presents a bandpass frequency selective surface (FSS) radome based on fully metallic gap waveguide (GW) technology. The element of the proposed FSS radome consists of a conventional cross-dipole slot etched on metallic plates and positioned over a groove GW cavity. A design with a single GW-cavity layer was initially produced which was later optimized for performance, to comprise a dual GW-cavity layer, while considering both functionality and manufacturability. It is shown that the proposed FSS element offers a stable and wide bandpass (from 26 to 30 GHz) performance in the broadside direction for both transverse electric (TE) and transverse magnetic (TM) polarizations. For oblique angle of incidence, the suggested FSS element works up to 30° with a reduction in usable bandpass bandwidth performance to 26–28 GHz for both TE and TM polarizations. A <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$20 \times 20$ </tex-math></inline-formula> -element GW-FSS array prototype has been fabricated and measured, which was integrated with a fixed-beam array antenna to further validate its functionality as a filtering radome. The findings show an excellent agreement between simulations and measurements. Hence, the proposed GW-FSS represents a great opportunity to develop an all-metallic FSS with low insertion loss, sharp-roll-off filtering, wideband performance, and inexpensive fabrication cost.

A Liquid Sensor Based on Frequency Selective Surfaces

A novel, simple and easy-to-fabricate liquid sensor using frequency-selective surfaces (FSSs) is proposed. The new sensor concept is based on modifying the capacitance between adjacent FSS elements when materials of different electrical characteristics are inserted. The change in capacitance produces a change in resonant frequency. The FSS design consists of a 9 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 9 array of square loops on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.31\lambda \times 0.31\lambda $ </tex-math></inline-formula> square unit cells with trenches between the loops. The trenches are filled with liquids under test (LTUs). The structure operates at 4.6 GHz without any liquid. When liquids are inserted in the trenches, the resonance frequency varies in relation to the dielectric constant of the liquid. This is observed by measuring the transmission coefficient ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{21}$ </tex-math></inline-formula> ). Butan-1-ol, ethanol, methanol, propan-2-ol, and xylene are used to demonstrate the sensing function. A maximum sensitivity of 8.65% for xylene was achieved. Furthermore, very low differences were observed between the measured and expected dielectric constant and loss tangent, thus validating the design. The device is inexpensive, compact, and easy to make and scalable for large-area operations in liquid detection for microwave sensing applications. This technique has potential applications in reconfigurable FSS.

Single-Polarized Low-Profile High-Order Bandpass Frequency Selective Surfaces Based on Aperture-Coupled Patch Resonators Under TM and TM Modes

This communication presents a methodology to construct the aperture-coupled dual-mode patch resonators (AC-DMPRs)-based periodic element to realize single- and dual-band high-order bandpass frequency selective surfaces (FSSs). Initially, a single patch resonator (PR) loaded with two sets of shorting pins is theoretically studied by using the characteristic mode analysis (CMA) method and by applying periodic boundary conditions, respectively. The results demonstrate that the radiation pattern and resonant frequency of TM00 mode in the PR can be effectively reshaped and controlled by the shorting pins, so that the TM00 and TM01 modes can be simultaneously excited under the normal incidence of a plane wave. As such, the single-mode PR is transformed into a DMPR. Moreover, such DMPRs are arranged in a back-to-back manner with coupling apertures etched on the common middle metallic layer to form an AC-DMPR-based bandpass FSS element. By reasonably reallocating the resonant frequencies of these two resonant modes and etching different apertures to provide magnetic and/or electric couplings in the AC-DMPR, single- and dual-band high-order bandpass FSSs can be conveniently and flexibly constructed, respectively. Finally, the conceptual designs are validated by measurements.

Arbitrary polygon shape optimization applied to multiband frequency selective surface inverse design

An arbitrary polygon shape optimization method is proposed for multiband frequency selective surface (FSS) inverse design in this paper. Different from the traditional deterministic methods, our approach uses the relationship between lumped parameter of equivalent circuit model (ECM) and distribution parameter which can readily determine the search direction. Meanwhile, the iteration step size is determined by the normalized current density. Polygon representation of FSS element can improve the degree of freedom of structural optimization to a great extent compare with parameterized method. This approach has the ability to avoid the conflict of multi-objective optimization and optimize multiband FSS effectively and accurately. A dual-band FSS is taken as a case to verify the method. The simulation results of optimized structure meet the expectation and are basically coincident with measurement results, thus verifying the feasibility of the optimization method.

Second-Order, Single-Band and Dual-Band Bandstop Frequency Selective Surfaces at Millimeter Wave Regime

This communication presents the miniaturized low-profile second-order (SO) bandstop frequency selective surfaces (FSSs) for single- and dual-band resonances. The unit cell comprises two identical first-order (FO) elements in a back-to-back configuration with the metal grid in between. Two thin dielectric substrates separate the three metal layers. The proposed FO unit cell consists of intercell-connected spiral resonators (SRs) with capacitively coupled cross loop dipole. The SO filter response of the proposed FSS exhibits a wide stopband ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{21} &lt; -10$ </tex-math></inline-formula> dB) bandwidth (BW) of 27.6%. The unit cell has the periodicity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.162\lambda _{0}$ </tex-math></inline-formula> , where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda _{0}$ </tex-math></inline-formula> is the free space wavelength at the lower passband. The novelty of the proposed work is a miniaturized simple unit cell geometry, at the millimeter-waveband but not limited to it, with polarization insensitive and high angular stable filter response up to 60° with small variations in the BW. An unconventional four-unit cell element FSS is designed for dual-band SO stopband resonances with the BWs of 8.3% and 13.8%. Prototypes of both FSSs were fabricated and their measured results were found consistent with the simulated filter responses.

Bandwidth analysis of microwave metamaterial absorber with a resistive frequency selective surface by using an equivalent circuit model

Bandwidth analysis of a resistive frequency selective surfaces (FSS) in microwave metamaterial absorber is very important for bandwidth-targeted design of the FSS pattern. A useful analysis for the case of a thin grounded dielectric substrate was performed a decade ago, however in the cases of thicker dielectric substrates it is not clear how to design the FSS pattern in order to achieve the largest bandwidth. Without the assumption that the grounded dielectric substrate is thin, we investigated relations between the bandwidth and the FSS reactance and revealed explicit expressions of the FSS reactance for the largest bandwidth. Although in full wave simulations for demonstrating the validity of the bandwidth analysis we assumed exemplary the square loop element, the bandwidth analysis is applicable for general FSS element structures. Our findings give design guidance for the FSS patterns of perfect absorbers in order to achieve the largest bandwidth.

Electromagnetic Backscattering Mechanisms of Frequency Selective Surfaces for Effective Analysis of Radar Cross Section by the First-Order Approximation of Floquet Modes

Full-wave analysis of finite but electrically large frequency selective surfaces (FSSs) substrates for the radar cross section (RCS) estimation is too time-consuming and unbearable for practical applications. Such FSS substrates can be either absorbing or reflective types and are widely used as radomes of antennas for stealth applications, where the electromagnetic (EM) backscattering is of much concern. This large EM problem is simplified by applying Floquet mode decomposition on the scattering fields, where the Kirchhoff approximation is employed to find the backscattering fields for RCS estimation. The resulted formula allows one to explore the scattering mechanism for RCS enhancement or reduction. Besides, the approximation is used to resemble the FSS radome by equivalent substrates. Thus, the transmission and reflection coefficients of the equivalent dielectric substrates are similar to those from FSS elements under an infinite array assumption. Afterward, the resulting EM problems of antenna radiations and scattering can be easily analyzed using the equivalent substrates of EM materials. Numerical examples of planar FSSs to form radomes demonstrate the feasibility of numerical simulations’ proposed work. The presented work can also be directly applied for fast analysis of EM shielding effectiveness by FSS substrates.

Dual-Polarized Filtering Transmitarray Antennas With Low-Scattering Characteristic

A filtering transmitarray antenna (TAA) with high-gain in-band radiations at the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$X$ </tex-math></inline-formula> -band and an out-of-band low-scattering characteristic is presented in this communication. The proposed composite TAA element consists of a dual-polarized resistive sheet, a fixed bandstop frequency-selective surface (FSS) element, and an adjustable bandpass FSS element also serving as a phase shifter. The resistive element is constructed by inserting a parallel inductor–capacitor (PLC) resonant structure into the center of a lumped-resistor-loaded metallic dipole. A transparent frequency-domain window at the center operating frequency ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{c}$ </tex-math></inline-formula> ) is created by the PLC resonance and is further shared by both FSS elements, hence allowing for high in-band transmission herein. At frequencies below or above <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{c}$ </tex-math></inline-formula> , the whole composite TAA element acts as an absorber with the bandpass or bandstop FSS elements as the ground planes for the resistive sheet, respectively. The measured results show that the composite TAA can achieve gain-filtering responses with out-of-band suppression levels ≥25 dB, ~8 dB scattering cross section (SCS) reductions in 4–7 and 14–20 GHz, and the radiation gain of 25.3 dBi simultaneously. Because of these merits, the proposed design can relieve the stress on filtering circuit designs, also offering an attractive solution in stealth technology.

Miniaturized frequency selective surface with high angular stability

Abstract In this paper, a miniaturized bandstop frequency selective surface (FSS) with high angular stability is presented. Each FSS element consists of four sets each consisting eight octagonal concentric interconnected loops. The four sets are connected with each other through outermost octagonal loop. The unit size is miniaturized to 0.066 λ0 at the resonant frequency of 1.79 GHz. The proposed configuration achieves excellent angular stability (only 0.025 GHz resonant frequency deviation is observed upto 83° oblique angles). The working mechanism of FSS is explained with the help of equivalent circuit model (ECM), electric field distribution, and corresponding surface current distribution. A prototype of the designed bandstop FSS is fabricated to verify the simulated frequency response. The experimental results are consistent with the simulation results. Simple geometry, low profile, high angular stability, and compact cell size are prominent features of the proposed structure.

Metamaterial-inspired optically transparent active dual-band frequency selective surface with independent wideband tunability.

This paper presents an optically transparent active bandstop frequency selective surface (FSS) with wideband tunability of two resonance frequencies using the concept of miniaturized element FSS (MEFSS). The proposed design consists of metallic square loop arrays on a new optically transparent substrate as the top layer, a glass interlayer, and periodic patterns of cross dipoles on the substrate as the bottom layer. Two kinds of resonant elements loaded with varactors and the designed bias networks achieve two independent tunable stopbands. The proposed FSS has two large tuning ranges, one is from 1.20 GHz to 2.63 GHz and another is from 2.0 GHz to 5.9 GHz (75% and 99% with respect to the center frequency, respectively). The wideband dual-tuning mechanism is theoretically analyzed and demonstrated by deriving its equivalent circuit (EC) model. The experiment results exhibit reasonable agreement with the numerical simulation responses. This proposed design, with low profile, angular stability, polarization insensitivity, optical transparency, and wideband dual-tunability can play an important role in manipulating electromagnetic wave propagation for manifold applications.

Approximate analytical method for hexagonal slot frequency selective surface analysis

In this article, an efficient approximate analytical method for hexagonal slot frequency selective surface (FSS) analysis is proposed. The main contribution of this article is to derive the tangential incident electric field along each oblique strip complementary to the hexagonal slot, so that Kirchhoff type circuit can be established according to the transmission line parameters and distributed sources. The current distribution is solved using Kirchhoff current law and continuity condition for potential. The transmission characteristic can then be calculated following Munk's scheme. Meanwhile, the physical nature of Munk's scheme is studied. The periodic circuit equations are derived from the corresponding electromagnetic field boundary conditions. The approximate closed-form expressions for aperture field and patch surface current distribution of common FSS elements can be calculated by the method of moments. From the analysis, it is known that the receiving current and transmitting current corresponds to the basis function and test function, respectively. Measured and simulated results of a symmetric hybrid radome composed of hexagonal slot are carried out to verify the accuracy and efficiency of the presented method. The CPU time is about 600 and 0.4 second for CST and the proposed method, respectively.

A novel 4-DOF wide-range tunable frequency selective surface using an origami “eggbox” structure

AbstractShape-changing mechanical metamaterials have drawn the attention of researchers toward the development of continuous-range tunable frequency selective surfaces (FSSs). In this paper, a novel tunable FSS utilizing an origami-inspired “eggbox” structure is presented featuring four-degrees of freedom that can change the frequency response of two orthogonal linear polarizations. The centrosymmetric “eggbox” structure can be folded or rotated along two axes that lead to unprecedented reconfigurability compared to traditional Miura-Ori-based structures which have fewer degrees of control. The utilized cross-shaped dipole FSS element shows enhanced bandwidth, support for orthogonal linear polarization, and ease of fabrication. The prototype is fabricated using a low-cost fully additive inkjet printing process with silver nanoparticle conductive ink. The outcome of this study shows a 25% frequency tunable range over two polarization directions. The design can be an ideal spatial filtering candidate for advanced ultra-wideband terrestrial and space applications.

Ultralow Scattering and Broadband Metasurface Using Phase Adjustable FSS Elements Embedded With Lumped Resistors

This letter presents a novel phase-cancellation and absorptive metasurface (PCAM) with ultralow scattering and broadband features. The PCAM is formed by two circular frequency selective surface elements with lumped resistors, which can not only reduce reflection amplitudes by transforming the incident waves into heat but also adjust reflection phase responses simultaneously. The two electromagnetic scattering mechanisms, phase cancellation and absorption, cooperate in the same band and single layer. This is the first proposed ultralow backscattering performance where the monostatic radar cross section (RCS) is reduced by more than 20 dB in 5.0–10.9 GHz. Simultaneously, an RCS reduction over 10 dB is achieved at a broad frequency band of 4.1–12.0 GHz under normal incidence. Both the analysis and measurements are demonstrated in detail. The good agreement between simulated and measured results validates the proposed PCAM.

Dual-Band Shared-Aperture Base Station Antenna Array With Electromagnetic Transparent Antenna Elements

An electromagnetic transparent antenna element is proposed for dual-band shared-aperture fifth-generation (5G) MIMO base station antenna array developments. The antenna element that operates in the low-frequency band (LB) of 1.8-2.7 GHz is placed above the aperture of the antenna array that operates in the high-frequency band (HB) of 3.3-3.8 GHz. Because of this antenna array configuration, the antenna elements in the LB have to be transparent to the electromagnetic waves radiated by the HB antenna array. This kind of electromagnetic transparent antenna element allows the HB antenna array working properly in the dual-band shared-aperture antenna array configuration. In this work, the electromagnetic transparent antenna element is inspired by the element of a typical wide-angle bandpass frequency selective surface (FSS). The radiation performance of the electromagnetic transparent antenna element at the LB is realized by properly combining the wide-angle bandpass FSS elements. A dual-band dual-polarized shared-aperture antenna array is then developed to demonstrate the concept and design. The experimental result validates the stable radiation patterns of each antenna element in both LB and HB. It indicates the proposed approach is promising for 5G MIMO base station antenna array developments.

2.5D frequency selective surface with quasi‐elliptical bandpass response

AbstractIn this article, a via‐based 2.5‐dimensional (2.5D) frequency selective surface (FSS) is proposed to realize a quasi‐elliptical bandpass response with high frequency selectivity. Each FSS element is composed of two parts. Part A is a double‐layer metallic circular patches, which are etched on both sides of a thick substrate and connected by a central big metallic via and several small vias. Part B is a single layer metallic circular patch etched on a thin substrate. The proposed FSS features several transmission/reflection zeros, exhibiting a flat transmission in the passband and fast roll‐off out of band. By investigating the electric field distributions, two paths of electromagnetic wave (EMW) propagation are observed, and the physical mechanism of the transmission/reflection zeros of the FSS is obtained. A prototype of the FSS is fabricated to verify the design, and the measured transmission responses agree with the simulated ones.

Miniature Convoluted FSS for Gain Enhancement of a Multiband Antenna

Convoluted elements on a frequency selective surface (FSS) allow for low frequency elements to be contained in physically smaller unit cells. Smaller unit cells give the FSS greater angular stability, especially where a curved FSS is required, and so unwanted grating effects are avoided. A convoluted element FSS with a frequency rejection band centred at 2 GHz and unit cell area of 15 mm by 15 mm ( $0.10\,\,\lambda \times 0.10\,\,\lambda$ ) has been developed. To test its usefulness, the full structure FSS is used as a parabolic reflector in a dual band FSS reflector antenna operating at 1 GHz and 2 GHz. Simulated and measured results are close at both bands. The reflector antenna has high gain at 2 GHz of 12.7 dBi (simulated) and 11.7 dBi (measured). To observe the angular stability of the FSS and therefore its effectiveness as a reflector, it was compared with a copper test reflector at both bands. Simulation of the reflector antenna with test reflector produced a 2 GHz gain of 13.3 dBi which is very close to that with the FSS reflector. The simulated 2 GHz gain plot of the reflector antenna with FSS reflector is very similar to that with the test reflector indicating that the FSS has good angular stability. The gain at 1 GHz is also high with 9.3 dBi (simulated) and 8.7 dBi (measured). Simulation of the reflector antenna with no FSS and only a rear test reflector produced a 1 GHz gain of 10 dBi which is very close to that with the FSS reflector in place indicating that the FSS causes no significant attenuation at that frequency. The convoluted element FSS would be useful as a curved reflector in the creation of high gain, multiband, conformal antennas.