Interaction of Electromagnetic Waves with Two-Dimensional Metal Covered with Radar Absorbing Material and Plasma

A two-dimensional metal model is established to investigate the stealth mechanisms of radar absorbing material （RAM） and plasma when they cover the model together. Using the finite-difference time-domain （FDTD） method, the interaction of electromagnetic （EM） waves with the model can be studied. In this paper, three covering cases are considered： a. RAM or plasma covering the metal solely; b. RAM and plasma covering the metal, while plasma is placed outside; e. RAM and plasma covering the metal, while RAM is placed outside. The calculated results show that the covering order has a great influence on the absorption of EM waves. Compared to case a, case b has an advantage in the absorption of relatively high-frequency EM waves （HFWs）, whereas case c has an advantage in the absorption of relatively low-frequency EM waves （LFWs）. Through the optimization of the parameters of both plasma and RAM, it is hopeful to obtain a broad absorption band by RAM and plasma covering. Near-field attenuation rate and far-field radar cross section （RCS） are employed to compare the different cases.

Microwave Absorption Properties of the Carbonyl Iron/EPDM Radar Absorbing Materials

Employing carbonyl iron powder and Ethylene-Propylene-Diene Monomer （EPDM） as the absorbent and matrix, rubber radar absorbing materials （RAM） were prepared. Effects of the carbonyl iron volume fraction and the thickness of the RAM on the microwave absorption properties in the frequency range of 2.6-18GHz were studied, and a mathematical analysis was made using the electromagnetic theory. The experimental results indicate that the minimum reflectivity of the radar absorbing materials continuously decreases with the increase of the carbonyl iron volume fraction, and the absorption peak also moves towards the low frequency for the same thickness of the RAM. The minimum reflectivity of the 3.0 mm RAM is -21.7dB at 3.5 GHz when the volume fraction of carbonyl iron is 45%. The reflectivity of the RAM is not in direct proportional to the thickness of the RAM, when the RAM has the same volume fraction of the carbonyl iron. The reflectivity of the RAM presents a regular trend at a given carbonyl iron volume fraction in the frequency range of 2.6-18 GHz. With the increase of the thickness, the maximum absorption peak moves towards low frequency band, the minimum reflectivity firstly decreases and then increases, and the absorption bandwidth for reflectivity〈-10 dB firstly increases and then decreases. The microwave absorption properties of the RAM are determined by the thickness and the composition of the radar absorbing materials. Theoretical analysis indicates that the reflectivity of the RAM is determined by the matching degree of the air＇s characteristic impedance and the input impedance.

Multi-objective optimization design of radar absorbing sandwich structure

By introducing a dimensionless parameter to couple the two objectives, weight and radar absorbing performance, into a single objective function, a multi-objective optimization procedure for the radar absorbing sandwich structure （RASS） with a cellular core is proposed. The optimization models considered are one-side clamped sandwich panels with four kinds of cores subject to uniformly distributed loads. The average specular reflectivity calculated with the transfer matrix method and the periodic moment method is utilized to characterize the radar absorbing performance, while the mechanical constraints include the facesheet yielding, core shearing, and facesheet wrinkling. The optimization analysis indicates that the sandwich structure with a two-dimensional （2D） composite lattice core filled with ultra-lightweight sponge may be a better candidate of lightweight RASS than those with cellular foam or hexagonal honeycomb cores. The 2D Kagome lattice is found to outperform the square lattice with respect to radar absorbing.

Preparation and radar-absorbing properties of Al2O3/TiO2/Fe2O3/Yb2O3 composite powder

Al2O3/TiOe/FeeO3/Yb2O3 composite powder was synthesized via the sol-gel method. The structure, morphology, and ra- dar-absorption properties of the composite powder were characterized by transmission electron microscopy, X-ray diffraction analysis and RF impedance analysis. The results show that two types of particles exist in the composite powder. One is irregular flakes （100-200 rim） and the other is spherical A1203 particles （smaller than 80 rim）. Electromagnetic wave attenuation is mostly achieved by dielectric loss. The maximum value of the dissipation factor reaches 0.76 （at 15.68 GHz） in the frequency range of 2-18 GHz. The electromagnetic absorption of waves covers 2-18 GHz with the matching thicknesses of 1.5-4.5 mm. The absorption peak shifts to the lower-frequency area with increas- ing matching thickness. The effective absorption hand covers the frequency range of 2.16-9.76 GHz, and the maximum absorption peak reaches -20.18 dB with a matching thickness of 3.5 mm at a frequency of 3.52 GHz.

Comparison the Theoretical and Experimental Results of Radio Absorbing Properties of Au Thin Films in 8-11.6 GHz Band

The comparison of theoretical and experimental results of measurements of absorbing properties of Au films of 10 nm thickness deposited on a polymer substrate in the frequency band 8-11.6 GHz electromagnetic radiation are presented. The new configuration of Au thin film in a rectangular waveguide had been considered. An abnormally high level of absorption of electromagnetic radiation throughout the range of wavelengths was obtained. This is apparently due to the lack of galvanic contact between the film and waveguide.

Metamaterial Inspired Radar Absorbers:Emergence,Trends and Challenges

The advances in metamaterial science and technology have raised the expectations of camouflage or stealth researchers to one order higher in terms of absorption characteristics.As metamaterial inspired radar absorbing structures are proving themselves as a good candidate with near unity absorption,feasibility towards hardware realization is necessary.Hence an extensive literature survey of metamaterial inspired radar absorbing structure has been carried out and reported in this paper along with the challenges and material issues.The various types of metamaterial structures that can be used as absorber have been provided along with simulation figures.To make the review more useful,graphene and carbon nanotube(CNT)based radar absorbing structures are also included along with their simulation and fabrication techniques.

The Optimizing Research about Radar Absorbent Material Parameters

From the Physical Optics theory (PO) and Leontovich Impedance Boundary Condition (IBC), We research RCS reduction (RCSR) of multilayer dielectric and magnetic medium on different shape conductors such as plate, cuboid and cone by use of Matlab programs. Some available RCS data and graph results are given. These show the connection between Radar Absorbent Material (RAM) parameters and the number of layers. In the mean time, the relation between RAM optimized parameters and RCS value is also shown. It has better practical significance.

Equivalent Currents on an Anisotropic Material Backed by a Metal Surface and Their Relation

Based on a first-order state-vector differential equation representation of Maxwell's equations, an analytical formulation is derived for the equivalent currents on an anisotropic material backed by a metal surface, and the relation between two currents is also considered. These expressions are degenerated into the common forms for some simple cases. This effort will provide the theoretical preparation for the approximate calculation of electromagnetic scattering from a conducting object coated by an anisotropic material.

EM Scattering from Conducting Flat Plates Coated with Thin RAM

According to the equivalence principles, high frequency approximation and boundary conditions, a method has been developed to deal with the EM scattering by a rectangular conducting flat plate coated with uniaxial anisotropic radar absorbing material (RAM). The simple and effective method is available to the system of RCS prediction in which the large complex targets modeled by facets and wedges. Numerical results show some properties of EM scattering by conducting plate coated with thin uniaxial anisotropic RAM.

Surface Reflection Coefficient of Impregnated RAM Honeycomb with Incident Normally Plan Wave to the Side Wall

The impregnated radar absorbing material(RAM) honeycomb is often used to fabricate parts of the war plane for reducing radar cross section. The incident wave vector may be divided into two components: one perpendicular to its hole and the other to its side wall. Until now, there has not been a program to calculate the input impedance or its equivalent electromagnetic parameters for the later case. In this paper, an approach for analyzing the reflection characteristics of the impregnated honeycomb when its side wall faces the incident plane wave is proposed. Experiments prove it an effective, accurate and fast solution to this subject.

Protection of armoured vehicles against chemical, biological and radiological contamination

The article presents problems related to mechanical protection of vehicles with different add-on armours against chemical,biological and radiological contamination.This applies to vehicles with additional passive,reactive and hybrid protection in the form of cassettes against piercing with anti-tank projectiles,piercing with their kinetic energy of impact,and as a result of chemical energy of shaped charges as well as explosively formed projectiles.It has been shown how increased ballistic protection of these vehicles at the same time reduces their decontaminability in various places of the vehicle due to the increased additional surface of the vehicle with cassettes.Prevention of contamination of these cassettes has been presented as a way of reducing hard to reach surface for decontamination and a method of insulating construction elements(stands),fixing these cassettes to the vehicle,from the environment to prevent contact with contaminated liquids and dusts.The selection of appropriate materials is shown,which may affect the improvement of the decontamination efficiency of the vehicle with such cassettes.This applies to the use of materials with low absorption of chemical warfare agents,which prevent the accumulation of large amounts of these agents on contaminated surfaces and improve the effectiveness of decontamination.It also shows how to ensure better access of the disinfectant to as much of the vehicle surface as possible,covered with cassettes that have been contaminated.It shows how a vehicle,in particular with such cassettes,can provide protection against radar detection when Radar Absorbent Material is used on vehicle cassettes.

Stealth Considerations for Aerodynamic Configurations Design of Missiles

The aerodynamic design of a strategic weapon is of interest, especially when the radar signatures are included in the conceptual design phase. The basics of stealth configurations and stealth mechanisms for missiles are reviewed. The Radar Cross Sections （RCS） of some generic missiles are predicted and compared to analyze the trade-offs involved between low RCS and aerodynamic performance. The consideration of RCS prediction in the conceptual design phase gives a quick insight into the stealth performance prior to detailed design.

Radar Cross Section Reduction of a Cavity in the Ground Plane

This paper investigates the reduction of backscatter radar cross section(RCS)for a rectangular cavity embedded in the ground plane.The bottom of the cavity is coated by a thin,multilayered radar absorbing material(RAM)with possibly different permittivities.The objective is to minimize the backscatter RCS by the incidence of a plane wave over a single or a set of incident angles.By formulating the scattering problem as a Helmholtz equation with artificial boundary condition,the gradient with respect to the material permittivities is determined efficiently by the adjoint state method,which is integrated into a nonlinear optimization scheme.Numerical example shows the RCS may be significantly reduced.

Hierarchical construction of CNT networks in aramid papers for high-efficiency microwave absorption

Carbon nanotubes(CNTs)incorporated polymeric composites have been extensively investigated for microwave absorption at target frequencies to meet the requirement of radar cross-section reduction.In this work,a strategy of efficient utilization of CNT in producing CNT incorporated aramid papers is demonstrated.The layer-by-layer self-assembly technique is used to coat the surfaces of meta-aramid fibers and fibrils with CNT,providing novel raw materials available for the large-scale papermaking.The hierarchical construction of CNT networks resolves the dilemma of increasing CNT content and avoiding the agglomeration of CNT,which is a frequent challenge for CNT incorporated polymeric composites.The composite paper,which contains abundant heterogeneous interfaces and long-range conductive networks,is capable of reaching a high permittivity and dielectric loss tangent at a low CNT loading,its complex permittivity is,so far,adjustable in the range of(1.20−j0.05)to(25.17−j18.89)at 10 GHz.Some papers with optimal matching thicknesses achieve a high-efficiency microwave absorption with a reflection loss lower than−10 dB in the entire X-band.

RCS Evaluation of Dihedral corner Renector by Complex Ray Method

With the techniques provided by the complex my method,like co,nplex ray expansion, complex mytracing,complex my paraxial approximation, etc.,the back scatter of dihedral corner reflectors is evaluated.Numerical results show that this method gives good and useful RCS prediction of the targets.

Influence of Unidirectional Tensile External-loading on Electromagnetic Property of the Perforated Frequency Selective Surface Illuminated by Electromagnetic Wave Incident at Arbitrary Angles

That whether unidirectional tension changes the influence of incident angle on FSS's(frequency selective surface) electromagnetic(EM) property and whether incident angle changes the effect of unidirectional intension on FSS's EM property is researched.The pattern and magnitude of these changes are revealed.Firstly,the analyses for the mechanism of unidirectional tension's influence and the mechanism of the coupling between the influences of unidirectional tension and incident angle are conducted.Then,according to the mechanism of these influences,Mechanics-Electromagnetics co-analysis is conducted based on mechanics finite element method and electromagnetics finite element method.The frequency responses of the FSS which is under different unidirectional tensions and illuminated by EM wave incident at different angles are gained.The concept"unidirectional tensile sensitivity of FSS"is put forward along with two unidirectional tensile sensitivity factors.By using angular sensitivity-factor and unidirectional tensile sensitivity-factor,the influences of unidirectional tension and incident angle,which are coupled with each other,are expatiated respectively.Research results show that these influences have two sides.Meanwhile,basing on Mode Matching Method and according to the results of numerical investigation,one principle for the layout of FSS with apertures is gained.Several suggestions for further research are given.

Determination of Interrogating Frequencies to Maximize Electromagnetic Backscatter from Objects with Material Coatings

The electromagnetic backscattering of a crosscut of a cruise missile coated by a thin homogeneous layer made of radar absorbent material is modeled using a finite element method.Based on the radar cross section and a reflection coefficient,optimization problems are formulated for evaders and interrogators leading to optimal material parameters for the coating and optimal monostatic radar operating frequencies,respectively.Optimal coating materials are constructed for several radar frequencies.Tuning only dielectric permittivity gives a narrow frequency range of high absorption while also tuning magnetic permeability widens it significantly.However the coating layers considered do not provide substantial reduction of backscattering in the entire frequency range from 0.2 to 1.6 GHz.Computational experiments also demonstrate that the reflection coefficient based on a simple planar geometry can predict well the strength of radar cross section in the sector of interest with a substantially reduced computational burden.