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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 article introduces a simple yet highly effective wideband microwave absorber, leveraging a resistive ink Frequency Selective Surface (FSS) in conjunction with Equivalent Circuit Modeling (ECM) and Deep Neural Network (DNN) techniques. This absorber design addresses critical challenges in the field by offering a versatile and efficient solution for wideband absorption while remaining lightweight and polarization-insensitive. The presented resistive FSS unit cell has an electrical length of 0.31 λ. The proposed square-loop FSS-based microwave absorber is designed and fabricated using Y-Shield HSF 64 resistive ink having a conductivity of 640 S/m. Experimental measurements are meticulously executed using the WR90 rectangular waveguide method, utilizing 1 × 2-unit cells. This configuration allows for broad bandwidth absorption with minimal weight and polarization sensitivity. Results indicate an impressive − 10 dB absorption bandwidth spanning 3.5 GHz (8.9–12.4 GHz) within the X-band of microwave frequencies. Beyond its wideband prowess, this absorber champions the attributes of simplicity and lightweight construction, rendering it an attractive candidate for diverse applications. Moreover, it showcases an inherent indifference to polarization, a pivotal feature for stealth applications.

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 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.

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.

By combining the technique of energy selective surface and frequency selective rasorber, an energy selective rasorber is proposed, which performs selective energy protection in the low communication frequency band (0.8–2 GHz) and wave-absorbing property in the high-frequency band (6–18 GHz). The design consists of two layers, of which the bottom one contains a lumped diode structure for energy selection function in the transmission band, while together with the top layer, they perform a wideband wave absorbing function. The simulated and measured results agree well with each other, and both show good absorption in 6–18 GHz and energy-selective property around 1.86 GHz. That is, when the incident power changes from −30 to 14 dBm, the reflection coefficient changes from below −22 dB to above −2 dB, while the transmission coefficient changes from above −3 dB to below −17 dB.

Surface modification of graphene oxide (GO) is a powerful strategy to develop its energy density for electrochemical energy storage. However, pre-modified GO always exhibits unsatisfactory hydrophilia and its ink-relevant utilization is extremely limited. Although GO ink is widely utilized in fabricating micro energy storage devices via extrusion-based 3D-printing, simultaneously obtaining satisfactory hydrophilia and high energy density still remains a challenge. In this work, an in-situ surface engineering strategy was employed to enhance the performance of GO micro-supercapacitor chips. Three dimensionally printed GO micro-supercapacitor chips were treated with pyrrole monomer to achieve selective and spontaneous anchoring of polypyrrole on the microelectrodes without affecting interspaces between the finger electrodes. The interface-reinforced graphene scaffolds were edge-welded and exhibited a considerably improved specific capacitance, from 13.6 to 128.4 mF·cm −2 . These results are expected to provide a new method for improving the performance of micro-supercapacitors derived from GO inks and further strengthen the practicability of 3D printing techniques in fabricating energy storage devices.

The open-source code PSSFSS for analysis and design of polarization selective surfaces (PSSs), and frequency selective surfaces (FSSs) is presented, beginning with an introduction to the Julia programming language in which the code is written. Analysis methods and algorithms used in PSSFSS are described, highlighting features of Julia that make it attractive for developing this type of application. Usage examples illustrate the code’s ease of use, speed, and accuracy.