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Diverse application scenarios demand frequency-selective surfaces (FSSs) with tailored center frequencies and bandwidths. However, their design traditionally relies on iterative full-wave simulations using tools such as the High-Frequency Structure Simulator (HFSS) and Computer Simulation Technology (CST), which are time-consuming and labor-intensive. To overcome these limitations, this work proposes an octagonal fractal frequency-selective surface (OF-FSS) composed of a square ring resonator and an octagonal fractal geometry, where the fractal configuration supports single-band and multi-band resonance. A physics-informed reinforcement learning (PIRL) algorithm is developed, enabling the RL agent to directly interact with CST and autonomously optimize key structural parameters. Using the proposed PIRL framework, the OF-FSS achieves both single-band and dual-band responses with desired frequency responses. Full-wave simulations validate that the integration of OF-FSS and PIRL provides an efficient and physically interpretable strategy for designing advanced multi-band FSSs.

Radar absorption structures made of an active frequency selective surfaces (AFSS) have enormous potential in the aviation, naval, and other industries. In this research paper, a systematic review (SR) is carried out in the field of the AFSS to bring uncertainties, obstacles, challenges, classifications, applications, and design issues that arrive in the development of the sub-6 GHz architecture. To bias the AFSS component, as per the signal requirements, a unique set of circuits (PIN diode) is required, with ON and OFF state and a transmission zone. The bandwidth of which is determined by the bias voltage supplied. It can behave as a complicated hybrid impedance structure by providing ON and OFF biasing voltage to a PIN diode embodied in an FSS structure. Higher manufacturing costs of AFSS components, more significant complexities involved, a large amount of power consumption, and reactive impedance losses are some common limitations faced while implementing and designing an AFSS. Many envisioned problems are corrected with the AFSS design, current or creative implementations, and processing parameters are investigated progressively. It implies that new AFSSs will be an alternative to regular FSSs in the future. This paper is based on Kitchenham’s three-phase review procedure and supplements it with results, views, and recommendations from other leading experts in the field.

A three-dimensional frequency selective surface (3D-FSS) that provides stability for an angle of incidence and miniaturised unit cell size is presented. The proposed 3D-FSS is easy to fabricate and is implemented using via holes in a multilayer printed circuit board structure. Frequency transmission characteristics for different angles of both TE and TM polarisations are presented through simulations. The proposed structure was fabricated so as to verify the simulation results. The comparisons between the simulation and the measured results show good agreement. The results also show that the proposed 3D-FSS can provide better frequency stability for different incidence angles and polarisations as well as miniaturised unit cell size.

A low radar cross-section (RCS) microstrip antenna is proposed. A polarisation-dependent frequency selective surface (PDFSS) structure is designed and used as the antenna ground. By properly etching the unit cells of the PDFSS with different orientations on the ground of the microstrip antenna, the RCS has been reduced in the angular range of −90 ° ≤ θ ≤ +90° in both the xoz-plane and the yoz-plane under the φ-polarised incident wave. The ‘Γ’ shape slots between the unit cells with different orientations are used to reduce their mutual coupling.

Here, a wideband absorptive frequency‐selective reflector (AFSR) based on a split‐ring resonator (SRR) embedded with chip resistors is proposed. The wide absorption band is achieved by the multimode resonances of two dipoles, and the insertion of SSRs is able to bypass the losses in the passband, thus allowing to open a wide window in addition to achieving broadband absorption performance. The upper structure consists of a dipole in series with two SSRs, while the lower dipole connects one SSR. The upper and lower structures are grouped together and rotated by 90° around the Z‐axis to form the final dual‐polarization 3D‐AFSR. The simulated results demonstrate that reflection coefficients below −10 dB are seen between 6.74–9.31 and 16.91–19.61 GHz under normal incidence, with a total fractional bandwidth of 46.8%. It also has a wide 3 dB reflection band which is obtained from 10.69 to 15.44 GHz, and the entire profile is 0.17λL, where λL is lowest operating frequency. A wideband absorptive frequency‐selective reflector based on split‐ring resonator embedded with chip resistor is proposed here. The simulated results demonstrate that, reflection coefficients below −10 dB are seen between 6.74–9.31 and 16.91–19.61 GHz under normal incidence, with a total fractional bandwidth of 46.8%. It also has a wide reflection band from 10.69 to 15.44 GHz.

This paper presents a wideband and high-gain rectangular microstrip array antenna with a new frequency-selective surface (FSS) designed as a reflector for the sub-6 5G applications. The proposed antenna is designed to meet the US Federal Communications Commission (FCC) standard for 5G in the mid-band (3.5–5 GHz) applications. The designed antenna configuration consists of 1 × 4 rectangular microstrip array antenna with an FSS reflector to produce a semi-stable high radiation gain. The modeled FSS delivered a wide stopband transmission coefficient from 3.3 to 5.6 GHz and promised a linearly declining phase over the mid-band frequencies. An equivalent circuit (EC) model is additionally performed to verify the transmission coefficient of the proposed FSS structure for wideband signal propagation. A low-cost FR-4 substrate material was used to fabricate the antenna prototype. The proposed wideband array antenna with an FSS reflector attained a bandwidth of 2.3 GHz within the operating frequency range of 3.5–5.8 GHz, with a fractional bandwidth of 51.12%. A high gain of 12.4 dBi was obtained at 4.1 GHz with an improvement of 4.4 dBi compared to the antenna alone. The gain variation was only 1.0 dBi during the entire mid-band. The total dimension of the fabricated antenna prototype is 10.32 λo × 4.25 λo ×1.295 λo at a resonance frequency of 4.5 GHz. These results make the presented antenna appropriate for 5G sub-6 GHz applications.

This paper presents a miniaturized, polarization insensitive and angularly stable frequency selective surface (FSS) for WiMAX (3.5 GHz) applications. The proposed FSS structure improves upon conventional curved units by incorporating 45∘ tilted dipoles with extended lengths to increase the effective electrical size. The proposed FSS is printed on float glass with a dielectric constant of 8. The unit cell dimensions are 0.062λ0×0.062λ0 (where λ0 is the free space wavelength at the first resonant frequency). It exhibits a bandstop characteristic at 3.5 GHz with a bandwidth of 540 MHz (-10 dB). This FSS demonstrates a stable frequency response under incident angles ranging from 0∘ to 80∘ for both horizontal and polarization angles. Furthermore, the proposed structure is further analyzed through the derivation of an equivalent circuit model. Finally, a prototype of adequate size is fabricated to validate the simulation results. Both the simulation and measured results confirm the stable performance of the proposed FSS.

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.

A novel compact coplanar waveguide (CPW) fed hexagonal shaped metamaterial unit cell inspired antenna for multi-band operation is proposed in this article. The gain of the proposed antenna is enhanced using frequency selective surface (FSS) as reflector. Initially, a hexagonal shaped CPW-fed monopole antenna is designed with the dimensions of 30 × 30 × 1.6 mm 3 on FR4 substrate for the resonant frequency of 4.5 GHz and a proposed unequal width of hexagonal metamaterial unit cell is inspired on the antenna for obtaining the other two resonating frequencies at 1.7 GHz and 3.4 GHz. The gain achieved by this antenna is 2.4 dBi. To enhance the gain of the antenna, the FSS is introduced as the reflector. The proposed antenna with FSS is providing the enhanced gain of 9 dBi. Antenna parameters such as reflection coefficient, E-plane and H-plane radiation patterns, gain and FSS characteristics such as band stop, band pass, reflection phase are analyzed through simulation and validated through measurements. In addition, the angular stability of the proposed FSS structure is analyzed and found as 40°. The proposed technique can be adopted for increasing the gain of the multi-band antenna.

Frequency-selective surfaces (FSSs) have attracted great attention owing to their unique feature to manipulate transmission performance over the frequency domain. In this work, a filtering antenna-filtering antenna (FA-FA) FSS with a wide passband and double-side sharp roll-off characteristics is presented by inter-using the filtering antenna and receiving–transmitting metasurface methods. First, a dual-polarized filtering antenna element was designed by employing a parasitic band-stop structure with an L-probe feed. Then, the FA-FA-based FSS unit was constructed by placing two such filtering antennas back to back, with their feedings connected through metallic vias. Finally, the FSS with a wide passband and high selectivity was realized by arraying the FA-FA units periodically. The full-wave simulation results demonstrated that the designed FA-FA-based FSS had a wide passband from 13.06 GHz to 14.46 GHz with a flat in-band frequency response. The lower and upper roll-off bandwidths were sharp, reaching 1% and 1.2% of the center frequency. The proposed FA-FA-based FSS was fabricated and measured, achieving the coincident performance according to the theoretical prediction. The wideband band-pass FSS obtained a sharp double-side roll-off feature, which can be applied in various studies such as an antenna array, metasurface, communication, etc.