不同吸波涂层对三维复杂目标的RCS影响的分析

利用有限元-快速多极子算法(FEM-FMM)分析了三维复杂目标涂有不同吸波涂层的雷达散射截面(RCS).以圆锥体为例,详细计算分析了涂敷有耗各向同性、正单轴各向异性、负单轴各向异性和纳米吸波材料4种典型吸波材料对目标电磁散射特性的影响,并首次讨论了每种吸波材料随不同涂层厚度对RCS的影响,得到了最佳隐身效果的吸波材料和涂层厚度的组合.结果表明,涂敷最佳吸波材料目标的后向RCS比目标没有吸波涂层时可以降低16.6dB.     

Coupling into and scattering from cylindrical structures coveredperiodically with metallic patches

Circular cylindrical structures covered periodically with metallic patches are considered. After an analogy to planar periodic surfaces is shown, formulations are presented for calculating induced currents on the curved surface. The equations are solved and results calculated for the specific case of periodic strips on the cylindrical surface. For a cylindrical structure a two-dimensional periodicity exists, as in a planar structure, while a spherical structure allows only a rotational periodicity. When the cylindrical structure is excited by the characteristic harmonic of the system, the spectral response of the transmitted field exhibits resonances that depend on the surface periodicity, as is known for planar structures. Since the cylindrical structure contains finite closed regions, the effects of resonances internal to the structure are seen and give additional information as compared to planar structures     

一种电大尺寸涂覆目标RCS 的算法

从Stratton-Chu 积分方程入手,推导出一种光滑凸体金属表面涂覆雷达吸波材料（RAM）的物理光学后向RCS计算公式,同时考虑边缘绕射的贡献,介质劈与金属劈的电磁散射特性是不同的,须通过等效电磁流法(EEC)来求解介质边缘散射加以修正。通过对涂覆平板、涂覆柱锥组合体及某导弹目标RCS 的计算,再与实测值和矩量法结果对比,它们均相吻合,从而验证了算法的有效性和准确性。本算法特别适合大尺寸目标RCS 计算。     

振子阵列天线的雷达截面积

本文用矩量法,对大型平面阵列和圆阵柱阵列天线在不同加载情况下的散射进行了研究。在考虑振子互耦情况下,计算出振子上的电流分布,并得到阵列的雷达截面积(RCS)。实验结果表明,理论值与实验值两者基本趋势一致。文中采用的平面阵加截和共形曲面阵技术,可大幅度地缩减振子阵列天线的RCS,具有实际的工程意义。     

Scattering of electromagnetic pulses by simple-shaped targets withradar cross section modified by a dielectric coating

We study the scattering interaction of electromagnetic pulses with a spherical target. The target is a perfectly conducting sphere coated with a thin dielectric layer. Two different hypothetical materials are specified: a lossy dielectric and a dielectric that also has magnetic losses. The monostatic radar cross section (RCS) is computed in each case and we examine the influence of the coating on the RCS. In particular, we compare the RCS of the coated sphere with the (normalized) backscattered power when a large perfectly conducting flat plate coated with the same dielectric layer is illuminated at normal incidence by the same waveform. In particular, we find that except for frequencies below those within the efficiency band of the absorbent material, the normalized RCS of the coated sphere agrees well with the power reflection coefficient of the plate covered with the same kind of coating. For low-frequency incidences, the peaks and dips in the RCS are more prominent for the coated target than they are for the bare one. Analyzing the response of the spherical targets in the combined time-frequency domain we demonstrate that the coating itself, although reducing the RCS could introduce additional resonance features in the target's signature at low frequencies that could be used for target recognition purposes. This observation is also confirmed by a study of the bistatic RCS of these coated objects, which we have displayed in various color graphs     

基于EBG结构吸波材料在螺旋天线阵中的应用

研究了基于EBG结构的超薄吸波材料在螺旋天线阵RCS缩减中的应用。通过采取在EBG贴片之间加载集总电阻的方式实现吸波材料,并将螺旋天线阵金属底面用EBG吸波材料代替,以期实现了天线阵工作频带内RCS减缩的目标。实验结果表明,在EBGRAM的吸收频段内该吸波材料能有效缩减螺旋天线阵的RCS,同时保持天线单元辐射性能,仅前向增益有所下降。     

Radar cross section of a finite planar structure coated with a lossy dielectric

An asymptotic high frequency estimation of monostatic radar cross section (RCS) of a finite planar metallic structure coated with a lossy dielectric is made theoretically and compared with experiments in theX- band. An expression for RCS is obtained by using a diffraction coefficient involving Maliuzhinets' function. A closed form expression of the Maliuzhinets' function has been developed. Corner diffraction effects are not considered. A continuous wave (CW) monostatic RCS measurement set up has been used to verify the theoretical results experimentally. A typical reduction with a single coating of epoxy resin of thickness equal tolambda /20is of the order of 5 dB for normal incidence compared to a conducting plate of the same size.     

具有超宽带RCS减缩特性的天线设计

该文设计了两种人工磁导体(AMC)单元,在8～20 GHz的超宽频带内,两种AMC结构能够实现180°±37° 的反射相位差,将这两种单元组成棋盘结构时,能够实现入射电磁波的散射场相消,从而在超宽的频带内实现棋盘表面法向雷达散射截面(RCS)的显著减缩。同时,利用超表面天线的概念,设计馈电网络,将设计的AMC结构用做天线,仿真发现在9.08～10.30 GHz的范围内,天线的S₁₁小于–10 dB,可以实现天线的有效辐射。实测结果和仿真吻合较好,因此该文的棋盘结构可以实现具有RCS减缩特性的天线设计。     

A review of high-frequency radar cross section analysiscapabilities at McDonnell Douglas Aerospace

Two basic types of physical-optics (PO)-based radar cross section (RCS) analysis codes have come to maturity in today's HF electromagnetic analysis environment. These are facet based and curved surface based codes. Facet codes have very fast analysis rates, while curved-surface codes are usually considered more accurate. At McDonnell Douglas Aerospace (MDA), the need for very reliable RCS results to guide the aircraft design process has led us to develop a curved surface based high-frequency code. This code is called CADDSCAT (computer-aided design drafting scattering). Various aspects of CADDSCAT's curved surface and facet based analysis capabilities are described and compared. Topics covered include the genesis of curved surface based PO techniques, a discussion of first-bounce PO computations, and multiple-bounce PO interactions. CADDSCAT's first-bounce and multiple-bounce PO algorithms include options for treating radar absorbing materials (RAM). The CPU needs for the ray-tracer algorithms, for curved and faceted models, are compared. Gap diffraction, resistive-card modeling, and sea-surface modeling are briefly discussed     

Application of curvilinear basis functions and MLFMA for radiation and scattering problems involving curved PEC structures

The curvilinear surface and wire basis functions have been applied to solve radiation and scattering problems of various shapes of curved metallic structures with the method of moments. However, a suitable form of junction basis function that can connect the curvilinear wire and the curvilinear Rao-Wilton-Glisson (RWG) surface basis functions has not been found. We present a novel curvilinear junction basis function that can fill the gap. The convergence tests reveal that the curvilinear junction basis function, in conjunction with curvilinear wire and surface basis functions, can reduce the radar cross section (RCS) error by one order of magnitude more than their reduced linear forms for a curved structure. The curvilinear basis functions are applied to solve the radiation from wire antennas attached to curved surfaces. We will demonstrate that the curvilinear basis functions with the multilevel fast multipole algorithm can significantly reduce the computational resources for simulating electrically large objects with curved features.     

Electromagnetic scattering from simple jet engine models

The mode-matching (MM) technique is employed for the evaluation of the radar cross section (RCS) of structures that simulate a jet engine inlet. The geometry consists of a perfectly conducting cylindrical inlet terminated by an array of blades mounted on a cylindrical hub. Comparisons of numerical results with actual measurements are presented for the first time     

A uniform geometrical theory of diffraction for an edge in a perfectly conducting surface

A compact dyadic diffraction coefficient for electromagnetic waves obliquely incident on a curved edse formed by perfectly conducting curved ot plane surfaces is obtained. This diffraction coefficient remains valid in the transition regions adjacent to shadow and reflection boundaries, where the diffraction coefficients of Keller's original theory fail. Our method is based on Keller's method of the canonical problem, which in this case is the perfectly conducting wedge illuminated by plane, cylindrical, conical, and spherical waves. When the proper ray-fixed coordinate system is introduced, the dyadic diffraction coefficient for the wedge is found to be the sum of only two dyads, and it is shown that this is also true for the dyadic diffraction coefficients of higher order edges. One dyad contains the acoustic soft diffraction coefficient; the other dyad contains the acoustic hard diffraction coefficient. The expressions for the acoustic wedge diffraction coefficients contain Fresenel integrals, which ensure that the total field is continuous at shadow and reflection boundaries. The diffraction coefficients have the same form for the different types of edge illumination; only the arguments of the Fresnel integrals are different. Since diffraction is a local phenomenon, and locally the curved edge structure is wedge shaped, this result is readily extended to the curved wedge. It is interesting that even though the polarizations and the wavefront curvatures of the incident, reflected, and diffracted waves are markedly different, the total field calculated from this high-frequency solution for the curved wedge is continuous at shadow and reflection boundaries.     

优化算法在手征媒质中的应用研究

由于手征媒质的手征参数可调性,使它成为一种新型的吸波材料,在减缩目标雷达散射截面方面有巨大的应用前景.文中运用传输线法分析了金属衬底上涂敷手征媒质的反射特性,计算了手征参数对手征媒质反射特性的影响;然后基于非均匀有理B样条建模,采用物理光学法比较了目标涂敷常规吸波材料与手征媒质时的雷达散射截面,结果说明了手征媒质减小反射电磁波的效果更好;最后运用粒子群算法和模拟退火算法在给定的范围内优化手征参数,并计算了在此参数下目标的散射截面.     

A computational methodology and results for quasistaticmultilayered magnetic shielding

A methodology for accurate calculations of shielding factors for quasistatic multilayered magnetic shields is described. “Transfer relations” for individual layers with specified magnetic permeabilities and electrical conductivities are spliced together. Specific transfer relations for four layer geometries (planar, cylindrical with transverse fields, cylindrical with axial fields, and spherical) and constraints at source and shielded surfaces for six source-shield configurations involving both externally applied fields and enclosed sources are developed. Limiting cases are extracted for magnetically permeable, nonconducting layers, and for thin, magnetically nonpermeable, conducting layers. Reciprocity conditions are identified for interchanged source and shielded regions in planar, transverse field cylindrical, and spherical geometries. Variations of magnetic field and flux density with position are shown for a specific planar example involving alternating layers of aluminum and steel, with the same total shield thickness occupied by either one or five layer pairs. Simulations with alternating layers of aluminum and steel for the four layer geometries are used to study the effects of material composition, number of layer pairs, and air gaps. An optimal number of layer pairs for a given total shield thickness is identified. Results from simulations where induced currents in the steel layers are neglected are compared with those for simulations with a realistic conductivity value for steel to assess the relative effects of flux shunting and induced current shielding     

曲面EBG结构带隙特性的研究

利用光学B ragg反射条件,设计了一种曲面电磁带隙结构。采用悬置微带线法,分析了其结构参数以及圆柱结构参数对带隙特性的影响。结果表明,结构参数对表面波带隙影响的规律与平面上一致,圆柱曲率对表面波带隙的影响不是太大,只是带隙中心频率略有下降。这一结论对在曲面共形天线上应用电磁带隙结构提高天线性能有重要的指导意义。     

求解埋入体电磁散射问题的矢量波函数展开法

本文利用矢量波函数展开法求解了任意激励原埋入体的电磁散射问题。通过导出圆柱和圆球矢量波函数的转换关系,使场量满足分界平面和数学球面边界条件,从而方便地利用矢量波函数展开求解了这一复杂边值问题。作为示例,本文计算了在平面波和偶极子激励下,埋入导体球和介质球的散射场。     

VECTOR WAVE FUNCTION EXPANSION FOR SOLVING ELECTROMAGNETIC SCATTERING BY BURIED OBJECTS

An analysis of solving the electromagnetic scattering by buried objects using vectorwave function expansion is presented.For expanding the boundary conditions both on the planarair-earth interface and on the spherical surface,the conversion relations between the cylindricaland spherical vector wave functions are derived.Hence the vector wave function expansion isconveniently applied to solve this complex boundary-value problem.For the excitation of the in-cident plane wave and the dipole above the earth,the scatterlng patterns of the buried conductingand dielectric spheres are presented and discussed.     

High-order impedance boundary conditions for multilayer coated 3-Dobjects

The scattering problem by a multilayer coated three-dimensional (3-D) object where the coating is modeled by an impedance boundary condition (IBC) is considered. First, the exact boundary condition is obtained for an infinite planar coating with an arbitrary number of layers. Then, various approximations for the pseudodifferential operators involved in this exact condition are proposed. In the expressions of the resulting IBCs, all tangential derivatives of the fields of order higher than two are suppressed. These IBCs are compared, in terms of numerical efficiency, by computing either the reflection coefficients on an infinite planar metal-backed coating or the radar cross section (RCS) of a perfectly conducting coated sphere using the tangent plane approximation. In both cases, it is found that the highest order IBC models the coating with a good accuracy. Finally, some guidance is given on how this IBC may be numerically implemented in an integral equation or a finite-element formulation for an arbitrarily shaped object     

一种利用外推获得飞行器目标远场RCS方法研究

提出了一种球面波照射下获取室内飞行器目标RCS的方法。室内RCS测试难以满足远场条件,造成散射方向图畸变。针对飞行器目标机身高度较小,可将入射球面波等效为柱面波,然后借助参考目标,导出其在柱面波与平面波照射下的修正系数g(x)。再次,建立被测目标近距散射场与g(x)之间的关系,从而得到目标等效平面波照射下的远场RCS。仿真表明:与理论值相比,外推后的结果在有限角域内吻合良好。实验在微波暗室测量了飞行器目标在两个不同距离下的散射场,外推后RCS具有一致的收敛性,并与电磁计算结果进行比较,验证了外推技术具有较高的精度。另外,该方法对目标口径、纵深尺寸没有苛刻的限制。     

RCS measurements and results of an engine-inlet system designoptimization

A number of papers have been published on the prediction of backscatter from engine-inlet-type structures. Measured data used to verify these predictions mostly consists of backscattered data from canonical structures, e.g., cylindrical waveguide cavities. In this paper, measured backscattered data for an unmanned aerial vehicle-type engine-inlet system is presented. The effects of the application of radar-absorbing materials to the engine-inlet system on the contribution to the RCS is discussed, and data measured in a compact-range facility are presented