For Frequency Selective Surfaces Research Articles

A Spatial Inverse Design Method (SIDM) Based on Machine Learning for Frequency-Selective-Surface (FSS) Structures

A Spatial Inverse Design Method (SIDM) Based on Machine Learning for Frequency-Selective-Surface (FSS) Structures

Miniaturization of frequency selective surface by 2.5-dimensional meandered split ring cells for application in L-band

In this research article, A miniaturized hexagonal split-ring 2.5-dimensional (2.5-D) unit cell is proposed for frequency selective surface (FSS) applications. The 2.5-D FSS provides efficient usage of the available surfaces and optimum increased current paths by connecting its two sides through vias. The size of the FSS unit cell is 0.026{{varvec{uplambda}}}_{0}times 0.026{{varvec{uplambda}}}_{0} at the resonance frequency of 1.25 GHz. The proposed FSS has good angle stability due to its small size. The effects of variation of FSS geometry parameters on transmission zero are investigated, namely, the control of bandwidth and roll-off of frequency response by these parameters is studied. An equivalent circuit is obtained for the proposed structure to predict its frequency response, which agrees very well with the results of full-wave simulations. A prototype model of the proposed FSS structure is designed, simulated, fabricated, and tested as proof of concept.

Independently switchable dual-passband frequency-selective surface design based on characteristic mode analysis.

High-order bandpass filtering responses are highly desirable for frequency-selective surfaces (FSSs) in narrowband antenna/radar systems. In this paper, the design process of a reconfigurable dual-band FSS with second-order response is presented. Initially, the basic dumbbell-shaped resonator used in this design is theoretically investigated using the characteristic mode analysis (CMA) method to study the relation between geometric design parameters and the excited orthogonal resonance modes in different frequency bands. Then, an additional CMA process was performed on a unit cell with four such patch resonators arranged with 90-degree rotation between adjacent ones. This detailed analysis leads to a polarization-independent FSS design with a high-order dual-band response. Two of these composite resonators are combined back-to-back through coupling apertures on the middle layer. Finally, PIN diodes are loaded on separate layers to realize independent pass-band switching. The loading place of the PIN diodes is carefully chosen based on electromagnetic field analysis. A prototype was also fabricated and experimentally tested. Experimental results show that this FSS has two independently switchable passbands centered at 3 GHz and 4.8 GHz.

Design of Wideband Frequency Selective Surface Based on the Combination of the Equivalent Circuit Model and Deep Learning

In this letter, a design method for frequency selective surface (FSS) based on the equivalent circuit model (ECM) and deep learning is proposed. In the proposed method, the ECM depicting the target transmission characteristic is first established. Then, with the aid of the established ECM, the target transmission characteristic is converted into the desired transmission curve and the initial topology of the FSS structure is determined. The cascade network is adopted to realize the rapid inverse design of the FSS structure. Benefiting from the established ECM, the high quality of the training data and proper description of the design target can be ensured in deep learning. For verification, an ultra-wideband FSS structure with a passband ranging from 3.1 to 12.05GHz and a stopband ranging from 12.85 to 20GHz is designed. Both simulated and experimental results show that the designed FSS provides an ultra-wide passband with flat-top and fast roll-off characteristics, covering 98.9% target passband and 95.3% target stopband.

An Extension of Partial Element Equivalent Circuit Method for Frequency Selective Surface Analysis

In this communication, an extension of a partial element equivalent circuit (PEEC) method for frequency selective surfaces (FSSs) modeling is presented. Based on Floquet’s theorem and dyadic periodic Green’s functions of a layered medium, the general PEEC formulations and models for periodical structures are deduced, and Poisson’s sum formula is used to improve the convergence speed. Furthermore, the extension is also performed by developing a normalized-current-based simplified PEEC (NCB simplified PEEC), which can accelerate the calculation of the scattering coefficients significantly. Also, the circuit parameters within the interested frequency band are related to the FSS geometric parameters, which is beneficial to the optimization of FSS. The scattering coefficients of both patch and slot FSSs with a dielectric substrate are calculated, which agree well with those calculated by electromagnetic simulation software Altair FEKO 2020.

High-Selectivity FA-FA-Based Frequency Selective Surfaces Using Magnetoelectronic Dipole Antennas

A new method for frequency selective surface (FSS) with high selectivity, namely, filtenna–filtenna (FA-FA)-based technique, is proposed, which consists of two filtering antennas and impedance matching network. Based on the reciprocity theorem, the radiation null of filtenna can provide a stopband of the proposed FSS. In this work, it is validated by using a periodic array of back-to-back magnetoelectronic (ME)-dipole antennas with split-ring resonators (SRRs), and a slotted GND plane is used to connect the two ME-dipole antennas for impedance matching. Bandpass response of the proposed FSS is realized by the intrinsic high-pass characteristic of ME-dipole antennas and the bandstop characteristic of SRRs. Four transmission zeros (TZs) and three transmission poles (TPs) are generated, exhibiting a high-order filtering response. TZs and TPs are analyzed by equivalent circuit model and current distributions. The 3 dB fractional bandwidth ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {FBW}}_{\mathrm {3\,dB}}$ </tex-math></inline-formula> ) of 38.5% in the passband and the 20 dB fractional bandwidths <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$({\mathrm {FBW}}_{\mathrm {20\,dB}})$ </tex-math></inline-formula> of 8.7% in the lower rejection band and 13.8% in upper rejection band are obtained in this design. Finally, a prototype is fabricated and measured to further validate the proposed FA-FA-based technique. Simulated and measured results show that the proposed FA-FA-based FSS is with the distinct advantages, including wide passband, low profile, tunable TZs, and dual-polarized application, which verifies the proposed method of FSSs.

A Simple Equivalent Circuit Approach for Anisotropic Frequency-Selective Surfaces and Metasurfaces

An equivalent circuit model for Frequency Selective Surfaces (FSS) comprising anisotropic elements is presented. The periodic surface is initially simulated with an arbitrary azimuthal incidence angle and its surface impedance matrix is derived. The impedance matrix is subsequently rotated by an angle $\varphi^{rot}$ on the crystal axes $\chi_1$, $\chi_2$ thus nullifying its extra diagonal terms. The rotation angle $\varphi^{rot}$ is derived according to the spectral theorem by using the terms of the matrix initially extracted. The diagonal terms of the rotated matrix, that is, the impedances $Z_{\chi_1}$ and $Z_{\chi_2}$, are finally matched with simple LC networks. The circuit model representation of the anisotropic element can be used to analyse anisotropic FSSs rotated by a generic azimuth angle. The methodology provides a compact description of generic FSS elements with only five parameters: the lumped parameters of the LC network $L_{\chi_1}$, $C_{\chi_1}$, $L_{\chi_2}$, $C_{\chi_2}$ and the rotation angle $\varphi^{rot}$. The circuit model can take into account the presence of dielectric substrates close to the FSS or a variation of the FSS periodicity without additional computational efforts. The equivalent circuit model is finally applied to the design of two transmitting polarization converts based on anisotropic metasurfaces.

Calibrated Parallel-Plate Waveguide Technique for Low-Frequency and Broadband Absorptivity Measurement

A broadband measurement technique for the normal-incidence absorptivity based on parallel-plate waveguide (PPW) is proposed. The method is well suited for frequency-selective surface (FSS)-based absorbers. By employing the PPW structure, only a small sample size, i.e., 1-D array of FSS, is required for the measurement. The thru-reflect-line (TRL) calibration is implemented to remove the time-gating ambiguity in the conventional PPW-based method. A measurement system working up to 6 GHz is designed such that its length is adjustable to ensure all three calibration standards (thru, reflect, and line) can be carried out in a single device. Moreover, the height of the PPW is also adjustable to allow the measurement of absorbers with a different periodicity of FSSs. Two single-layer FSS-based absorbers with different periodicity are fabricated and measured. The results agree well with the conventional free-space measurement system, which validates the proposed method.

An improved particle swarm optimization algorithm for frequency-selective surface design

In this paper, an improved particle swarm optimization (PSO) algorithm, which can perform a global search over the search space with a faster convergence speed, is proposed for frequency-selective surface (FSS) design. Four improvement strategies including better particle initialization, acceleration coefficient dynamic adjustment, search space reset, and particle density control are applied to PSO algorithm aiming to accelerate convergence and, at the same time, improve its search capability. For verification, the improved PSO algorithm has been successfully implemented to the square loop FSS design. Then, the optimized results are compared to those obtained by the standard PSO algorithm as well as those existing in the literature, respectively, proving that the improved PSO algorithm has the capacity for accelerating convergence on the premise of ensuring the optimization accuracy.

NUFFT for the Efficient Spectral Domain MoM Analysis of a Wide Variety of Multilayered Periodic Structures

In this paper the Method of Moments (MoM) in the spectral domain is used for the analysis of multilayered structures containing periodic arrays of either patches or apertures. The patches and apertures may have many different geometries including complex surfaces limited by two parallel lines and two arbitrary curves, circular and elliptic rings, circular and elliptic arcs, and circular and elliptic sectors. Basis functions accounting for edge singularities are used in the approximation of the electric/magnetic current density on the patches/apertures, which enables a fast convergence of MoM with respect to the number of basis functions. Since the 2-D Fourier transforms of the basis functions cannot be obtained in closed-form, these Fourier transforms are efficiently computed by means of the nonuniform fast Fourier transform (NUFFT) algorithm. Results have been obtained for frequency-selective surfaces (FSSs), and for the elements used in the design of both reflectarray and metasurface antennas. The results obtained indicate that the software based on the NUFFT is only 15% slower than the standard spectral domain MoM software used for structures in which the 2-D Fourier transform of the basis functions is analytical, and between 50 and 80 times faster than CST.

A hybrid encoding method for frequency selective surface optimization design with angular stability property

In this letter, a hybrid encoding method of topology optimization for frequency selective surface (FSS) with high angular stability is proposed. By combining two different encoding method of binary and real-number, both bandpass and bandstop FSS with angular stability can be obtained. Topology and configuration parameters can be obtained with genetic algorithm from the desired resonate frequency and corresponding bandwidth. By adopting the hybrid encoding method, the optimization space has been reduced with compressed encoding length and the optimization accuracy can also be improved. For verification, two FSSs with bandpass and bandstop property have been designed, fabricated, and measured. Good agreement between the simulation and measurement results can be observed.

Approximate Analysis Method for Frequency-Selective Surface Based on Kirchhoff-Type Circuit

A novel approximate method based on the Kirchhoff-type circuit for frequency-selective surface (FSS) analysis is presented in this paper. Instead of splitting up the array element and solving a higher order impedance/admittance matrix equation in the periodic moment method, the equivalent transmission line method is adopted to obtain analytical solutions of the normalized receiving current (or magnetic current) distribution for pattern functions. The impedance/admittance elements and transmission coefficients can then be analytically calculated using Munk’s scheme. The proposed method can be applied to generic FSS elements composed of relatively narrow straight slots/strips. The revision scheme of the equivalent transmission line lengths is proposed to account for discontinuity effects at corners of the element. The transmission coefficients of the six biplanar symmetric hybrid radomes are computed to verify the accuracy and efficiency of the presented method.

Design of Frequency Selective Surface Structure With High Angular Stability for Radome Application

A design method for frequency selective surface (FSS) structure with high angular stability has been proposed in this letter. In the proposed method, bandwidth angular stability of FSS structure has been improved by adopting the bandwidth compensation technique, and structural parameters can be obtained with a curve-fitting method from the desired resonate frequency and bandwidth. Discussions on improving the bandwidth design accuracy have been presented. By taking bandwidth angular stability into consideration, the structure designed by the proposed method is more suitable to construct FSS radome. For verification, an FSS structure operating at 15 GHz with a bandwidth of 2 GHz has been designed, fabricated, and measured. Good agreements between the simulated and measured results can be observed.

Polarization rotator of arbitrary angle based on simple slot-array

A novel polarization rotator of arbitrary angle was proposed and realized based on simple slot arrays. To achieve the rotation of an arbitrary angle α, the slots on the first layer have to be at an angle of α to the slots on the second layer. Consequently, 90° rotation can be realized using two perpendicularly oriented slot arrays, which overturns the conventional notion of that perpendicular slot arrays are not possible to pass electromagnetic wave. In addition, such structure provides the same bandwidth comparing to its counterpart utilized for frequency selective surface (FSS). Furthermore, such structure is much easier to be fabricated compared to the substrate integrated waveguide (SIW) array. Moreover, low insertion loss can be achieved based on metallic material.

Simple, Compact, and Multiband Frequency Selective Surfaces Using Dissimilar Sierpinski Fractal Elements

This paper presents a design methodology for frequency selective surfaces (FSSs) using metallic patches with dissimilar Sierpinski fractal elements. The transmission properties of the spatial filters are investigated for FSS structures composed of two alternately integrated dissimilar Sierpinski fractal elements, corresponding to fractal levelsk=1, 2, and 3. Two FSS prototypes are fabricated and measured in the range from 2 to 12 GHz to validate the proposed fractal designs. The FSSs with dissimilar Sierpinski fractal patch elements are printed on RT/Duroid 6202 high frequency laminate. The experimental characterization of the FSS prototypes is accomplished through two different measurement setups composed of commercial horns and elliptical monopole microstrip antennas. The obtained results confirm the compactness and multiband performance of the proposed FSS geometries, caused by the integration of dissimilar fractal element. In addition, the proposed FSSs exhibited frequency tuning ability on the multiband frequency responses. Agreement between simulated and measured results is reported.

A Rigorous Equivalent Network for Linearly Polarized THz Absorbers

Linearly polarized electromagnetic absorbers are often used in THz space science as means of power detection. In this paper, we present a rigorous and analytical equivalent network suitable for the analysis of these structures. The model includes, on one hand, the vectorial representation of the general propagation and scattering of the dominant electromagnetic waves and, on the other hand, details of the absorber geometry. Unlike existing models for frequency selective surfaces (FSS), the present network highlights the full interaction between TE and TM modes and the current in the strips that couples them. The network is elegant, simple and accurate, providing the explicit description of the dominant physical mechanism involved. The tool derived from the network is used for designing several THz absorber based detectors showing broadband absorption efficiencies. The network is validated with full wave simulations.

Analysis and design of frequency selective surfaces using teragon patch elements for WLAN applications

This paper describes a fractal design methodology for frequency selective surfaces (FSSs) composed of teragon metallic patches on a single-layer fiberglass dielectric. Shapes presented by teragons are exploited to design compact FSSs acting as dual-band band-stop spatial filters. An FSS parametric analysis is performed in terms of the fractal dimension and unit cell size of fractal patches. The transmission properties of the proposed periodic arrays are initially investigated through simulations performed by Ansoft DesignerTM commercial software. To validate the used methodology, three FSS prototypes are selected for fabrication, and one of them is designed to reject wireless local area network signals in the bands of 2.4–2.5 GHz (IEEE 802.11b) and 5.0–6.0 GHz (IEEE 802.11a). Experimental characterization of the FSS prototypes is accomplished through an alternative setup with elliptical monopole microstrip antennas for low-cost FSS measurements. The obtained results point to interesting features for FSS spatial filters: compactness, with high values of frequency compression factor (up to 80.4%); dual polarization; dual-band frequency responses; and stable resonance frequencies at oblique incidence.

Frequency selective surface for radio frequency energy harvesting applications

A novel application for frequency selective surfaces (FSSs) is presented. The novel FSS design can harvest power from arbitrarily polarised incident waves. The design does not involve a matching network, which results in a simple, polarisationindependent power harvester. An accurate analytical procedure, to calculate the impedance of the commercially available Schottky rectifiers, is presented. The results are validated by full-wave simulations and by measurements. A simple equivalent circuit model to predict the transmission and reflection characteristics of the gridded-square-loop FSS is employed. The design method is validated for different polarisations and for different incident angles. The addition of lumped elements (R, L, C) in the metallic conductive grid of the FSS is investigated. The waveguide simulator method and full-wave simulations are employed to validate the derived analytical equations. A 3 x 3 and a 5 x 5 RF harvesting FSS have been designed, simulated, fabricated and validated. A radio frequency (RF) to DC conversion efficiency of 25% for the 3 x 3 RF harvester and 15.9% for the 5 x 5 RF harvester is measured at an RF input power level of -6 dBm.

EFFICIENT MULTI-OBJECTIVE OPTIMIZATION OF FREQUENCY SELECTIVE RADOME WITH NONUNIFORM WALL THICKNESS

An e-cient optimization technique for frequency selective surface (FSS) radome with nonuniform wall thickness is proposed to improve the power transmission e-ciency and the boresight error (BSE) of FSS radome simultaneously. The high-frequency method based on the approximate locally planar technique is used to evaluate the transmission performance of FSS radome. An e-cient multi-dimensional adaptive sampling method combined with spectral domain method of moment (MoM) is employed to analyze transmission performance of FSS structure. The immune clone algorithm (ICA) is applied to the design of a FSS radome, in which the linear combination of the maximizing power transmission e-ciency and the minimizing BSE is adopted as the a-nity function, and the radome wall thickness is optimized. A design example for the three-dimensional tangent ogive radome with nonuniform thickness is given. The results show that the power transmission e-ciency is improved signiflcantly and the BSE of the optimal antenna-radome system is also reduced over the antenna scan volume.

Stable and compact multiband frequency selective surfaces with Peano pre‐fractal configurations

This work presents a fractal design methodology for frequency selective surfaces (FSSs) with Peano pre-fractal patch elements. The proposed FSS structures are composed of periodic arrays of metallic patches printed on a single-layer fibreglass dielectric. The shapes presented by pre-fractal patches allow one to design compact FSSs that behave like dual-polarised band-stop spatial filters. On the other side, the space-filling and self-similarity properties of Peano fractals became possible various configurations for patch elements. An FSS parametric analysis is performed in terms of the fractal iteration-number and cell-size of pre-fractal patches. To validate the used methodology four FSS prototypes are built and tested in the range from 1.0 to 13.5 GHz. Experimental characterisation of the FSS prototypes is accomplished through three different measurement setups with commercial horns and circular monopole microstrip antennas. Results show that the proposed FSS presents most of the desired features for spatial filters: compact design, multiband responses, dual-polarisation, excellent angular stability and facility for reconfiguration.