Conformal Frequency Selective Surface Research Articles

Conformal Frequency Selective Surfaces in Radome Design: A Mini Review

In aerospace engineering, radomes play an important role in protecting antenna systems from external interference and minimizing air resistance during flight. However, traditional radomes often fall short of providing the electromagnetic filtering and stealth capabilities important for modern aircraft applications. Conformal Frequency Selective Surfaces (FSS) radomes represent an evolutionary step beyond their conventional counterparts. These structures not only continue to protect antenna structures from environmental adversities, but also adapt seamlessly to non-planar surfaces. This adaptation is not only functional but also covers aerodynamic and aesthetic considerations, improving stealth capabilities and multi-band frequency operability, which are key factors for state-of-the-art communications and radar systems. However, design and implementation of conformal FSS radomes have some challenges. Considering that one of the authors has focused on such challenges for new generation air platforms, this study examines the contemporary advancements in conformal FSS radome technology, highlighting the latest research and breakthroughs. It covers latest findings on cutting-edge materials, design methodologies, simulation techniques for performance prediction, and the practical applications and advantages of these radomes. Building on a theoretical foundation of FSS and radome integration, the article discusses material and design innovations, simulation challenges, and practical implementations. It is hoped that this review will act as a bridge for knowledge sharing and a stimulant for continued research within the domain of conformal FSS radome technology

Ultrathin and conformal frequency selective surfaces bandpass filter to eliminate the 5G bands on radio altimeters

AbstractThis work proposes a frequency‐selective surface‐based radome structure to selectively allow the radar altimeter operating band and also minimize the interference from Sub‐6GHz 5 G frequency bands. A Jerusalem cross and combinational loop (JCL) based frequency selective surfaces (FSS) unit cell of overall dimension 0.37λ was synthesized using a thin flexible substrate for use in fuselage of airborne systems. The proposed FSS unit cell exhibits transmission zero at 3.61 and 6.025 GHz and highly selective pass band at 4.3 GHz. The effect of bends and angle of incidence on the frequency response of the synthesized FSS was studied in transverse electric and transverse magnetic modes. A lumped element‐based equivalent circuit model for the proposed design was also presented. Finally, the fabricated prototype was experimentally validated in an Anechoic chamber setup and the suitability of the proposed FSS layers for aviation radar systems was confirmed.

Conformal Frequency Selective Surfaces for Arbitrary Curvature

An algorithm is introduced for generating frequency selective surfaces (FSSs) capable of conforming to any curvature while maintaining the proper size, shape, and spacing of the elements. Compared to traditional projection and mapping methods, the presented algorithm maintains the electromagnetic properties of the FSS array despite the curvature. The algorithm can be used to conform to radomes, parts of autonomous vehicles, or any surface. The algorithm is agnostic to both element design and surface curvature. This allows the user to design a FSS for any curved surface while maintaining its response comparable to a flat array. The algorithm outputs two standard tessellation language (STL) files, one describing the curved surface and the other the elements of the FSS placed onto the curved surface. This makes the algorithm suitable for 3-D printing using systems with more than three axes or for flexible electronics. Several examples of arbitrary surfaces are shown. Lastly, the algorithm was applied to a Jerusalem-cross (JC) FSS on a nonsymmetrical parabolic dome. The dimensions of the parabolic dome were chosen to test the response of the array on a rather extreme surface against a projected array on the same surface. Simulations were carried out using Ansys HFSS from the infinite array to finite arrays to confirm the operation. Three test surfaces were manufactured with measured results found to be in good agreement with the simulation.

Conformal Multifunction FSS With Enhanced Capacitance Loading for High Angle Stable Stopband Filtering and Microwave Absorption

This article presents a new method of enhancing the capacitance of a single-layer conformal frequency selective surface (FSS) for multifunctional operation. The unit cell of the FSS consists of gridded square loop-loaded modified Jerusalem cross (JC) element. End capacitance of the conventional JC FSS is enhanced by placing parallel diagonal strips, resulting in a pair of reflection zeros near the sidebands. The modified geometry provides high selective stopband filtering that reflects the signal at 20.16 GHz with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> < −10 dB stopband bandwidth of 20.52%. The FSS shows stable stopband reflection up to a high incidence angle of 85°. The stability in the filter response is also achieved at various conformal angles. The proposed FSS is optimized for dual-band near-perfect absorption. At two resonances 12.76 and 22.54 GHz, the absorption are 99.88% and 99.1%, respectively. Prototypes of the stopband FSS and the dual-band absorber are fabricated to verify the feasibility of the proposed concept. The measured filter and absorber responses are found consistent with the simulated results. As a conformal multifunctional structure, the proposed FSS has the merits of electromagnetic interference shielding and radar cross section reduction in stealth application.

3D conformal bandpass millimeter-wave frequency selective surface with improved fields of view

Conventional planar frequency selective surfaces (FSSs) are characterized in the far-field region and they are sensitive to the incidence angle of impinging waves. In this paper, a spherical dome FSS is presented, aiming to provide improved angular stable bandpass filtering performance as compared to its planar counterpart when the FSS is placed in the near-field region of an antenna source. A comparison between the conformal FSS and a finite planar FSS is presented through simulations at the frequency range between 26 to 40 GHz in order to demonstrate the advantages of utilizing the conformal FSS in the near-field. The conformal FSS is 3D printed and copper electroplated, which leads to a low-cost and lightweight bandpass filter array. Placing it in the near-field region of a primary antenna can be used as radomes to realize compact high-performance mm-wave systems. The comparison between simulated and measured conformal FSS results is in good agreement. The challenges that arise when designing, manufacturing, and measuring this type of structure are reported and guidelines to overcome these are presented.

Optimization of Frequency Selective Surfaces for the Design of Electromagnetic Mantle Cloaks

A full optimization procedure that combines the parallel self-adaptive low-high evaluation-evolutionary algorithm with the finite element method-robin boundary condition iteration numerical method is proposed for the design of electromagnetic mantle cloaks enclosing arbitrary objects. The procedure is efficiently applied to optimize an ultrathin, conformal frequency selective surface (FSS), placed around an inhomogeneous dielectric cylinder to be made invisible. Two very different optimal solutions have been found, and this can be a great advantage for the designer using the proposed optimization strategy.

Mantle Cloaks Based on the Frequency Selective Metasurfaces Designed by Bayesian Optimization

We propose a full optimization procedure for designing mantle cloaks enclosing arbitrary objects, using sub-wavelength conformal frequency selective surface (FSS). Rely on the scattering cancellation principle of mantle cloak characterized by an average surface reactance, a personal computer can achieve this design procedure. By combing a Bayesian optimization (BO) with an electromagnetic solver, we can automatically find the optimal parameters of a conformal mantle cloak which can nearly cancel the scattering from the enclosed objects. It is shown that the results obtained by our method coincide with those from a rigorous analytical model and the numerical results by full parametric scanning. The proposed methodology opens up a new route for realizing ultra-wideband illusion scattering of electromagnetic wave, which is important for stealth and microwave applications.

Beamwidth Control of Omnidirectional Antenna Using Conformal Frequency Selective Surface of Different Curvatures

Development and implementation of different curvatures of a conformal frequency selective surface (FSS) for achieving beam diversity in a dual-band monopole antenna are demonstrated. The FSS based on a flexible dielectric substrate (polymer) is reflective at 3.5 GHz. The curved FSS when incorporated in the antenna gives different beamwidths (3 dB) between 68°–130° for concave shape and 144°–210° for convex shape at 3.5 GHz. In both the cases, omnidirectional radiation from the antenna is maintained at 5.8 GHz. Later, a corner reflector based on the FSS instead of a metal, operating at 5.8 GHz, is integrated in the antenna along with the convex shaped curved FSS. The new antenna system exhibits a beamwidth of 190° with a peak gain of 6.8 dBi at 3.5 GHz and the beamwidth of 58° with a peak gain of 8.3 dBi at 5.8 GHz. The proposed method is useful for achieving beam diversity in multiband antennas. Simulation results from ANSYS HFSS and experimental measurements of the fabricated prototypes are provided.

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Frequency-Dependent Directive Radiation of Monopole-Dielectric Resonator Antenna Using a Conformal Frequency Selective Surface

Development of a conformal frequency selective surface (FSS) for radiation diversity of hybrid monopole-dielectric resonator antenna has been demonstrated in this paper. In the proposed method, a planar FSS screen with meandered unit cell is designed to be reflective at 5 GHz followed by its mapping on a cylindrical surface of flexible dielectric material. The proposed conformal reflector when kept at a specific distance from the radiating element improves the bandwidth of the antenna from 26.8% to 53.67% in 4–6 GHz band. A significant enhancement of 5–6 dBi in gain is also achieved over this band. Gain of the antenna with reflector is maintained around 9.5 dBi with a variation of ±1.5 dB. On the other hand, omni-directional radiation of the antenna is maintained at an upper band of 7–9 GHz. Simulations have been performed using ANSYS High Frequency Structure Simulator (HFSS). Experimental measurements of the fabricated prototype have been provided. The proposed design is useful to achieve pattern diversity in multiband antenna system.

Efficient Numerical Model to Analyze the Conformal Frequency-Selective Surfaces

A modified model is presented in this letter to efficiently analyze the electromagnetic properties of the conformal capacitive frequency-selective surfaces (FSSs). By employing a dual set of quasi-orthogonal rooftop basis functions for the surface current distribution over the FSS conducting surface, the conditioning of the system matrix is significantly improved without decreasing the solution accuracy or increasing the computational costs. As a result, the iteration steps and CPU time are dramatically reduced in comparison with the conventional rooftop-expanded scheme. Besides, without subdividing the narrow side of the conducting cells of the FSS in the space discretization, the proposed model is also geometrical-adaptive and engineering-accordant, which results in a significant reduction of the number of unknowns compared to the conventional RWG-expanded scheme. Moreover, the multilevel fast multipole algorithm is employed to further improve the computational efficiency. Numerical experiments demonstrate the satisfactory accuracy and much higher efficiency of the proposed scheme.

Design and Modelling of the Cylindrical Conformal FSS with Mechanical Bending Cover Method

Traditional Design of frequency selective surfaces (FSSs) often involves the patterning of planar arrays of 2D structures that act as stop or pass band filters for electromagnetic waves. Using the mechanical bending cover method, cylindrical conformal FSS is constructed and then simulated by the discrete FITD model. The results show that the center frequency offsets to the low frequency with the decrease of curvature, but the in-band flatness decreases and the out-band appears severe concussion. The Wood's anomaly exists at high frequency where the transmission will be decreased. Moreover, compared with cylindrical metal plate, the cylindrical conformal FSS behaves good at the RCS reduction performance of single and double stations within the 5 20GHz bands with the incident angles between -150 -50 and 50 150.

DESIGN AND PERFORMANCE OF A W-BAND MMW/IR COMPOUND CASSEGRAIN ANTENNA SYSTEM WITH A HYPERBOLIC SUB-REFLECTOR BASED ON FREQUENCY SELECTIVE SURFACE

A MMW/IR compound Cassegrain antenna system for mono-pulse radar applications is presented in this paper. By comparing different modeling methods of conformal frequency selective surface (CFSS), a sub-reflector, with a good performance of reflection at 93 GHz and transparency at the wavelength of 1.06µm, is achieved according to sputtering technique. At the wavelength of 1.06µm, transmittance of the sub-reflector is 67%. Compared to a Cassegrain antenna system consisting of a metallic sub-reflector with identical size, the gain of the compound antenna system has a negligible loss (less than 0.4 dB) at 93 GHz. Compared with the patent in (13), the design can improve the limited size of receiving system and the utilization of aperture of the compound detection system at IR region, and can also enhance the heat dissipation.

Analysis of Finite Conformal Frequency Selective Surfaces via the Characteristic Basis Function Method and Spectral Rotation Approaches

An efficient characteristic basis function (CBF)-based method is proposed to analyze conformal frequency selective surfaces (FSS) that are not amenable to analysis by using conventional numerical methods typically used to model infinite, planar, and doubly periodic FSSs. The technique begins by employing the CBFs to describe the currents induced on the elements. The reaction integrals needed to derive the reduced matrix elements are computed either in the spatial domain, or spectral domain, depending on the separation distance between the blocks, so as to make the process numerically efficient. The spectral domain integrals are evaluated by making use of the spectral rotation (SR) on the spectra of the CBFs to alleviate the computational burden to be associated with full three-dimensional (3-D) Fourier transform, which is required in the conventional spectral domain approach applied to nonplanar geometries. Numerical results from the new method have good agreement with the fully spatial characteristic basis function method (CBFM) and the conventional method of moments (MoM). However, the required central processing unit (CPU) time is greatly reduced.

Analytical modeling of conformal mantle cloaks for cylindrical objects using sub-wavelength printed and slotted arrays

Following the idea of “cloaking by a surface” [A. Alù, Phys. Rev. B 80, 245115 (2009); P. Y. Chen and A. Alù, Phys. Rev. B 84, 205110 (2011)], we present a rigorous analytical model applicable to mantle cloaking of cylindrical objects using 1D and 2D sub-wavelength conformal frequency selective surface (FSS) elements. The model is based on Lorenz-Mie scattering theory which utilizes the two-sided impedance boundary conditions at the interface of the sub-wavelength elements. The FSS arrays considered in this work are composed of 1D horizontal and vertical metallic strips and 2D printed (patches, Jerusalem crosses, and cross dipoles) and slotted structures (meshes, slot-Jerusalem crosses, and slot-cross dipoles). It is shown that the analytical grid-impedance expressions derived for the planar arrays of sub-wavelength elements may be successfully used to model and tailor the surface reactance of cylindrical conformal mantle cloaks. By properly tailoring the surface reactance of the cloak, the total scattering from the cylinder can be significantly reduced, thus rendering the object invisible over the range of frequencies of interest (i.e., at microwaves and far-infrared). The results obtained using our analytical model for mantle cloaks are validated against full-wave numerical simulations.

A Low-Profile Third-Order Bandpass Frequency Selective Surface

We demonstrate a new class of low-profile frequency selective surfaces (FSS) with an overall thickness of lambda/24 and a third-order bandpass frequency response. The proposed FSS is a three layer structure composed of three metal layers, separated by two electrically thin dielectric substrates. Each layer is a two-dimensional periodic structure with sub-wavelength unit cell dimensions and periodicity. The unit cell of the proposed FSS is composed of a combination of resonant and non-resonant elements. It is shown that this arrangement acts as a spatial bandpass filter with a third-order bandpass response. However, unlike traditional third-order bandpass FSSs, which are usually obtained by cascading three identical first-order bandpass FSSs a quarter wavelength apart from one another and have thicknesses in the order of lambda/2 , the proposed structure has an extremely low profile and an overall thickness of about lambda/24 , making it an attractive choice for conformal FSS applications. As a result of the miniaturized unit cells and the extremely small overall thickness of the structure, the proposed FSS has a reduced sensitivity to the angle of incidence of the EM wave compared to traditional third-order frequency selective surfaces. The principles of operation along with guidelines for the design of the proposed FSS are presented in this paper. A prototype of the proposed third-order bandpass FSS is also fabricated and tested using a free space measurement system at C band.