Frequency Selective Surface Absorber Research Articles

Accelerating multilayer frequency selective surface absorber design by combining equivalent circuit models with machine learning

Designing high-performance electromagnetic functional materials and artificial structures is of great significance for electromagnetic wave modulation applications. Optimizing performance in the high-dimensional parameter space of electromagnetic functional materials has posed a challenging problem. This paper proposes the use of reinforcement and transfer learning methods to facilitate the quick and accurate design of wideband circuit analog absorbers (CAA). A trained reinforcement learning network is utilized to comprehend the relationship between reflectivity performance and changes in impedance parameter. Furthermore, the transfer learning method is applied to accelerate the training process, leading in a 20-fold reduction in the number of simulations. To validate the effectiveness of this approach, a double-layer absorber aiming for reflectivity less than −20 dB at 3–10 GHz is designed, requiring only 50 simulations. This demonstrates that the proposed method provides a flexible strategy for the interactive design of equivalent circuit and electromagnetic structural parameters. Moreover, it presents a novel and efficient solution for microwave absorber design, with potential applications across various microwave design fields.

Design, analysis and validation of pinnacles impedance FSS absorber based on La-doped mullite ceramic

The bandwidth adaptation factor (BAF), which is calculated as BW • (λL - λH): (d • fr), is a crucial evaluation parameter for designing ultrathin broadband absorbers. In this paper, a high BAF absorber is designed by utilizing an impedance frequency selective surface (FSS), which includes a six pinnacles FSS, a La-doped mullite ceramic dielectric layer and a metal backplane. The optimal numbers of pinnacled FSS is determined through comparison and optimization, and the power monitor results indicate that the main loss modes are electrical and ohmic loss. Subsequently, the impedance analysis reveals that the favourable absorption performance is attributed to good impedance matching and high loss coefficient. Furthermore, the observation of the current distribution on the surface confirms that the primary loss appears at the pinnacles. The proposed absorber has below −10 dB reflectivity in 12.26–18.00 GHz. Meanwhile, the BAF is 3.36 and the thickness is only 0.061 λL (1.5 mm). The measured results are in good agreement with the simulation results, demonstrating an excellent BAF characteristic.

Wideband dual‐polarized rasorber design with genetic algorithm synthesis

AbstractIn this letter, an 11 GHz centered wideband absorptive frequency selective surface (FSS) design is presented. The absorptive FSS is dual polarized, and it has wide transmission band and a wide absorption band at the lower frequency side of the transmission band. An octagon shape with parallel microstrips toward center connected with lumped element resistors at the edges is proposed for the resistive layer unit element. Also, a multilayer octagon patch and aperture resonator are proposed for the wideband bandpass layer. As an alternative design method, the unit element of resistive layer of absorptive FSS is synthesized by a genetic algorithm code successfully. Genetic algorithm synthesis provides straightforward synthesis process compared to conventional method. Designed absorptive FSS is manufactured and frequency response is measured with WR‐90 waveguide setup. The measurement result of 1 dB transmission bandwidth is between 9.43 and 12.50 GHz with a minimum insertion loss of 0.65 dB.

Ultra-wideband thin frequency-selective surface absorber against sheet resistance fluctuation

The design of thin frequency selective surface (FSS) absorber based on resistive film that meets the requirements of broadband, polarization independence, incident angle stability, and strong absorption is a challenging task. Fabrication tolerance of resistive film can result in fluctuations in sheet resistance, which negatively affects the absorber performance. To tackle these problems, this work firstly investigates how sheet resistance fluctuations affect the absorbing performance of resistive film FSS absorber. The analysis of simulated surface current density distribution and impedance reveals that the diversity of current paths provides an effective way to mitigate the influence of sheet resistance fluctuation. This is achieved by enabling flexible variation of surface current in response to sheet resistance fluctuations. Consequently, the variation of input impedance of the FSS absorber due to the fluctuation of sheet resistance is suppressed within a small range. Then, a method of extending bandwidth is proposed by employing the complementary variation of FSS impedance with frequency at different layers. By combining this approach with a miniaturization design, a thin and light FSS absorber is developed that exhibits ultra-wide bandwidth, polarization independence and angle stability while mitigating the effects of sheet resistance perturbation. The proposed FSS absorber achieves a 90% absorption bandwidth from 1.50 GHz to 20.50 GHz, covering Ku, X, C, S bands and part of the L and K bands, with a relative bandwidth reaching 173%. The absorber has a thickness of 0.093<i>λ</i><sub>L</sub> for both transverse electric (TE) polarization and transverse magnetic (TM) polarization, yielding a figure of merit (FoM, the ratio of the theoretical minimum thickness to the actual thickness) of 0.95, indicating that the thickness is close to the theoretical limit. The absorber maintains over 90% absorption rate for TM polarization at an incidence angle of up to 70°, and 80% absorption for TE polarization at 45°. Furthermore, the 90% absorbance bandwidth of the absorber remains at 167.0% when the sheet resistance of any FSS layer fluctuates within a range from 12 to 30 Ω/sq. A prototype of the proposed FSS absorber is fabricated and measured, and the experimental results are in good agreement with the simulation results, thus validating the effectiveness of the proposed method.

An Angle-Stable Ultra-Wideband Single-Layer Frequency Selective Surface Absorber

An ultra-wideband polarization-insensitive frequency selective surface (FSS) absorber is proposed for S to K-band applications. The absorber comprises two compensation slabs, a lossy FSS layer and a grounded dielectric plate. The FSS unit cell is a combination of a second-order Chinese knot and a cross. To enhance the bandwidth and angular stability of the single-layer FSS absorber, a compensation layer composed of FR4 and polymethyl methacrylate (PMMA) slabs is incorporated. The proposed FSS absorber demonstrates a remarkable absorption rate of over 90% within the frequency range of 3.1–22.1 GHz, exhibiting a fractional bandwidth of 150.8%. Even when subjected to an oblique incidence of 45°, the absorber maintains an 80% absorption rate in the frequency range of 4.4–19.1 GHz for both TE and TM polarizations. The total thickness of the FSS absorber is 0.0848 λL (the wavelength at the lowest cutoff frequency), and only 1.08 times the Rozanov limit. To validate the design, a prototype of the proposed absorber was fabricated and measured. Good agreements were observed between the simulations and measurements.

Design and Manufacturing of a Hexapattern Frequency Selective Surface Absorber for Aerospace Stealth Application.

Integrated frequency selective surface (IFSS) absorbers with larger bandwidth, effective reflection loss, polarization-insensitive characteristics, angular stability with compact/thin design, and ease of fabrication have captivated significant importance in stealth technology. Herein, we report on an IFSS absorber that has been designed, simulated, and implemented for manufacturing to achieve effective stealth properties. Initially, frequency selective surface (FSS) layers have been designed that comprise a closed centroid honeycomb structure surrounded with four annular hexagonal rings, splitted, alternatively, and enveloped with four L-shaped elements. The simulated pattern has been optimized on glass fabric for reflection loss (RC, dB) at a thickness of ∼0.1 mm by choosing sheet resistance of pattern 110 Ω/□. A FSS layer combined with interlayer lossy dielectric laminates (1.8 mm) and a carbon-fabric-reinforced-plastic ground has been simulated as an IFSS absorber. The performance of RC, in normal and angular configuration (0-60°), under transvers an electric/magnetic mode of polarization, including analysis of the displacement current, volume power loss distribution, and complex admittance has been carried on IFSS. Subsequently, the proposed absorber has been fabricated using customized carbon-based resistive ink imprinted on glass fabric by mask lithography compounded with laminates (a carbon black powder/epoxy composite) and ground. Their manufacturing details, including free space and anechoic chamber RC measurements, have been presented. The simulated and experimental RC performances of the absorber are found to be in good agreement, possessing minimal 10 dB reflection loss (90% absorption) with a sample thickness of 1.9 mm (0.05λL, where λL corresponds to a lower operating frequency), covering 76% fractional bandwidth in X and Ku bands. The proposed design architecture of the IFSS is ideally suitable for aerospace stealth platforms.

Inverse design of ultra-wideband transparent frequency selective surface absorbers based on evolutionary deep learning

Conventional frequency selective surface (FSS) absorbers design is time-consuming, involving multiple electromagnetic (EM) simulations for parameter scanning. A novel reverse design method is proposed utilizing evolutionary deep learning (EDL) based on an improved bacterial foraging optimization (IBFO) algorithm and a deep belief network. It establishes the relationship between the geometric structure and EM response. The combination of IBFO and EDL facilitates an efficient optimization for structural parameters, mitigating the ‘one-to-many’ problem and accelerating the design process. An optically transparent FSS absorber with an ultra-bandwidth of 8–18 GHz is designed to verify the proposed method’s capability. The simulation and experimental results demonstrate that the absorber displays exceptional characteristics such as polarization insensitivity and robustness under a 45° oblique incidence angle, making it a suitable candidate for radar stealth and photovoltaic solar energy applications. The proposed method can be applied to the design and optimization of various absorbers and complex EM devices.

A novel dataset sampling method for deep learning-based absorption prediction of FSS absorbers

Combining the deep learning (DL) model to design frequency selective surface (FSS) is a promising research topic, while the DL model usually requires massive datasets. In order to reduce the size of the dataset while maintaining the performance of the DL model used to predict absorptivity of FSS absorbers, a novel dataset sampling method is proposed. Based on the bounded-loss theory, it is proposed that the performance of the DL model is determined by the sampling density and the difference between absorptivity of neighboring samples. This difference is measured by the variations of absorptivity corresponding to these samples. The absorptivity are acquired through the equivalent circuit model (ECM) analysis instead of full-wave simulation to provide immediate results during the sampling period. Numeric validations of the single-layer absorber show that the method requires a much smaller dataset size than the conventional random sampling method with the same Mean Square Error performance of the DL model. The three-layer results show that the method is more accurate in predicting the absorption rate when trained with 20% of the total dataset compared to the random method and the ECM method.

Tapered Resistive Sheets and FSS Absorber Loading for the Wideband RCS Reduction of the Elliptic Surface at TM Polarization

A wide-band radar cross section (RCS) reduction of the elliptic surface at TM polarization is reported in this communication. The main scattering sources and the corresponding scattering mechanisms of the elliptic surface are accurately identified based on the ISAR imaging technique and the high-frequency approximation theory. The frequency selective surface (FSS) absorber is employed to suppress the specular reflection, and the tapered resistive sheets (TRSs) are used to handle the scattering from the trailing edges. Samples have been fabricated and measured in the anechoic room, both the simulation and measurement results illustrate that 10-dB RCS reduction is achieved within 1.35-4.9 GHz at TM polarization.

Compact wideband FSS-absorber-based low-RCS reflectarray antenna

This paper presents a method to realize compact broadband low-RCS ReflectArray (RA) antenna based on a Frequency Selective Surface (FSS) absorber and a reflective metasurface. Such an FSS absorber consists of a resistance-loaded lossy layer and an FSS layer, which is utilized to reach an absorption-transmission response. The bottom reflective metasurface works as a phase array, reshaping the quasi-sphere wave from the feeding antenna into the quasi-plane wave. As a demonstration, the low-RCS RA antenna is simulated, fabricated, and measured. The simulated and measured results show that the developed low-RCS RA antenna has an aperture efficiency of 42.7% and a gain of 25.4 dBi in the X band. In the meantime, it simultaneously reaches the 10 dB RCS reduction for the orthogonal polarizations at the S and C bands, corresponding to a fractal bandwidth of 120%. Specifically, the adopted patch-feeding antenna makes the RA antenna more compact than the horn-feed conventional ones. Furthermore, the proposed RA antenna uses a few layers of substrates, making it lower in cost and easier for fabrication. The proposed design may have potential application in integrated stealth communication systems.

An Ultrawideband Frequency Selective Surface Absorber With High Polarization-Independent Angular Stability

An ultrawideband frequency selective surface (FSS) absorber with high angular stability and polarization independency applied in a microwave band is proposed. The absorber is composed of three identical Teflon layers and dual FSS layers with distinct patterns. The miniaturized FSSs with unique patterns demonstrate advantages of facile manipulation of the impedance and multiple resonances in an ultrawide frequency range. The use of identical dielectric substrate greatly reduces the complexity in the absorber design, compared with the conventional way that requires variable dielectric constants for a multilayer structure. Experimental results reveal that more than 90% absorption rate in the frequency range of 2.79–20.62 GHz for normal incidence is achieved. The absorption rate can still retain more than 80% in the range of 4.41–20.93 GHz for both TE and TM polarizations with an incidence of 60°.

High-Selectivity Frequency-Selective Rasorber With Tunable Absorptivity

This article presents a high-selectivity frequency-selective rasorber (FSR) with an adjustable out-band absorptivity property. This rasorber is composed of a dynamic absorptive frequency-selective surface (AFSS) on the top, with p-i-n diodes employed to achieve tunable absorptivity, and a high-quality ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> )-factor bandpass FSS in the bottom, with an aperture-coupled resonators (ACRs) structure to obtain a high-selectivity transparent window. To begin with, theoretical analyses and extensive simulations are performed to evaluate the performance. Subsequently, a prototype is fabricated and measured, showing a 1 dB transmission window with high selectivity at 8.43–8.51 GHz and electrical tuning on the absorption rate response with a wide tunable span of 0.9 at out bands of 4.43–7.76 and 9.21–13.45 GHz, respectively. The proposed rasorber with dynamic tunability of absorptivity can provide radar cross Section (RCS) consistent with the environment, which is potential for radar stealth, deception, and anti-identification.

Tapered Resistive Sheets Loading to the FSS Absorber for the Wide-Angle Monostatic RCS Reduction Applications

In this communication, wide-angle monostatic radar cross section (RCS) reduction of a metal plate is reported. The whole angular range (0°–90°) is divided into three parts according to the different dominant scattering mechanisms. Each dominant scattering mechanism is accurately identified and suppressed effectively. Frequency selective surface (FSS) absorber is employed to reduce the specular echo intensity within the first angular part. The tapered resistive sheets (TRSs) are loading as an edge termination to the metal layer and FSS layer, respectively, providing a slowly varying surface current to taper the transition between the free space and the targets. Significant RCS reduction in the rest two angular parts is achieved. Simulation results clearly show that more than 10 dB monostatic RCS reduction is realized within a wide angular range (0°–10° and 30°–85°) for the frequency band ranging from 1.3 to 2 GHz. Samples have been fabricated and measured in the anechoic chamber, and a remarkable agreement between the simulation and measurement is obtained. The effectiveness of the design is fully verified.

Frequency Selective Surface Composites With Honeycomb Absorbing Structure for Broadband Applications

A novel composite of an area-coated grading honeycomb (ACGH) structure and a frequency selective surface (FSS) was theoretically and experimentally investigated for use as broadband microwave absorbing. The main difference from the traditional FSS absorber was that the aramid honeycomb was replaced by an absorbing honeycomb structure, which increased the absorption bandwidth by suppressing the half-wavelength total reflection point. The physical mechanism was analyzed using an equivalent circuit model and the impedance range diagram. The designed absorber had a −10 dB fractional absorption bandwidth of 154.3% from 2.0 to 15.5 GHz, and it had a thickness of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.113\lambda _{L} $ </tex-math></inline-formula> at the lowest frequency. The ACGH structure was integrally formed to ensure high structural strength. Reflectivity measurements were consistent with calculated values, validating the design principle and preparation technique for the composite.

Partition Layout Loading of Frequency Selective Surface Absorbers on the Curved Surfaces for the Significant RCS Reduction

In this article, the significant radar cross section (RCS) reduction of curved surfaces in the low microwave frequency band is reported. The design method of partition layout loading of frequency selective surface (FSS) absorbers to control the surface current is put forward. Comparison of the scattering field intensities of different zones on the curved surface clearly shows that surface current should be suppressed in a large zone to effectively reduce the RCS of the curved surface at these frequencies. FSS absorbers under normal and 45° angle of incidence are designed and placed on the different zones of the curved surface to realize the partition layout loading, and remarkable RCS reduction has been obtained. Specifically, more than 15-dB RCS reduction is achieved in 1.05–1.45 GHz, and absorption above 90% is obtained from 0.95 to 1.5 GHz for TE polarization. The great agreement between the simulated and measured results validates the proposed design method.

A filling-factor engineered, perfect metamaterial absorber for multiple applications at frequencies set by IEEE in C and X bands

In this paper, by engineering the filling factor of the resonator of a hybrid frequency selective surface (FSS), a low-profile and application-targeted microwave metamaterial absorber is proposed with polarization and angle insensitivity. The proposed hybrid FSS of the absorber on FR4 substrate comprises four identical concentric copper sub SRRs and a defected copper ground. The square-cut area at the center of the ground and the filling factor of the resonator were tuned to get desired absorption peaks at some IEEE defined frequencies at C and X bands, which any other available perfect metamaterial absorbers cannot obtain. The measurement of the array absorber in anechoic chamber and the equivalent circuit analysis has demonstrated that the filling factor engineering technique illustrated an efficient way to achieve maximum absorptivity with the negative refractive index for perfect absorptions at the desired frequencies. The Wide-angle and polarization insensitivity to both co- and cross-polar waves are described by numerical analysis and EM responses. The thickness and dimension of the unit cell at the lowest operating frequency are 0.022 λ and 0.22 λ, respectively, and make it subwavelength compact. Besides normal and oblique incidence up to 80° and 180°, respectively, the absorber showed similar performance for cross-polar waves with an average of 99.5% absorptivity at 4.196 GHz, 5.24 GHz, 8.632 GHz, 9.264 GHz, and 10.152 GHz. The absorber has the potential for real-life RCS reduction, stealth, and sensing applications at C and X band antenna, satellite, and fighter aircraft stealth systems.

A Multispectral Single‐Layer Frequency Selective Surface Absorber for Infrared and Millimeter Wave Selective Bi‐Stealth

AbstractThe multispectral electromagnetic (EM) absorber based on the frequency selective surface (FSS) has been applied with interest in the field of stealth technology due to its ability to avoid detectors or guided weapons at multiple wavelengths simultaneously. The multispectral absorption performance is generally accomplished by stacking different types of FSS absorbers in multiple layers. Herein, a thin and flexible multispectral single‐layer FSS (MSLF) absorber is presented using a micro‐holed, macroscale FSS combined with an infrared (IR) absorbing ground. The FSSs are simplified to MSLF by changing the absorbing layered microstructure to a transmitting microstructure. Dual‐band millimeter waves (MMWs) are absorbed by the macroscale resonance cavity, while the target IR waves are absorbed by the IR absorbing ground after penetrating the micro‐hole array pattern. Thermal emission is reduced significantly due to its low emissivity at IR bands other than the target IR. It is confirmed analytically and experimentally that patterns of various sizes do not exhibit functional crosstalk. The demonstrated multispectral absorption and low thermal emission make this a very promising material for IR−MMW selective bi‐stealth. Furthermore, the proposed structure can be applied to existing macroscale patterns in order to increase their applicability by providing additional selectivity without any functional interference.

Graphene-FSS Hybrid Absorptive Structure With Amplitude/Frequency Dual-Modulated Passband

In this letter, a reconfigurable absorptive frequency selective surface (FSS) based on graphene sandwich structure (GSS) and varactor-tunable FSS is proposed. The frequency and amplitude of transmission window can be tuned continuously and independently by applying bias voltages to GSS and FSS, with absorption bands below and above the passband maintained. The feeding network is integrated with the GSS-FSS hybrid structure creatively, maintaining desirable responses and symmetry of the structure for dual-polarized functionality. Moreover, the transmission window can be turned off by controlling the polarity of the biasing voltages, rendering the rasorber with the switchable properties. In the absorber mode, absorptivity can be manipulated by varying the bias voltage of GSS. The measured results agree with the simulation results well, validating the desired dual-reconfigurability of our design.

Active Frequency Selective Surface Absorber With Point-to-Point Biasing Control System

A frequency selective surface (FSS) absorber with a biasing control system is presented in this letter. The absorber consists of the top-layer FSS structure, the bottom-layer metal sheet, and an air spacer in between. The biasing circuit is controlled by a single-chip microcomputer with constant current outputs and each output port is connected to one p-i-n diode, which is employed to realize different absorption bands by controlling the induced current distribution. Both simulated and measured results demonstrate that fast switching function can be realized within certain frequency ranges (3.62-5.35 GHz and 7.92-9.27 GHz) with the absorption depth below -10 dB.

Device modeling of RF varactor diodes for adjustable microwave reflectivity in active frequency selective surfaces

At present, the design of absorbers with broadband electromagnetic absorption performance in the P/L band (0.3 ∼ 2 GHz) is a huge challenge. The frequency selective surface (FSS) loaded with active RF devices utilizes the characteristics of electrically adjustable device parameters, thereby a P/L band broadband active FSS absorber can be designed. It is obvious that device parameter characteristics are crucial to design excellent absorbing performance. Unfortunately, due to the industrial packaging characteristics of the current application of surface mounted devices, there are parasitic parameters in addition to the main impedance parameters. Here, an equivalent circuit model of a varactor diode is proposed. We combine thru-reflect-line calibration technology and advanced design system fitting to obtain the parasitic parameter values. Then a classic FSS structure is used to verify the consistency of the simulation and measurement for the varactor diode model. We hope that the model of the varactor diode we proposed will help the design of the new broadband adjustable absorber in the P/L band.