A Tri-band Angularly Stable Frequency Selective Surface with Controllable Resonances for EM Shielding

A wide stop-band angular stable frequency selective surface (FSS) is presented. This FSS consist of two spiral structures separated by a substrate in the middle. It exhibits a wide stop-band of 20 GHz in the frequency range of V-band. Furthermore it produces a stable angular response upto 80 degree for both TE and TM incidence modes. Since it provides a wide stop-band and good angular stability, it is suitable for millimeter-wave shielding applications.

In this paper, a low profile, ultrathin, wide passband, polarization insensitive, wide angular stable frequency selective surface (FSS) is proposed. The structure is realized on a two-sided copper on Roger’s RT 5880 substrate with 0.51 mm thickness. The top layer design consists of a combination of slotted circle with quad arcs along with cross bars at the center with diagonal tilts at the corner. On the bottom layer, an elliptical-shaped patch is placed at the center. This elliptical patch at the bottom layer of FSS helps to realize the wide pass-band response. The pass-band bandwidth ranges from 8.45 to 9.45 GHz centered at 8.96 GHz. The proposed FSS is polarization insensitive. The structure provides angular stability till 45° of incident angle for Transverse Electric (TE) and Transverse Magnetic (TM) modes of operation. The prototype of FSS is fabricated with an array of 10 x 10 unit cells. The simulated and measured results are found to be in good agreement.

This work presents a miniaturized angularly stable frequency selective surface (FSS) on a single layer substrate. Unit cell comprises of convoluted circular rings connected cross dipole and linear arrow headed dipole printed on both sides of the substrate. A closely spaced dual stop band resonances are obtained at 10 and 14.3 GHz with frequency ratio of 1.43. FSS shows a − 10 dB stopband bandwidth (BW) from 7.32 to 11.05 GHz (40.61%) and 12.68 to 15.97 GHz (22.97%) with >30 dB stop band rejection at center frequencies. FSS poses the features of stable performance under change in polarization and angularly stable response up to 60° incident angle. Later the proposed FSS is printed on grounded thick dielectric substrate and it shows broadband reflection type co to cross polarization conversion from 7.2 to 17.32 GHz (82.54%) with polarization conversion ratio >80% and oblique angle stability up to 35°. To the best of authors' knowledge, this is the first time in literature a single unit cell geometry with two different properties, spatial filtering on transmission and polarization conversion on reflection have been designed and analyzed in details. A finite array with size 25 × 25 of the proposed FSS is fabricated and the measured filter response shows a good agreement with simulated performance at normal and oblique incidences. The proposed FSS is used as a reflector of a dual band monopole antenna and the simulation results shows gain improvement of 5.51 and 3.8 dB at the two stopband frequencies.

Professor Yongmeng Liu from the Harbin Institute of Technology in China talks about the research behind his Letter ‘Optical-transparent Wi-Fi bandpass mesh-coated frequency selective surface’ on page 381. Professor Yongmeng Liu Our group's work is focused on designing novel frequency selective surfaces (FSSs) as security wallpaper for optical windows of electromagnetic filtering and interference shielding. A FSS is an array of periodic apertures perforated on a conducting sheet or metallic patches which exhibits bandpass or bandstop responses. A traditional FSS generally has very low optical transmittance and cannot be easily applied as a dual-mode filter to achieve simultaneous high optical transparency and Wi-Fi bandpass filtering. The solution is to create novel periodic structures that have unique and fascinating features attracting us to this field of research. We have reported a FSS periodic structure fabricated using a UV lithography technique to achieve simultaneous high optical transparency and Wi-Fi bandpass filtering. The FSS is a resonance cavity structure with a periodic array of apertures removed from an optically transparent conductive mesh coating. Simulation and experimental results show that the resonance centre frequency of the mesh-coated FSS is about 5.3 GHz with transmittance loss of −2dB, while its optical transmittance is greater than 87.5% in a waveband of 1.35 to 3.3 µm. Such a FSS has the merits of ameliorable electromagnetic shielding effectiveness and improvable optical transmittance by optimising the thickness of the mesh layer. Our FSS can be used as security wallpaper for optical windows to solve the problem of transmission security of Wi-Fi communications. Generally, transparent conductive mesh coating is widely used to provide both strong RF/microwave shielding and optical transmittance, however this seriously attenuates Wi-Fi transmittance. A bandpass FSS is used to provide spatial filtering, but suffers from low optical transmittance due to the large fraction of the area obscured by metallic films. Neither of these existing technologies can meet the dual requirement of optics/Wi-Fi filtering for security wallpaper. It is a significant advancement to develop a periodic structure to achieve simultaneous optical transparency and stable Wi-Fi bandpass. One potential application is as security wallpaper for securing confidential business data. Another application is to enhance the transmission of the specific signal of interest through optical windows. State-of-the-art FSS technology includes indium tin oxide (ITO). This kind of ITO-FSS is used for energy saving windows since the ITO layer reflects a significant portion of long and short infrared wave energy. However, the restrictive relationship between conductivity and optical transparency of ITO is the fatal flaw in this implementation. We are working on a project to design conductive metallic meshes to shield radio-frequency/microwave interference for optical windows to optimise mesh patterns and parameters to improve optical transmission and to minimise the high-order optical diffraction effect, as well as enhance electromagnetic shielding effectiveness. Challenges will be to further optimise the structure of the mesh-coated FSS to improve the performance, specifically the sheet resistance, bandwidth and the transmission stability of incidence angles and polarisations. The FSS has high sheet resistance which will impact Wi-Fi transmission, however the FSS has adjustable thickness to enhance its conductivity and improve Wi-Fi transmission performance while having no impact on optical transparency. Additional challenges include developing ways to integrate the mesh-coated FSS with optical windows of existing buildings. Fortunately we can fabricate the FSS on flexible PET films and add the composite films to each side of existing optical windows. Over the next few years novel periodic electromagnetic structures will integrate metamaterials, active circuit elements and other new techniques to develop security wallpapers to protect communications security. The trend of periodic electromagnetic structures for electromagnetic filtering and shielding will evolve from expensive, heavy and passive to low-cost, lightweight, active and self-tuning. We would like to see increasing numbers of new periodic electromagnetic structures being developed throughout numerous frequency ranges, perhaps even the entire electromagnetic spectrum. We believe all these beautiful periodic electromagnetic structures will be brought into real-world use and will make our lives better!

Frequency Selective Surfaces (FSSs) are engineered periodic structures that function as spatial filters, enabling the selective transmission, reflection, or rejection of electromagnetic (EM) waves. These structures are widely utilized in advanced RF and microwave systems for applications such as filtering, electromagnetic shielding, and stealth technology. In this manuscript, we propose an angularly stable and polarization-insensitive band-stop Frequency Selective Surface (FSS) optimized for applications in the lower Ka-band. The proposed FSS is constructed from a periodic two-dimensional array of compact unit cells, each with dimensions of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$5.5 ~\text{mm} \times 5.5 ~\text{mm}$</tex>. This compact configuration supports seamless integration into high-density millimeter-wave (mmWave) systems. Each unit cell incorporates a concentric dual-ring geometry embedded with orthogonally intersecting cross-slots. This unique configuration enhances the EM wave rejection capability of the surface by inducing multiple resonances. As a result, the FSS achieves a wideband stopband response ranging from 20 GHz to 28.5 GHz, corresponding to a notable fractional bandwidth (FBW) of approximately 34%. The dual-ring structure promotes resonance coupling, while the cross-slot inclusions facilitate effective suppression of incident waves, regardless of polarization orientation. The concentric design further contributes to polarization insensitivity, ensuring consistent stopband performance for both vertically and horizontally polarized waves. In addition, the symmetrical layout enhances angular stability, a critical requirement for real-world deployment. To verify the robustness of the design, full-wave electromagnetic simulations were conducted under varying angles of incidence for both Transverse Electric (TE) and Transverse Magnetic (TM) polarizations. The results reveal negligible shifts in the resonance frequency and stable stopband attenuation across angles up to 60°, confirming the angular resilience of the structure. With its compact form factor, wideband rejection characteristics, and strong angular and polarization stability, the proposed FSS is highly suitable for advanced mmWave applications, including EM interference suppression, co-site interference mitigation, and radar cross-section (RCS) reduction in defense, aerospace, and next-generation communication systems.

Single-layer dual-band frequency selective surface for coverage-enhancing applications in millimeter-wave 5G

In this paper, a dual-band Frequency Selective Surface (FSS) absorber is presented for Wireless Local Area Network (WLAN) security. The FSS absorber is designed for two frequencies of interest, i.e., 2.45 GHz (lower band) and 5.25 GHz (upper band), respectively. FSS design consists of two layers; first layer consists of conducting square-loop elements while second layer consists of resistive square-loop elements, placed in-front of a conducting FSS layer. The proposed FSS design provides good frequency stability for oblique incidences for perpendicular (TE) and parallel (TM) polarizations. It provides better absorption rate for 2.45 GHz and 5.25 GHz frequency bands and allow the transmission of other microwave frequencies. Also, the proposed FSS design reduces multipath fading of additional WLAN signals which occurred due to the placement of conducting FSS layer. Presented results demonstrate that the proposed FSS design has a stable frequency response for the desired frequency bands for TE and TM polarizations with good absorption rate when the incident wave angle varied from 0<SUP>0</SUP> to 45<SUP>0</SUP>.

Purpose The purpose of this paper is to present the results of a particle swarm optimization (PSO) method applied in the design of a square-loop frequency selective surface (FSS) via the equivalent circuit model (ECM), considering the dielectric effective permittivity as a variable in the optimization problem. Design/methodology/approach In the optimization process considered, besides the FSS square loop geometric parameters, the thickness and relative permittivity of dielectric material used as support are included as variables in the search space, using for this a model of dielectric effective permittivity introduced by the authors in a previous work. Findings Square loops were designed and the obtained results were compared with designs reported in literature for applications in wireless local area network and long-term evolution 4G systems. The low computational cost is remarkable as well as the acceptable accuracy obtained with the proposed approach. The PSO method results were implemented with the ECM and compared with those obtained via Ansys – high frequency structure simulator commercial software simulations. Originality/value The lack of a model of dielectric effective permittivity for the ECM causes a restricted search space in the stochastic FSS design process limited to only geometric parameters, as it is reported in the available literature. The proposed approach simplifies and makes more flexible the design process, and allows guiding the FSS design to unit cell surface and/or dielectric thickness of small dimensions.

A method for designing miniaturized frequency selective surface (FSS) using equivalent circuit model (ECM) and multi-objective particle swarm optimization (MOPSO) algorithm is proposed in this paper. In the proposed method, ECM describing the desired frequency response is established, and parameters of the ECM are determined using the MOPSO algorithm. Combining the ECM analysis and the MOPSO algorithm, the requirements of desired frequency response can be converted into the constraints of equivalent circuit parameters, which simplifies the optimization process of FSS design. For verification, a single-layer bandstop-bandpass FSS composed of convoluted elements with the unit cell size of 0.073λ 0 × 0.073λ 0 is designed. Simulation results show that the −15 dB bandwidth is from 2.62 to 2.83 GHz, and the −0.5 dB bandwidth is from 3.84 to 5 GHz. Meanwhile, the resonant frequency is stable at different incident angles (up to 80°) under TE and TM polarization. Finally, the proposed FSS prototype is fabricated and measured. The measured results agree well with the simulated ones.

In this paper, a liquid crystal tunable frequency selective surface (FSS) in S-band is proposed. The relative permittivity of liquid crystal can be changed from 2.5 to 3.3, with the change of bias voltage applied to it. In that way, the FSS proposed in the paper has a tunable feature. The full-wave simulation of the tunable FSS is carried out in the paper and the equivalent circuit model (ECM) is extracted. The SPICE-based simulation results are consistent with the full-wave simulation, which verify the validity of the ECM of the FSS. The pass band shifts from 3.612 GHz to 3.954 GHz with a relative bandwidth of 9.04% when the bias voltage of liquid crystal changes.

This paper deals with a novel Frequency Selective Surface (FSS) Electromagnetic shield that operates in the low 5G vertical applications. This FSS is designed for the purpose of serving as an electromagnetic shield to eliminate EMI. The FSS designed operates at a frequency of 5GHz with a bandwidth of 1.45GHz providing a shielding of more than 40dB. The proposed FSS uses a polyester sheet as substrate and uses silver for the metallization of the FSS. Since the proposed FSS is symmetric, it behaves similarly when a Transverse Electric (TE) or a Transverse Magnetic (TM) wave is incident. The structure was designed and simulated using CST software. The results show that this FSS blocks electromagnetic radiation at lower frequencies as intended.

This paper presents the first additively manufactured, single-layer, polarization-insensitive, fluidically switchable band stop frequency selective surface (FSS) for electromagnetic (EM) shielding applications. The unit cell features a meandered square-loop metallic pattern with a fluidic cavity embedded within the substrate via fused deposition modeling. Frequency tuning is realized by controlling the fluidic state of the cavity through an automated pump system. In the absence of water, the structure resonates at 3.5 GHz; introducing distilled water into the cavity lowers the resonance to 2.6 GHz, achieving a 29.5% tuning range. The proposed FSS provides a minimum shielding effectiveness of 30 dB for both cases. An equivalent circuit model is developed to interpret the tuning mechanism, and both simulation and experimental results confirm the effectiveness of the design. The proposed FSS maintains stable performance up to 60 ∘ under both TE and TM polarizations. With its simple, low-profile, single-layered design, this FSS offers strong potential for EM shielding and interference mitigation applications.

In this paper, a low-profile tunable bandstop frequency selective surface (FSS) is presented for S-band applications. The FSS consists of square loops connected through varactor diodes across the diagonals. By controlling the reverse voltage of the varactors, the resonance frequency can be tuned for a wide frequency range. Full-wave simulation shows 21% tuning range from 2.31 GHz to 2.79 GHz with respect to lower resonance frequency. The novelty of the design lies in its four-fold symmetry, which makes the structure polarization-insensitive. The FSS is also angularly stable under oblique incidence for both TE and TM polarizations. Additionally, an equivalent circuit model has been introduced to explain the resonance mechanism of the proposed FSS. The structure is also fabricated and measured, where good agreement is observed between simulated and measured responses.

Frequency selective surfaces (FSS) have been widely used for many years in antennas applications, like beams multiplexer, or filtering for microwave and optical signals. We present a new application for FSS. For the former applications, FSS are free standing or backed by a perfect dielectric. It can be also embedded into a dielectric medium or sandwiched between different dielectric media. In the new application, FSS are used as components of radar absorbing materials (RAM). Classical RAM are composed of several layers of dielectric or magnetic media. For example, the two most famous absorbing screens are the Dallenbach and the Salisbury screens. The first is a short circuited lossy dielectric (or magnetic) layer, while the second is composed of two layers. The Salisbury screen consists of a thin resistive film deposited on a perfect metal-backed dielectric layer. We show how a FSS can modify and improve the absorption performance of Dallenbach screens. The FSS is deposited on the outer surface of the Dallenbach screen in this new application. We show numerically that the absorption band can be either shifted or broadened by the presence of an FSS. We illustrate the numerical results with measurements. The theoretical aspects of the computation of the interaction between the electromagnetic field and the structure are discussed, and the properties of the new absorbing screen are examined.

This article describes a frequency selective surface (FSS) for shielding with an improved angular stability over a wide frequency range. The proposed unit design consists of a circular ring in a square element, two cross dipoles, and a T-type slotted structure printed of size 0.052 lambda0 × 0.052 lambda0 on a layer of FR4 substrate having a thickness of 0.0075 lambda0. Here lambda0 is the free-space wavelength of lower cut-off frequency. It has improved the bandwidth range from 2.84 GHz to more than 20 GHz enough to provide effective shielding against the entire ultra-wideband (UWB) range, as well as the X– and Ku–bands. Up to 75 degrees of oblique incidence, the proposed design is polarisation independent and angularly stable. The structure is full wave simulated and an equivalent circuit model is realised. The findings are validated through the use of a fabricated prototype. The applications of shielding are in reducing electromagnetic interference in civil and military applications as well as improving the gain of UWB patch antennas.