Lossy Sheet Research Articles

A HYBRID STRUCTURE OF FREQUENCY SELECTIVE SURFACE AND MAGNETIC ABSORBING SALISBURY SCREEN FOR WIDEBAND RADAR CROSS SECTION REDUCTION

This article presents a novel hybrid physical mechanism that combines frequency selective surface (FSS) and Salisbury-type absorption to create a structure that can reduce the radar cross section (RCS) over a wide frequency range. The proposed design integrates a lossy sheet composed of a magnetic absorbing material (MAM) with a traditional Salisbury screen and a FSS of a T-ring resonator. The Salisbury screen effectively absorbs incoming waves when illuminated by a plane wave with normal incidence. Furthermore, the integration of a T-ring resonator generates high-order reflection beams, thereby enabling the selection of absorption band. Simulation results demonstrate that the proposed structure enables tunable RCS reduction between 2 and 18 GHz under various conditions with normal incidence. The combination of broadband absorption mechanism with frequency selective resonance allows to construct a perfect structure both in terms of bandwidth and frequency selection of absorption.

Optimal Thickness of Double-Layer Graphene-Polymer Absorber for 5G High-Frequency Bands

A new analytical approach to optimize the thicknesses of a two-layer absorbing structure constituted by a graphene-based composite and a polymer dielectric spacer backed by a metallic layer acting as perfect electric conductor (PEC) is proposed. The lossy sheet is made by an epoxy-based vinyl ester resin filled with graphene nanoplatelets (GNPs) characterized by known frequency spectra of the complex permittivity. The optimal thicknesses are computed at the target frequencies of 26, 28, and 39 GHz in order to obtain a –10 dB bandwidth able to cover the 5G frequency bands between 23.8 and 40 GHz. The resulting absorbing structures, having a total thickness lower than 1 mm, are excited by transverse magnetic (TM) or electric (TE) polarized plane waves and the absorption performances are computed in the 5G high frequency range.

New Sensing and Radar Absorbing Laminate Combining Structural Damage Detection and Electromagnetic Wave Absorption Properties

Within the paradigm of smart mobility, the development of innovative materials aimed at improving resilience against structural failure in lightweight vehicles and electromagnetic interferences (EMI) due to wireless communications in guidance systems is of crucial relevance to improve safety, sustainability, and reliability in both aeronautical and automotive applications. In particular, the integration of intelligent structural health monitoring and electromagnetic (EM) shielding systems with radio frequency absorbing properties into a polymer composite laminate is still a challenge. In this paper, we present an innovative system consisting of a multi-layered thin panel which integrates nanostructured coatings to combine EM disturbance suppression and low-energy impact monitoring ability. Specifically, it is composed of a stack of dielectric and conductive layers constituting the sensing and EM-absorbing laminate (SEAL). The conductive layers are made of a polyurethane paint filled with graphene nanoplatelets (GNPs) at different concentrations to tailor the effective electrical conductivity and the functionality of the material. Basically, the panel includes a piezoresistive grid, obtained by selectively spraying onto mylar a low-conductive paint with 4.5 wt.% of GNPs and an EM-absorbing lossy sheet made of the same polyurethane paint but properly modified with a higher weight fraction (8 wt.%) of graphene. The responses of the grid's strain sensors were analyzed through quasi-static mechanical bending tests, whereas the absorbing properties were evaluated through free-space and waveguide-based measurement techniques in the X, Ku, K, and Ka bands. The experimental results were also validated by numerical simulations.

Closed-Form Analytical Design of Optically Transparent Wideband Absorbers for 5G Technology

The design of new transparent absorbers for the upcoming fifth-generation (5G) technology is investigated. The absorbers consist of a matching layer, a lossy sheet, a spacer, and a back conducting layer (BCL). Both the matching layer and the spacer are one-quarter-wavelength thick and made of polyethylene terephthalate (PET). The lossy sheet and the BCL consist of indium tin oxide (ITO) thin films or graphene-PET laminates in order to avoid the use of any metal and to guarantee the optical transmittance. An innovative closed-form formulation is developed with the aim of evaluating the optimal values of sheet resistances of the lossy layers through analytical expressions. The absorption performances are investigated considering impinging plane waves with an incidence angle between 0° and 30°. The computed absorption coefficients in transverse magnetic (TM) or transverse electric (TE) modes are greater than 0.8, and the transmission coefficient is lower than 0.1 in the 5G frequency range from 25 up to 47 GHz, assuming that the matching layer and the spacer are 1.3 mm thick. The optical transmittance of the absorbers is evaluated in the wavelength range from 400 up to 700 nm by means of an accurate matrix simulation model, considering both optical polarizations, S (i.e., TE) and P (i.e., TM) for different incidence angles. The computed average optical transmittances of the designed absorbers are greater than 80% at 550 nm, i.e., at the wavelength corresponding to the maximum eye sensitivity.

Broadband and Tunable RCS Reduction using High-order Reflections and Salisbury-type Absorption Mechanisms

In this paper, a broadband and tunable radar cross section (RCS) reduction structure is proposed by using the hybrid physical mechanism that is based on high-order reflections and Salisbury-type absorption. Our design combines the high-index grating structure with a traditional Salisbury screen in which the lossy sheet is made of a graphene structure. When it is illuminated by a plane wave with normal incidence, the Salisbury screen can absorb the incoming wave, and the introducing high-index grating structure could further reduce the backward scattering wave by generating high-order reflection beams, which broadens the RCS reduction bandwidth. In addition, the RCS reduction level can be dynamically controlled by tuning the surface resistance of the graphene layer. Simulated results show that the proposed structure can realize tunable RCS reduction between 4.1 and 18 GHz under normal incidence with different graphene resistances. Experimental results are in accordance with those of the simulation results. In addition, the scattering field distributions and the plots of surface power loss density have been illustrated to analyze the RCS-reduction mechanism of our structure.

Tunable graphene/dielectric laminate for adaptive low-gigahertz shielding and absorbing screens

Shielding and absorbing screens made of tunable graphene/ dielectric laminate (GL) doped by an electrostatic field bias are designed applying simple modelling procedures in the low-gigahertz frequency range. The adaptive response of both types of screens is achieved through the control of the effective sheet resistance of the GL, consisting of a proper number of doped graphene layers separated by thin films of polyethylene terephthalate (PET). The performances of the shielding screen are predicted in the frequency range up to 1 THz, using simple approximate expressions and rigorous simulation models. The multilayer absorber is a twoperiod dielectric Salisbury screen, in which the outer lossy sheet is made of a single-layer graphene and the inner one is a GL with two electrically doped graphene layers. The reflection coefficient is minimized at each frequency through the proper setting of the electrostatic field bias. The optimal absorbing performances of the screen are predicted in the frequency range between 2 GHz and 25 GHz.

Adaptive Broadband Radar Absorber Based on Tunable Graphene

An innovative broadband adaptive absorber, providing a reflection coefficient lower than $-10\,{\rm dB}$ in the frequency range from 7 to 22 GHz, is proposed. The new absorber is a two-period dielectric Salisbury, in which the outer lossy sheet is made of a graphene monolayer, and the inner one consists of a tunable graphene/dielectric laminate (GL). The two dielectric spacers are made of a commercial low-loss polymer. The total thickness is $\sim 6\,{\rm mm}$ . The adaptive broadband response of the absorber is achieved tuning the effective sheet resistance of the GL through an applied electrostatic field bias. The sensitivity analysis of the reflection coefficient of the absorber illuminated by a plane wave with normal incidence is carried out with respect to the spacer thickness and the graphene carrier charge mobility. By applying a dc-voltage source always below 15 V, the minimum reflection coefficient of the adaptive absorber is centered around 14 GHz, and it is characterized by a bandwidth at $-10\,{\rm dB}$ of $\sim 15\,{\rm GHz}$ .

Simultaneous Measurement Method for Complex Permittivity and Permeability of Lossy Sheet

Simultaneous Measurement Method for Complex Permittivity and Permeability of Lossy Sheet

Analysis and design of triple-band high-impedance surface absorber with periodic diversified impedance

In this paper, a triple-band planar absorber with high-impedance surface (HIS) is designed and fabricated. The absorber structure is composed of polyurethane foam sandwiched between a lossy sheet of frequency selective surfaces (FSS) and a perfect electric conductor. The lossy FSS possesses different resistances in a periodic composite unit as compared with typical HIS absorber. Losses in the FSS are introduced by printing the periodic composite square ring pattern on blank stickers using various resistive inks. Physical mechanism of the HIS absorbers is analyzed by equivalent circuit model and electric field distribution studies. The proposed absorber with periodic composite units offers superimposed triple-band absorption as compared with that of the single units having single- or dual-band absorption characteristics. The reflection loss measurements show that the 90% absorption bandwidth of the HIS absorber is increased by 42% by the proposed composite periodic units.

Salisbury Screen Absorbers of Dielectric Lossy Sheets of Carbon Nanocomposite Laminates

In this paper, we present an innovative radar absorber composed of a dielectric spacer and a dielectric lossy sheet (DLS), instead of the resistive sheet of the conventional Salisbury screen absorber. We propose carbon nanocomposites as DLS to utilize their structural robustness and advantages in large-area applications, while maintaining a relatively precise thickness. These are laminates made of E-glass fabric/epoxy prepregs containing carbon black (CB), carbon nanotube (CNT), and carbon nanofiber (CNF). The spacer is a pure E-glass fabric/epoxy composite laminate. The optimal design process based on a numerical model of the complex permittivity of the carbon nanocomposites, and a genetic algorithm shows that the real part of the complex permittivity of the sheet is closely related to the reduction of the spacer thickness. In contrast, the variation of the imaginary part is very marginal. The design results show that, in the X-band, the absorber thickness is 3.1 mm for the CB-composites, 2.76 mm for the CNT-composites, and 2.5 mm for the CNF-composites. The 10-dB bandwidth of every absorber is about 3.8 GHz. In the Ku-band, the absorber thickness is 1.95 mm for the CB-composites, 1.85 mm for the CNT-composites, and 1.7 mm for the CNF-composites. The 10-dB bandwidth of every absorber is broader than 5.5 GHz.

Effect of short carbon fibers and MWCNTs on microwave absorbing properties of polyester composites containing nickel-coated carbon fibers

Multiphase composite materials filled with multiwall carbon nanotubes (MWCNTs), short nickel-coated carbon fibers and millimeter-long carbon fibers with various weight fractions and compositions are developed and used for the design of wide-band thin radar-absorbing screens. The effective complex permittivity of several composite samples is measured in the frequency range from 8 GHz to 18 GHz. The obtained results show that the addition of the MWCNTs into the mixture allows tuning the EM properties of the composite filled with the short nickel-coated fibers. Numerical simulations are also performed in order to design new radar-absorbing shields. Single-layer and bi-layer thin dielectric Salisbury screens are designed to exhibit minimum reflection coefficient at 10 GHz and at 15 GHz, and maximum bandwidth at −10 dB. It results that the total thickness of the screen can be reduced below 2 mm by using a lossy sheet made with the composite filled with MWCNTs and nickel-coated carbon fibers, whereas the bandwidth at −10 dB can exceed 6 GHz in a bi-layer structure.

Electromagnetic Design and Realization of Innovative Fiber-Reinforced Broad-Band Absorbing Screens

This paper presents the design and the realization process for radar absorbing panels made of composite materials. It is demonstrated that the proper selection of the carbon fiber length and volume fraction allows reducing sensibly the overall thickness of the screen compared to a standard absorber, and obtaining at the same time broad-band absorption response in the X- and Ku-bands. To this end, the carbon fiber composite material realizing the lossy sheet of the absorbing screen is designed by simulation in order to have tailored complex effective permittivity. Two prototypes of absorbing screens are realized and tested experimentally. The first one is a three-layer panel, characterized by reflection coefficient lower than -10 dB in the frequency range from 8 to 14 GHz, and total thickness of 4.5 mm. The second one is a five-layer panel with reflection coefficient less than -20 dB in the range 9-18 GHz, and total thickness of 5.5 mm.

Analysis of hybrid EM wave absorber using thin corrugated dielectric lossy sheet based on equivalent permittivity representation

AbstractThe 10‐m and 3‐m semianechoic chambers are in widespread use for the electromagnetic compatibility (EMC). The hybrid EM wave absorber consisting of the sintered ferrite tiles and the dielectric lossy materials has been usually used for these semianechoic chambers. In the past, Suetake, Naito, and Shimizu's studies played a key part in providing the basis for the hybrid EM wave absorber. The reflectivity of the hybrid EM wave absorber can be calculated from the multilayered approximate structure. In particular, each layer occupied partly by the dielectric lossy materials can be replaced by the homogeneous plate having the equivalent relative complex permittivity, which is determined through the synthesized capacitance model in the large wavelength range. But, there was a problem that the equivalent relative complex permittivity differs with the process of the synthesized capacitance model.We propose a new method which can be applied to the equivalent relative complex permittivity by the finite difference method (FDM). Moreover, the reflectivity of the hybrid EM wave absorber using the thin corrugated dielectric lossy sheet is calculated and measured. As a result, it is confirmed that the measured reflectivity agrees with the calculation one. © 2009 Wiley Periodicals, Inc. Electron Comm Jpn, 92(7): 12–20, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10082

Realization of 3 m Semi Anechoic Chamber by Using Crossed-Wedge Shaped Hybrid EM Wave Absorber Consisting of Thin Corrugated Dielectric Lossy Sheet

This letter proposes a new hybrid EM wave absorber with the crossed-wedge shape, which can be applied to 3m semi anechoic chambers. In this study, we designed a new hybrid EM wave absorber with the crossed-wedge shape, which consisted of the inorganic and organic thin corrugated dielectric lossy sheet containing organic conductive fibers. Then the 3m semi anechoic chamber is constructed in the size of 9.0m × 6.0m × 5.7m (L × W × H) using these absorbers, and also the normalized site attenuation (NSA) is measured according to ANSI C63.4 in the frequency range of 30MHz to 1GHz. As a result, the measured NSA is obtained within ±3dB of the theoretical one.

EMC Impact of Advanced Carbon Fiber/Carbon Nanotube Reinforced Composites for Next-Generation Aerospace Applications

This paper presents a comparative analysis of the electromagnetic properties of new composite materials that are of interest to future aircraft/aerospace structures. The fabrication process of single-phase and new multiphase micro/nanocomposites is described. Carbon black, carbon fibers, and multiwall carbon nanotubes are randomly mixed into an epoxy resin matrix at various weight fractions and compositions. The experimental characterization in the frequency range 8-18 GHz shows that the dispersion characteristics of short-carbon-fiber-reinforced composites can be properly controlled by the addition of nanopowders and nanotubes into the mixture. Numerical simulations demonstrate the feasibility of the fabricated materials for the design of new electromagnetic micro/nanostructured shields and radar-absorbing laminates. Thin dielectric Salisbury screens are especially designed to exhibit minimum reflection coefficient at 15 GHz. It shows that the total thickness of the screen can be reduced below 2 mm by using a lossy sheet made of three-phase composites.

FACTORS AFFECTING THE PERFORMANCE OF THE RADAR ABSORBANT TEXTILE MATERIALS OF DIFFERENT TYPES AND STRUCTUS

In the military applications, electro-optics and electromagnetics features of the cloths made of special textile structures and certain fibers play an important role in producing textile characterized with high ability of camouflage. This means not allowing enemy to precisely determine the moving and mute targets using available radar or any other electro-optics sensors. The aim of this paper is to determine the effect of the composite fabrics structure parameters on the fabrication of a resistive sheet used in many radar applications. The objective sheet can be used as a lossy sheet in absorbing mechanism. The radar absorbing mechanism has been studied with emphasis on the single layer structure mechanism with a resistive sheet. Fabric structure of the resistive sheet was accurately chosen and coated

A Study on Gain Characteristics of the Dipole Antenna Used for the RFID System in UHF Band

The RFID system in UHF band is a problem that deterioration of the radiation gain characteristic is caused by the attachment place of RFID tags. In this paper, we purpose to confirm the effect with some objects arranged in the back of the antenna in the RFID system. We carried out the study on changes of radiation gain characteristics when an antenna is attached a metal plate and when a magnetic lossy sheet is arranged between an antenna and a metal plate by analysis and measurement.

薄い波状誘電損失体を用いた複合電波吸収体の等価誘電率表現による解析

The 10m and 3m semi anechoic chambers are in widespread use for the electromagnetic compatibility (EMC). The hybrid EM wave absorber consisting of the sintered ferrite tiles and the dielectric lossy materials has been usually used for these semi anechoic chambers. In the past, Suetake, Naito, and Shimizu's studies played a key part in providing the basis for the hybrid EM wave absorber. The reflectivity of the hybrid EM wave absorber can be calculated from the multi-layered approximate structure. Especially, each layer occupied partly the dielectric lossy materials can be replaced by the homogeneous plate having the equivalent relative complex permittivity, which is determined through the synthesized capacitance model in the large wavelength range. But, there was a problem that the equivalent relative complex permittivity differs with the process of the synthesized capacitance model.We propose a new method which can be applied to the equivalent relative complex permittivity by the Finite Difference Method (FDM). Moreover, the reflectivity of the hybrid EM wave absorber using the thin corrugated dielectric lossy sheet is calculated and measured. As the result, it is confirmed that the measured reflectivity agree with the calculation one.

Simultaneous measurement of permittivity and permeability of lossy sheet using flanged rectangular waveguide

AbstractWe describe a method for simultaneously measuring the complex permittivity and the complex permeability of a lossy sheet that is inserted between two flanged rectangular waveguides. The equivalence principle of the electromagnetic fields is applied to the boundary surfaces between the sheet and the waveguides to derive the integral equations while taking the high‐order modes excited at the boundary surfaces into account. Then the method of moments is used in these equations to formulate the direct problem. Newton's method is applied to the inverse problem, and the convergence performance of the solution is examined. Furthermore, the reflection coefficients and the transmission coefficients are measured for some samples, and their complex permittivities and complex permeabilities are estimated from these coefficients. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 86(7): 32–41, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.10104

Finite element analysis for complex permittivity measurement of a dielectric plate and its application to inverse problem

AbstractThe resonant frequency and unloaded quality factor of a cylindrical cavity resonator are computed accurately by using the axisymmetric finite element method. Here the loss of both a dielectric sample and a shielding conductor is taken into account. A term with 1/r among the components of the finite element matrix is analytically integrated over a triangular ring quadratic element. Also, the inverse problem, that is, the estimation of the complex permittivity of a dielectric from a resonant frequency and an unloaded quality factor, is formulated and its usefulness is confirmed through some computed results for dielectric plates and deformed lossy sheets. © 2001 Scripta Technica, Electron Comm Jpn Pt 2, 84(5): 10–19, 2001