Frequency Selective Surface Pattern Research Articles

Microstructure FSS patterning to improve 5G microwave signals through low-e plastic windows

Low emissivity (low-e) windows are widely used in the architectural sector to block the infrared (IR) radiation from the Sun. The windows contain a multilayer nanoscale coating of metallic and dielectric layers. The metallic layer, which is responsible for the IR reflection, also attenuates the radio and microwave frequencies used for modern-day technologies such as Fifth Generation (5G) communications. As there is an ever-increasing demand for a reliable interior-to-exterior signal coverage, low-e windows should be transparent to such signals. A class of surface modification — Frequency Selective Surface (FSS) is the technique of choice to be applied on the thin metallic coating to transmit the wireless signal.In this work, a thin silver (Ag) film — 10 nm thick was deposited by electron beam evaporation on polycarbonate substrates as an example low-e coating. Despite excellent IR blocking (64 %) and visible light transmittance (60 %), it also presented a high attenuation of 20 dB at 5G signal bands (72–82 GHz).​ FSS patterns of various geometries and sizes were applied via laser ablation and evaluated to provide the lowest attenuation. We demonstrated that the application of the hexagonal pattern provided largest improvement reducing the 5G attenuation value from 20 dB to 1 dB, without compromising the visible transmittance and having only a minor effect on IR reflection.

Flexible and Reconfigurable Frequency Selective Surface With Wide Angular Stability Fabricated With Additive Manufacturing Procedure

In this letter, a flexible and frequency-reconfigurable bandstop frequency selective surface (FSS) is studied theoretically and experimentally. The FSS patterns are fabricated on an ultrathin flexible substrate through screen printing, a low-cost, high-efficiency additive manufacturing methodology. With the embedded varactors biased from 0 to 20 V, resonant frequency tuning from 3.5 to 5.7 GHz was measured. Outstanding wide angular stability under different bias voltages has also been examined in theory and measurement by changing the incidence angle from 0° to 60°. As a demonstration of the flexible FSS in the conformal application, the FSS is tightly covered on a half-cylinder with a diameter of 10 cm, which agrees well with the case when placed on a flat surface. Furthermore, the unit cell has a size of λ/15 at 3.5 GHz, featured with miniaturization. The mechanical flexibility, frequency reconfigurability, and angular stability make it meaningful in applications of flexible/conformal covering for aiming objects, including electromagnetic scattering control, electromagnetic compatibility (EMC), etc.

Chipless RFID sensor for corrosion characterization based on frequency selective surface and feature fusion

Chipless RFID sensors attract attention to structural health monitoring (SHM) because of its advantages of being low-cost, wireless, passive, and having multiple resonances for sensing. Its application for corrosion sensing, however, receives little attention and faces challenges in terms of sensitivity and reliability. This paper proposes a chipless RFID sensor for corrosion characterization based on frequency selective surface (FSS) and feature fusion. An FSS pattern on a substrate is designed to generate three resonances within 2–6 GHz. The ability of the FSS to characterize corrosion thickness was simulated and validated in the experiments. The experimental results using dedicated corrosion undercoating samples show that the FSS based chipless RFID sensor can be used to characterize corrosion, where the three resonance frequency features provide sensitivity and consistent monotonic relations to the corrosion progression. Furthermore, feature fusion using simple sum and confidence weighted averaging (CWA) can enhance the sensitivity and reliability of the sensor. With the low-profile and printability of the sensor, this work paves the way for smart coatings for corrosion sensing and monitoring on metallic structures.

Fast Semi-Analytical Design for Single-FSS-Layer Circuit-Analog Absorbers

In this article, analytical, empirical, and numerical techniques are integrated for analyzing and synthesizing circuit analog (CA) absorbers based on a single-layer frequency-selective surface (FSS). The proposed design approach dramatically reduces the number of full-wave simulations required for global optimization, so that the potential of fundamental FSS geometries can be exhaustively exploited. With the semi-analytical algorithm, the near optimal bandwidth-thickness can be quickly and reliably calculated for a given single-FSS-layer topology. To demonstrate the robustness of our semi-analytical approach, a square-patch and square-ring FSS absorbers at 10 dB level of absorption are revisited and optimized. The designs are constrained by available materials and standard tolerances for experimental validation. A manufactured prototype achieves a relative bandwidth of 144.15% and a normalized thickness of 0.0972λ L , which is superior to existing designs with more complicated FSS patterns in the literature.

Conceptual design, control, and simulation of a 5-DoF robotic manipulator for direct additive manufacturing on the internal surface of radome systems

In this paper, a novel concept of robotic manipulator is developed for direct additive manufacturing on non-planar surfaces. The application scenario is the metal coating of the internal surface of radome systems, using frequency selective surface patterns. The manipulator is presented from the design, modeling, and control point of view. It is developed following an application-driven approach, meaning that the requirements from the application and the additive manufacturing technology are translated into the design specifications of the robotic system. Simulation results demonstrate that the proposed control strategy based on a decentralized architecture is satisfactory to accurately control the motion of the robotic mechanisms along the trajectory foresees by the direct additive manufacturing task.

Design of Frequency Selective Surface-Based Hybrid Nanocomposite Absorber for Stealth Applications

This paper presents the design, fabrication, and electromagnetic testing of a frequency selective surface (FSS)-based novel hybrid nanocomposite absorber for enhanced electromagnetic absorption. The proposed hybrid absorber consists of an FSS pattern printed over a dielectric substrate backed by aluminum foil, which is integrated together with a customized dielectric nanocomposite sheet. The dielectric sheet comprises the conducting carbon black (CB) nanopowders embedded into the epoxy matrix. The fabricated nanocomposite sheet is first characterized for the effective complex permittivity using the free-space measurement technique, and the measured electromagnetic properties are then used for the design of a hybrid absorber. The proposed hybrid absorber having overall thickness of 2.6 mm shows enhanced −10-dB absorption bandwidth of 3.5 GHz, which is much higher than that of the FSS and CB nanocomposite samples measured individually. The proposed work provides an effective way to combine the absorption properties of FSS with that of a customized dielectric absorber sheet in order to design an efficient hybrid absorber having reduced thickness with enhanced absorption bandwidth for stealth technology and various strategic applications.

Thermal residual stresses in FSS layer-embedded composite plates in co-cure bonding

The residual stresses occur in the frequency selective surface (FSS)-embedded composite structures after co-curing due to mismatch between the coefficient of thermal expansions between the FSS and composite materials. It is well known that residual stress has a great influence on the strength and fatigue life of the FSS-embedded composite structures. Numerous researches have been reported about residual stresses in the composite structures. However, studies on the residual stresses in the FSS-embedded composite structure have not been widely investigated. Therefore, in this study, the thermal residual stresses of FSS-embedded composite structures were analyzed using finite element analysis, with considering the effect of FSS pattern and size. Various FSS patterns, such as square loop, gridded square loop, and double square loop, are considered in the analysis. The effects of dimensional change of design parameters of FSS pattern on the residual stresses in the hybrid composites were also investigated.

CHARACTERISATION OF COPPER NANOPARTICLE INK PRINTED FSS FOR CELLULAR SIGNALS SUPPRESSION

This paper proposes a copper nanoparticle ink printed frequency selective surface (FSS) for cellular signals suppression. The FSS pattern is deposited on a polyimide film by using an inkjet printing technique. The printed FSS elements undergo the post-processing called sintering, where the optimum exposure duration and temperature are determined in order to form a conductive path across the metal pattern. Later, the conductivity of the printed FSS structure deposited on polyimide film is observed. The signal suppression ability of the printed FSS is conducted using the Computer Simulation Technology (CST) Microwave Studio software.

Change of transmission characteristics of FSSs in hybrid composites due to residual stresses

The frequency selective surface (FSS) embedded hybrid composite materials have been developed to provide excellent mechanical and specific electromagnetic properties. Radar absorbing structures (RASs) are an example material that provides both radar absorbing properties and structural characteristics. The absorbing efficiency of an RAS can be improved using selected materials having special absorptive properties and structural characteristics and can be in the form of multi-layers or have a certain stacking sequence. However, residual stresses occur in FSS embedded composite structures after co-curing due to a mismatch between the coefficients of thermal expansion of the FSS and the composite material. In this study, to develop an RAS, the thermal residual stresses of FSS embedded composite structures were analyzed using finite element analysis, considering the effect of stacking sequence of composite laminates with square loop (SL) and double square loop (DSL) FSS patterns. The FSS radar absorbing efficiency was measured in the K-band frequency range of 21.6 GHz. Residual stress leads to a change in the deformation of the FSS pattern. Using these results, the effect of transmission characteristics with respect to the deformation on FSS pattern was analyzed using an FSS Simulator.

Residual stress and electromagnetic characteristics in loop type frequency selective surface embedded hybrid structures

Residual stresses occur in frequency-selective surface (FSS)-embedded composite structures after co-curing due to differences between the coefficients of thermal expansion between composite skins and FSSs. Furthermore, the electromagnetic characteristics may be affected by the deformation of the FSS pattern by residual stresses. Therefore, we studied the changes in electromagnetic characteristics due to the deformation of FSS, using residual stresses to deform loop-type FSS-embedded hybrid composites. We considered the effects of loop-type FSS patterns of equal dimension as well as the stacking sequences of composite laminates on the electromagnetic characteristics of FSSs: Square loop, triangular loop and circular loop. The stacking sequences of composite laminates considered in this study were [0]8, [0/90]4, [±45]4 and [0/±45/90]2. The FSS was located between composite laminates in the middle plane. To determine the residual stresses and deformations in the FSS embedded laminate structures, the thermal loading condition in the finite element analysis was induced by cooling the hybrid structures from 125°C to 20°C based on the cure cycle of the composite. Also, the electromagnetic reflection characteristics of the hybrid structures were predicted using deformed models by residual stresses, considering the effects of stacking sequence of composite laminates. The results showed that the maximum residual stresses and deformations were produced in the [0]8 composites with all three loop-types of FSS pattern. However, the maximum resonance frequency shifts occurred in the square and triangle loop-types with stacking sequence of [0]8, while the maximum resonance frequency shift occurred in the circular loop-type with stacking sequence of [0/±45/90]2.

주파수 선택적 투과막이 결합된 복합재료의 잔류응력평가 및 선택적 투과막 설계

본 연구에서는 최적화 알고리즘 중 하나인 PSO기법을 이용하여 주파수 선택적 표면을 설계하였으며, 설계된 주파수 선택구조를 적용한 하이브리드 레이돔의 잔류응력을 예측하였다. 주파수 선택 구조로서 Square Loop의 등가회로모델에 PSO를 적용하여, K-band(23GHz)에서 차단특성을 갖는 최적의 설계값을 구하였다. 또한 FSS와 복합재료의 하이브리드 레이돔을 제작할 때 발생하는 잔류응력의 효과가 클 경우 구조적 안정성이 떨어질 뿐만 아니라 층간분리가 발생하여 FSS의 전파투과특성에 영향을 미칠 수 있으므로, 하이브리드 레이돔의 제작 시에 동시경화 후 상온으로 냉각되는 과정에서 발생하는 잔류 열응력에 대하여 예측하였으며, FSS패턴과 복합재료의 적층각 변화에 따른 영향을 비교하였다. In this paper, Particle Swarm Optimization(PSO) is applied to the design of the Frequency Selective Surface(FSS) and residual stresses of hybrid radome is predicted. An equivalent circuit model with Square Loops arrays was derived and then PSO was applied for acquiring the optimized geometrical parameters with proper resonant frequency. Residual stresses occur in the FSS embedded composite structures after cocuring and have a great influence on the strength of the FSS embedded composite structures. They also effect transmission quality because of delamination. Therefore, the thermal residual stresses of FSS embedded composite structures were analyzed using finite element analysis with considering the effects of FSS pattern, and composite stacking sequence.

Geometric transition radar absorbing material loaded with a binary frequency selective surface

In this study, a geometric transition radar absorbing material loaded with a binary frequency selective surface (FSS) is presented. The FSS is designed to improve the absorber's reflectivity at low frequencies. A genetic algorithm (GA) was used to optimise the binary FSS pattern and its position within the absorber. The binary FSS loaded absorber was optimised for performance for both single- and dual-polarisation schemes and was subsequently evaluated against a previously published square conductive loop FSS loaded absorber. An image processing technique was utilised to assess the sensitivity of the FSS design to variations of small geometric features.

Preparation and Microwave Absorbing Property of Microwave Absorbers with FSS Embedded in Multilayer Composites

In this paper, we present the experimental results of microwave absorbing materials with a frequency-selective surface (FSS) embedded in the multilayer structural composites. Microwave absorbing materials with the FSS pattern embedded in multilayer structural epoxy resin matrix composites were fabricated. The effects of the FSS on multilayer structural composites for microwave absorbing material design have been investigated. Comparisons were made with the microwave absorptions of the multilayer composites with FSSs of different positions in composites. The microwave absorption properties of the multiplayer composites samples with FSSs are higher than those of the samples without FSSs. The experiment results show that the incorporation of a FSS can effectively change the reflection property of the composite.

Experimental investigations of microwave absorber with FSS embedded in carbon fiber composite

AbstractIn this paper, we present the experimental results of microwave absorbers with a frequency‐selective surface (FSS) embedded in an impedance‐graded carbon fiber (CF) composite. Samples with different FSS patterns embedded in CF composites are fabricated, and experiments are carried out to find the effects of embedded FSSs. It is found that both the FSS pattern and its location in the composite are critical for the reflection property of the sample. The results are compared with the theoretical predictions using the microgenetic algorithm (MGA), and are in good agreement. © 2002 John Wiley & Sons, Inc. Microwave Opt Technol Lett 32: 245–249, 2002.