Circuit Analog Absorber Research Articles

Impact damage characterization for varying areal-weight unidirectional carbon fiber-reinforced polymer circuit-analog absorbers

This paper numerically characterizes the effects of normal, blunt impact damage on unidirectional carbon-fiber reinforced polymer (CFRP) circuit analog absorbers (CAA). Single-layer CFRP CAAs are fabricated using a wet-layup technique and are then subjected to controlled impact scenarios. A free-space measurement system is implemented to track and quantify changes to the complex permittivity and sheet impedance of the CFRP and absorption capabilities of the CAA. Accuracy and repeatability analysis for both the damaging apparatus and the material characterization interface are presented. Alternative simulation methods and damage quantification techniques for impact damage in CFRP CAAs are developed.

Hybrid Frequency-Selective Rasorber With Low-Frequency Diffusion and High-Frequency Absorption

In this article, a novel frequency-selective rasorber (FSR) is proposed, which provides a high-efficiency transmission band and a broadband radar cross section (RCS) reduction by hybridizing low-frequency diffusion and high-frequency absorption. The FSR comprises three layers. A broadband second-order bandpass frequency-selective surface is designed as the bottom layer, which has a reflection-transmission-reflection frequency response, and acts as an equivalent ground plane for the upper layers outside the transmission band. A circuit analog absorber and a coding metasurface are designed as the middle and top layers, respectively. They use absorption and diffusion to reduce the RCS at high- and low-frequency bands. In the coding metasurface, miniaturized and absorptive techniques are utilized to suppress the harmonic resonances. The whole FSR unit cell exhibits a cross-polarized reflection-transmission-absorption frequency response. A co-polarized transmission magnitude larger than -1.5 dB from 7.4 to 12.1 GHz and a co-polarized reflection magnitude less than -10 dB from 3.1 to 20 GHz are observed in the full-wave simulation under the normal incidence. An FSR sample with 20 x 20 unit cells is designed and measured for verification. The measurement agrees well with the simulation, and it shows a significant monostatic RCS reduction from 3.3 to 20 GHz in comparison with a metallic plane of the same size.

Statistical Analysis of Fabrication Discrepancy Effects on Periodic Circuit Analog Electromagnetic Absorbers

Periodic circuit analog absorbers (PCAAs) are widely used to reduce electromagnetic (EM) scattering fields in various applications. Their performances are sensitive to various geometric parameters. Thus, the stability and sensitivity of the EM absorbing characteristics of PCAAs during a fabrication discrepancy due to the manufacturing uncertainty were investigated. These discrepancies are large in terms of the wavelengths at high frequencies. This article considered randomly produced nonuniform thicknesses of a conductive ink to fabricate the array structure of a PCAA and its two sandwiching dielectric substrate layers on both sides. To examine the discrepancies in fabrication, the thicknesses of the conductive ink were generated from random numbers with a desired mean value and a reasonable standard deviation of the normal distribution. To indicate the EM-field absorbing capability, we studied characteristics, such as S-parameters (reflection and transmission coefficients) and the resulting power efficiency, using numerical high frequency structure simulator (HFSS) full-wave simulations at a frequency band of 10 GHz. They were further validated by conducting experiments on prototype samples. We observed that the resonant frequencies were considerably affected by the thickness of the conductive ink in the PCAA structures. A fabrication accuracy of less than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> - 4</sup> λ was required to attain a stable performance.

Broadband radar absorbing composites: Spatial scale effect and environmental adaptability

Radar absorbing structures with multiple resistive frequency selective surfaces (FSSs) are of great potential in the aerospace and marine fields. However, reliable theoretical prediction models of multilayer circuit-analog (CA) absorbers together with proper optimizing programs have not yet well been established despite the large number of reported investigations. Herein, a precise and comprehensive optimization method was proposed for the design of lightweight and broadband absorbing structures based on improved genetic algorithm. To this end, the spatial scale effect of the entire structures and interlayer interference effects between FSS films were firstly considered. Several absorbing sandwich structures composed of fiber-reinforced epoxy facesheets, polyvinyl chloride (PVC) foam and FSS films with square patterns of various periods were then fabricated. The environmental adaptabilities of the as-obtained absorbing structures were assessed through electromagnetic and load-bearing experimental tests under different ambient temperatures and marine corrosion durations. The optimized absorbing sandwich structures displayed ultra-broadband absorbing capabilities from 2 to 17 GHz and average bending strength of 80 MPa. The influence of ambient temperature on absorbing capability was slight due to the thermostability of carbon conductive ink. In addition, degeneration effect of marine corrosion on the absorbing capability was initially severe but tended to stabilize as corrosion process proceeded. The results of this study provide a useful guideline for the design of integrated multi-functional absorbing composite structures for actual applications.

Miniaturized-Element Frequency-Selective Rasorber Design Using Characteristic Modes Analysis

A dual-polarization frequency-selective rasorber (FSR) with two absorptive bands at both sides of a passband is presented. Based on the characteristic mode analysis, a circuit analog absorber is designed using a lossy frequency-selective surface (FSS) that consists of miniaturized meander lines and lumped resistors. The positions and values of resistors are determined according to the analysis of modal significances and modal current. After that, the presented rasorber is designed by cascading of the lossy FSS and a lossless bandpass FSS. Equivalent circuits of the FSR are modeled, and surface current distributions of both FSSs are illustrated to explain the operation mechanism. Measurement results show that, under the normal incidence, a minimum insertion loss of 0.27 dB is achieved at a passband around 6 GHz, and the absorption bands with an absorption rate higher than 80% are 2.5–4.6 GHz in the lower band and 7.7–12 GHz in the higher band, respectively. Our results exhibit good agreements between measurements and simulations.

On the Design of Ultrawideband Circuit Analog Absorber Based on Quasi-Single-Layer FSS

In this letter, an ultrawideband and single-layer circuit analog absorber with a low profile and small unit size is proposed. The absorber is composed of a Rogers 4003 dielectric with two square loop arrays, respectively, printed on the top and bottom, and a metal ground below. Lumped resistors are embedded on the four edges of the square-loop arrays to introduce resistance loss. Equivalent circuit, input impedance, and current distribution are then presented to provide great insight into the existence of three resonances and wideband. The absorber offers a fractional absorption bandwidth of 132.3% (4.91:1) with at least 10 dB reflection reduction. The thickness and the unit size are, respectively, reduced to $0.075\lambda _{L}$ (wavelength at the lowest frequency) and $0.13\lambda _{L}$ , leading to a good performance under oblique incidence. A good agreement between the calculated, simulated, and measured results validates the proposed design.

A novel wideband absorptive frequency selective surface based on circuit analog absorber

In this paper, a single polarized absorptive frequency selective surface (AFSS) with ultra-wide transmission band and broad absorption band has been proposed numerically and experimentally. Based on the principle of circuit analog absorber (CAA), the proposed wideband AFSS is realized via replacing the ground plane with a band-stop frequency selective sturcture. The proposed AFSS is finally a hybrid structure composed of a wideband lossy frequency selective surface (FSS) in the top and a 3D band-stop frequency selective structure in the bottom. The wideband lossy FSS with thinner profile in the top is realized by utilizing a low pass capasitive elements to compensate the equivalent load inductance arising from the air space that is less than quarter wavelength installing between FSS and ground plane, and the low pass capactive lossy FSS is synthesized using “Gong”-shaped patterns with lumped resistors. Meanwhile, the unit cell of 3D frequency selective structure consists of two electrical coupling C-shaped patterns is investigated to achieve a wide stop band with high selectivity, which can be equivalent to the ground plane in the absorption band. By cascading the wideband lossy FSS and the 3D band-stop frequency selective structure, a thinner hybrid AFSS is presented with ultra-wide transmission band and broad absorption band. The results obtained from simulation shown that an absorption band with absorptivity above 90% is obtained from 6.6 to 11.6 GHz, and the 1 dB transmisson band is from 1 to 3.5 GHz with minimum insertion loss of 0.21 dB. Moreover, the performance can be guaranteed for TE-polarized oblique incidence up to 30°. Finally, a prototype is fabricated to validate its performance, the measured results shown that the 1 dB transmission band ranges from 1 to 3.5 GHz, and the absorption band is operating from 6.3 to 11 GHz at normal TE-polarized incidence. A good agreement between the experiment and simulation results is achieved, which verifies the effectiveness of the design.

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.

Perforated Multilayer Ultrawideband Absorber Based on Circuit Analog Absorber With Optimal Air Spaces

A multilayer ultrawideband circuit analog absorber is proposed in this letter. The absorber comprises three layers: two absorbing layers and a metal ground layer. With the whole profile unchanged, the effects of the air spaces between absorbing layers on bandwidth enhancement are mainly addressed. The equivalent circuit is employed to analyze the appearance of reflection maximum poles, which limit the absorption at high frequencies. Based on the optimal equivalent circuit, two single-polarization perforated absorbing layers consisting of interdigital capacitors, meander lines, and lumped resistors are designed to achieve an ultrawide absorption band. For demonstration, a multilayer absorber is manufactured to validate the approach, where a bandwidth ratio of 1: 12.88 with a total profile of $0.101\lambda _{L}$ at the lowest operating frequency is achieved under normal incidence. The simulated results are experimentally verified by the measured results.

The Design of Optical Circuit-Analog Absorbers through Electrically Small Nanoparticles

In the last few years, the perfect absorption of light has become an important research topic due to its dramatic impact in photovoltaics, photodetectors, color filters and thermal emitters. While broadband optical absorption is relatively easy to achieve using bulky devices, today there is a strong need and interest in achieving the same effects by employing nanometric structures that are compatible with modern nanophotonic components. In this paper, we propose a general procedure to design broadband nanometer-scale absorbers working in the optical spectrum. The proposed devices, which can be considered an extension to optics of microwave circuit-analog absorbers, consist of several layers containing arrays of elongated nanoparticles, whose dimensions are engineered to control both the absorption level and the operational bandwidth. By combining a surface-impedance homogenization and an equivalent transmission-line formalism, we define a general analytical procedure that can be employed to achieve a final working design. As a relevant example, we show that the proposed approach allows designing an optical absorber exhibiting a 20% fractional bandwidth on a thickness of λ/4 at the central frequency of operation. Full-wave results confirming the effectiveness of the analytical findings, as well as some considerations about the experimental realization of the proposed devices are provided.

Polymer matrix composites as broadband radar absorbing structures for stealth aircrafts

ABSTRACTStealth or low observable technology has been one of the most critical requirements for military aviation sector to counter the ever‐advancing target detection technologies worldwide. Among many, Radar, which operates in the broadband spectrum, is one of the principal threats, which measures the radar cross section (RCS) of the aircraft. Hence, researchers are putting immense efforts to develop low RCS materials based on polymer matrix composites (PMCs) called as radar absorbing structures (RAS), to replace highly reflective conventional metallic aircraft structures. Though many researchers have been successful in RAS realization using radar absorbing materials (RAMs) as fillers, this review article reckons the state‐of‐the‐art advanced research on the realization of RAS using a combination of RAMs and multilayered grid devices such as, frequency selective surfaces, circuit analog absorbers, metamaterials, and so on, and their effect on mechanical properties to deliver superior performance RAS for stealth aircrafts. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47241.

Application of Unidirectional Carbon-Fiber-Reinforced-Polymer Laminas in Circuit-Analog Absorbers

This paper explores the characteristics of unidirectional carbon-fiber-reinforced-polymer (CFRP) laminas and their use in electromagnetic absorber design. Unidirectional CFRP composites have been used for decades in structural applications. The electromagnetic properties of unidirectional CFRP composites have been well documented in the open literature. Since many structural applications may also have specific electromagnetic compatibility requirements, it is logical to exploit the useful electromagnetic properties of unidirectional CFRP. Specifically, this paper investigates circuit-analog absorbers (CAAs) made from a unidirectional CFRP lamina, foam spacer, and a ground plane. The unique properties of the unidirectional CFRP sheet provide the necessary resistive and reactive properties for the development of a CAA. To investigate such a CAA, the electromagnetic properties of several off-the-shelf unidirectional CFRP laminas were measured from 4 to 18 GHz using a free-space measurement system. A surface-impedance representation of the unidirectional CFRP lamina was used to build rigorous transmission-line models that provide insights about the properties of the absorbing structure. Transmission line as well as full-wave models were compared to measured absorption performance of fabricated prototypes. Maximum absorption configurations were found to be highly dependent on incident polarization relative to fiber orientation.

A Novel Polarization Independent Wideband Circuit Analog Absorber Using Crossed Loops

A Novel Polarization Independent Wideband Circuit Analog Absorber Using Crossed Loops

&lt;italic&gt;X&lt;/italic&gt;-Band Circuit-Analog Absorbers Using Unidirectional Carbon Fiber

This letter explores single-ply pre-impregnated (pre-preg) unidirectional carbon-fiber-reinforced polymer (CFRP) laminas for use in X -band (8.2–12.4 GHz) absorbers. Specifically, the unidirectional CFRP sheets were used as lossy frequency selective surfaces in single- and two-layer circuit-analog absorbers (CAAs). The aim of this letter was to assess the feasibility of using only the fiber orientation and areal weight of the material as a tuning aid. A free-space materials characterization system was used to characterize the anisotropic complex permittivity and anisotropic surface impedance of two different areal weight unidirectional CFRP samples. The absorbers were prototyped in a mechanically reconfigurable fixture with the aid of a vacuum-bagging system. The reconfigurable approach allowed reflectivity measurement performance to be measured without the need to fabricate cured sandwich constructions. Reflectivity measurements were compared to transmission-line models as well as full-wave models developed using commercial simulation software. Regions of validity for the transmission line models were identified. The results show that single-ply pre-preg unidirectional CFRP laminas have the potential to be used in single- and two-layer CAA configurations for use in X -band absorbers.

Double-Layer Circuit Analog Absorbers Based on Resistor-Loaded Square-Loop Arrays

Based on the equivalent circuit model, it is demonstrated that five resonances can occur within the absorption band of a circuit analog absorber containing two lossy layers. The lossy layers are composed of resistor-loaded square-loop and double-square-loop arrays. In the absence of a dielectric skin, an initial design with four resonances observed from its reflectivity exhibits a simulated bandwidth ratio (BWR) of 1:8.45 for at least 10 dB absorption. To facilitate physical implementation of the wideband absorber, a modified design is proposed to compensate the effect of parasitic parameters of actual resistors on reflectivity. It is shown through measurement that an operating BWR of 1:8.78 is achieved with the reflectivity smaller than −10 dB under normal incidence. The total thickness of the absorber prototype is only 0.096 λ at the lowest operating frequency. The agreement between simulation and measurement validates the proposed design.

A Polarization-Independent Broadband Multilayer Switchable Absorber Using Active Frequency Selective Surface

In this letter, a polarization-insensitive broadband switchable absorber has been presented, which exhibits wide absorption bandwidth switching between two frequency bands ( S -band in one biasing state, and C - and X -bands in other state). The structure utilizes the circuit analog absorber concept to achieve broadband absorption, whereas switchable mechanism is obtained by implementing active p-i-n diodes in multilayer geometry. The novelty of the proposed design lies in its four-fold symmetrical configuration and simplified biasing network as compared to the earlier reported switchable absorbers. Several parametric variations have also been investigated to explain the switching and the broadband absorption mechanisms of the structure. Furthermore, a prototype has been fabricated and experimentally demonstrated under normal incidence.

A Fast and Efficient Design Method for Circuit Analog Absorbers Consisting of Resistive Square-Loop Arrays

This paper proposes a novel optimization design method for circuit analog absorbers consisting of resistor-loaded square-loop arrays. First, new precise equivalent circuit models (ECMs) for square-loop-array-based absorbers are derived through the synthetic asymptote method. The presented models are applicable for multilayered absorber with printed single or double SLAs on each layer. Then, the combination of this ECM and conventional genetic algorithm (GA) is proposed and provided a fast and efficient way to design broadband and low-profile SLA absorbers. For demonstration, four absorbers are designed and optimized. Finally, the agreement between the simulated and measured reflection coefficients indicates the validity of the presented method.

Design, characterisation and fabrication of a broadband polarisation‐insensitive multi‐layer circuit analogue absorber

A broadband polarisation-independent circuit analogue absorber comprising multi-layer resistive frequency selective surfaces (FSS) has been presented in this study. The proposed structure consists of a periodic arrangement of square loops loaded with lumped resistors and these square loops are printed on dielectric substrates separated by an air spacer. The simulated result shows the reflectivity below -10 dB in the frequency range from 4.96 to 18.22 GHz (fractional bandwidth of 114.40%) under normal incidence, covering C, X, and Ku bands. An equivalent circuit analysis has been introduced to characterise the proposed absorber, which shows good matching with the full-wave analysis. The effects of the individual FSS layers and the air spacer have been studied and several parametric variations have been carried out to examine the sensitivity of the design parameters on the absorption bandwidth. Finally, the designed absorber has been fabricated and measured in anechoic chamber, which shows good agreement between the experimental results and the simulated responses, under different angles of incidence as well as for various polarisation angles.

Design of wideband radar absorbing material with improved optical transmittance by using printed metal‐mesh

A radar absorbing structure was designed on the basis of a circuit analogue absorber for the implementation of broadband characteristics by using a metal-mesh transparent electrode with an improved optical transmittance. To realise a high optical transmittance and broadband absorbing performance in the X-band, a resistive frequency selective surface, in which soda-lime glass, a transparent dielectric, was patterned with a metal-mesh, was used. The simulation for the reflection characteristics of the proposed absorber is performed and showed its validity through experiment. The measurement results were in good agreement with the simulation results, confirming the broadband reflection loss characteristics. Since the proposed structure has a high optical transmittance and stable broadband absorbing performance, it can be used in various fields including military applications, where facilities requiring secure wireless signals or transparent radar absorbers are necessary.

Optimal Design of Broadband Radar Absorbing Sandwich Structure with Circuit Analog Absorber Core

Circuit analog (CA) absorbers have advantages of broadband radar absorption and feasibility in realizing the design blueprint. In this paper, a kind of radar absorbing sandwich structure (RASS) with rigid CA absorber core is proposed, which is capable of broadband absorption as well as load bearing. The numerical approach to predict the broadband reflectivity of CA absorber is introduced into an adaptive genetic algorithm (AGA) so as to derive the optimal RASS design within a certain frequency range. Furthermore, mechanical constraints including facesheet yielding, core shearing, and facesheet wrinkling, are incorporated into the optimization procedure to derive the optimal design of a one-side clamped RASS panel subjected to uniformly distributed loads. The results indicate that, when the layer number of lossy films is fixed, the radar absorbing performance (RAP) is not improved monotonically with the structure thickness, and by the present optimization procedure, an optimal structure thickness together with position, sheet resistance and geometry of the lossy films can be expected for such kind of RASS.