一种旋转对称涂覆导体电磁散射高效分析方法

为改善传统方法分析旋转对称涂覆导体电磁散射问题的效率,提出了一种高效分析方法.该方法在介质表面建立电磁流混合场积分方程(Electric and Magnetic Current Combined Field Integral Equation,JMCFIE),在导体表面建立混合场积分方程(Combined Field Integral Equation,CFIE),利用了旋转对称体在空间上的旋转周期性,只需要对表面的母线进行剖分,具有未知量少且阻抗矩阵条件数好的特点.根据等效原理与边界条件推导了JMCFIE-CFIE方程,并与传统的PMCHW-CFIE方法对比了求解效率.数值算例表明该方法能明显改善方程的收敛性.     

The scattering of electromagnetic pulses by a slit in a conducting screen

A direct method for calculating the impulse response of a slit in a conducting screen is presented. The method may be used to study the scattering of electromagnetic pulses by conducting objects with planar boundaries and straight edges. The impulse response is obtained by assuming that the total response is composed of two sequences of diffracted waves. The solution is determined for the first two diffracted waves in one sequence by using Green's functions and the equivalence principle. The method is applied iteratively to obtain the solution for additional waves in the sequence. The solution for the other sequence is obtained by a transformation of coordinates. The total response is the sum of the two sequences of waves. The cases ofE-polarization andH-polarization are considered. Curves are presented to depict 1) the successive diffractions of the pulses by the edges of the slit, and 2) the impulse response of the slit.     

Transform approach to electromagnetic scattering

In this paper, we present a comprehensive review of the Fourier transform technique as applied to the problem of high-frequency scattering and introduce the concepts of the spectral theory of diffraction (STD). In contrast to the more commonly employed ray-optical method for high-frequency scattering, viz. the geometrical theory of diffraction (GTD), the STD approach interprets the scattered field as the spectrum, or the Fourier transform of the induced current on the scatterer. Such an interpretation offers several important advantages: uniform nature of representation, capacity to improve and extend the ray-optical formulas in a systematic manner, and convenient accuracy tests for the results. Methods for combining integral equation methods with the Galerkin procedure and asymptotic techniques in the transform domain are described, and representative examples illustrating the application of the spectral approach are included.     

高阶基尔霍夫法求解导体粗糙表面的散射特性

在分析粗糙表面电磁散射特性的基础上,提出了一种考虑粗糙表面协方差函数Taylor展开的高阶基尔霍夫近似(KA)法,解决了经典KA近似的大逼近误差问题.应用9阶高度的KA和传统的KA,对比研究了不同照射频率和均方根高度下的后向散射系数,并比较了遮挡函数的修正效应.同时应用高、低阶KA计算了典型粗糙面的后向散射系数,并分别与测量值和矩量法的数值解进行了比较.结果表明,9阶表面高度展开的切平面KA不仅拓宽了KA的适用条件,还具有更高的精度范围,从而证明了高阶均方根高度展开的KA近似的有效性.     

Scattering of electromagnetic waves by an anisotropic plasma-coated conducting cylinder

This paper considers the scattering of electromagnetic waves by an infinitely long anisotropic plasma-coated conducting cylinder. The source is assumed to be a magnetic current filament which gives rise to an incident magnetic field with only an axial component. Complete expressions for the scattered electric and magnetic fields are obtained. Scattering by an anisotropic plasma column and that by an isotropic plasma-coated conducting cylinder are special cases of the present problem.     

Scattering of electromagnetic waves by rough perfectly conducting circular cylinders

A numerical method is proposed for computing the normalized correlation functions of the real and imaginary parts of the field scattered from a statistically rough perfectly conducting circular cylinder. The deviation of the surface from its mean radius is assumed to be small. The correlation function of the far-field is related to the correlation function of the scattering object by an integral equation. Far-field correlation functions are found for two types of surface correlation functions: the delta function and a periodic exponential function.     

Resonance scattering of electromagnetic waves by a Kerr nonlinear dielectric layer

Diffraction of a plane electromagnetic wave by a dielectric layer is considered in the resonance region without allowance for multiple frequencies. The layer is assumed to be transversely inhomogeneous, isotropic, nonmagnetic, linearly polarized, and weakly Kerr nonlinear. A method based on solution of a nonlinear Fredholm integral equation of the second kind is developed. Sufficient conditions for the existence and uniqueness of a solution are obtained.     

New equations for electromagnetic scattering by small perturbationsof a perfectly conducting surface

New equations are derived for the electromagnetic scattering produced by a small perturbation of a general perfectly conducting surface. These equations explicitly incorporate the magnetic field boundary condition on the general surface, which implies that the new Born term by itself leads to conventional backscatter formulas. The accuracy of the new equations is demonstrated by an example     

Scattering of electromagnetic waves by a layer of air plasma surrounding a conducting cylinder

In this paper, the total scattering and back-scattering cross-sections (respectively represented by σ and σ_b) of an air plasma layer surrounding a conducting cylinder are studied. The plasma layer can be turned ON and OFF to allow for a comparison between the scattering cross-section of the bare cylinder and the plasma covered cylinder. The plasma layer is generated at atmospheric pressure, which results in a very highly collisional case. The scattered fields are calculated using a cylindrical expansion, with coefficients satisfying the appropriate boundary conditions, and which are a function of the refractive index of the air plasma. The results of our study are presented as plots of the total scattering cross-section, σ, and back-scattering cross-section, σ_b, versus frequency. The scattering cross-section gives an average characteristic of the scattering process from obstacles. Once the scattering cross-section is known, the actual scattered energy per unit length per second can be calculated by multiplying σ by the incident energy per unit area per second.     

Backscattering of electromagnetic waves by the vertex area of a thin conducting screen

The backscattering of the electromagnetic waves by the vertex area of a thin metal screen is investigated. A special case when the magnetic field of the incident wave is perpendicular to the diagonal of the vertex area is considered. The dependence of the vertex-scattered signal on the grazing angle is experimentally found. At a certain angle, an extremum in the radar scattering is discovered. At the extremum, good agreement between the results predicted by the asymptotic theory and the experimental results is observed.     

带电粒子的Mie散射研究

利用电磁波散射理论研究了带电粒子的电磁波散射特性,给出了散射系数与电磁阻抗系数及表面电导率的关系,计算和对比了不同面电导率的粒子与中性粒子对电磁波散射系数和散射能量分布的差别.计算结果表明:面电荷使面电导率达到微西门子量级时,会有明显的影响随着面电导率的增加,散射系数发生较大变化,但当面电导率达到一定值时,散射系数趋于恒定;对尺度较大的粒子,带电后的散射系数减小,但沿不同方向的散射能量会重新分配,一些方向散射增强,另一些方向散射减小;尺度较小的粒子带电后,散射系数增大,但能量沿不同方向的重新分配不明显.     

Scattering of electromagnetic waves by arrays of conducting and dielectric cylinders

A method is developed that enables the solution of scattering by a finite number of cylinders in a convenient and accurate form. It is based on an analytic formulation using the boundary value problem and an application of a point matching technique. The method is then applied to study the scattering properties of both conducting and dielectric cylinders. It is also used to determine the size and spacing of passive scatterers to improve the directivity of linear antennas. In the latter case, a combination of the present method and a space perturbation technique is utilized to optimize the antenna gain.     

The inverse electromagnetic scattering problem for a perfectly conducting cylinder

The problem of determining the shape of the cross section of a simply connected perfectly conducting infinite cylinder from a knowledge of the far-field pattern for all angles of observation and small values of the wavenumber is considered. The method proposed relies heavily on conformal mapping techniques. In particular it is shown that if the transfinite diameter is known each Fourier coefficient of the far-field pattern of the electric field determines a Laurent coefficient of the conformal mapping taking the exterior of the unit disk onto the exterior of the unknown cross section. The transfinite diameter is determined by changing the polarization of the incoming wave and measuring the far-field pattern of the resulting magnetic field. Of particular interest is the case when only a finite number of the Fourier coefficients of the far-field pattern are known. In this situation error estimates are obtained by using results on coefficient estimates for univalent functions.     

Depolarisation of electromagnetic waves backscattered by finitely conducting gently curved surfaces

The vector solution for the field backscattered by a finitely conducting surface with radii of curvature much larger than the incident wavelength obtained by the Stratton-Chu method is expressed in a form which gives insight into the dependence of depolarisation on angle of incidence, incident polarisation and on the electrical and geometrical properties of the backscattering surface.     

Numerical Simulation of scattering of electromagnetic waves from traveling and/or vibrating perfect conducting planes

This paper presents the one-dimensional computational results for electromagnetic waves scattered from traveling and/or vibrating perfect conducting planes via the application of the characteristic-based method. Relativistic boundary conditions combined with the characteristic variable boundary conditions are employed to account for relativistic effects due to the very high-speed motion of the conductor. The variations in both magnitude and frequency of the reflected electric field were investigated by comparing the computational results with the theoretical double-Doppler shift values. A maximum error percentage of less than 0.50% was found. It is also concluded that when the perfect conducting plane travels and vibrates simultaneously, the Doppler effects in frequency that are impressed on the reflected fields can be predicted, on the basis of computational results, simply dividing the vibrating frequency of perfect conducting plane by the factor (1+/spl beta//sub t/) where /spl beta//sub t/ is the ratio of the traveling velocity to the speed of light.     

Analysis of the electromagnetic scattering by perfectly conducting convex polygonal cylinders

An effective method for the analysis of the scattering by a perfectly conducting convex polygonal cross-section cylinder is presented. The effectiveness stems from the generalization of the Neumann series, factorising the right edge behavior of the electromagnetic field, thus leading to a quickly convergent method. The induced currents, the radar cross section (RCS) and the induced field ratio have been evaluated.     

NEW APPROACH TO ELECTROMAGNETIC SCATTERING FROM A CONDUCTIVE DISC-RING STRUCTURE

A one-dimensional modified integral equation for electromagnetic scattering from a metallic concentric disc-ring structure is derived by using Fourier expansion and separating the contribution of charge density from conventional EFIE. In the case of normal incidence, the derived equation is very simple and easy to be solved numerically regardless of the electric dimension of the structure. The comparison between the data of RCS of a disc by using this method and analytical method shows the effectiveness of this approach. Numerical results are also given for current distributions and the scattered fields for a disc-ring structure.     

Solution to the problem of scattering of electromagnetic waves by layered inhomogeneous bodies

The pattern-equation method is generalized for solution of the problem of scattering by layered inhomogeneous bodies. The algorithm of the method and calculated scattering patterns for bodies of various geometries are presented. The convergence rate of the numerical algorithm is demonstrated for bodies of various dimensions, and the method is compared to other numerical techniques of solution of the considered problem.     

Scattering of electromagnetic pulse waves by conducting wedge in uniaxially anisotropic plasma

Asymptotic expansion for the transient response of radiation fields due to electric dipole located above the conducting wedge with an arbitrary angle embedded in a simple plasma is derived by using the saddle point method of integration. The transient time, which is defined as the time required to reach steady state, is discussed for various kinds of wedge angles and plasma frequencies.     

Scattering of electromagnetic waves from dielectric coated conducting spheres

Several series of rigorous numerical calculations of the backscatter cross section of a conducting sphere with a thin lossless dielectric coating were carried out. The ratio of the radius to wavelength was varied from about 0.02 to 10.0; the dielectric constant of the coating was taken to be 2.56, 4.0, or 6.0; and the thickness of the coating was 0.1 or 0.05 times the outer radius of the coated sphere. Curves of the results are presented which indicate that the backscatter cross section of a coated sphere may be increased by as much as a factor of ten over that of an uncoated sphere of the same size, and, due to interference effects, an even greater decrease may be obtained. Further, small changes (less than one per cent) in the thickness or dielectric constant of the coating, or in the wavelength, may bring about large changes in the cross section. The numerical results are also compared with some experimental measurements, and with predictions of a "creeping-wave" type of analysis carried out by Helstrom.