Electromagnetic scattering from composite objects using a mixtureof exact and impedance-boundary conditions

Different surface integral equations for characterizing the electromagnetic scattering from a surface impedance object partially coated with dielectric materials are presented. The impedance boundary condition (IBC) is applied on the impedance surface and the exact boundary condition is applied on the dielectric surface. The resulting integral equations are solved for bodies of revolution using the method of moments. The numerical results are compared with the exact solution for a sphere. Other geometries are considered, and their results are verified by comparing results of the numerical solutions which were obtained using different formulations. The internal resonance problem is examined. It is found that the combined field integral equation (CFIE) can be used at any frequency and with any surface impedance     

Electromagnetic scattering by an inhomogeneous penetrable material with an embedded resistive sheet

This paper presents the method of moments (mom) formulation for the electromagnetic scattering by an inhomogeneous penetrable material with an embedded resistive sheet. Triangular surface facets and tetrahedral volume cells are used to discretize the scatterer allowing for greater flexibility in the geometric modeling of the material body. The formulation is very general in that it allows for a variety of material configurations : open or closed conducting surfaces, open or closed resistive (thin dielectric) surfaces, solid dielectric/ferrite material volumes, embedded conducting/resistive surfaces in material volumes, and partially embedded conducting/resistive surfaces (cladded materials). Results for a material coated resistive spherical shell and a material propeller blade are presented.     

On the numerical computation of scattering from inhomogeneous penetrable objects

A formulation of electromagnetic scattering problems involving a penetrable inhomogeneous scatterer is presented. The formulation combines the concepts of invariant imbedding and surface equivalent currents with the method of moments. It effectively circumvents the problem of intractable matrix size which has been a major deterrent in the past.     

Electromagnetic scattering from objects composed of multiplehomogeneous regions using a region-by-region solution

The paper presents an efficient procedure to calculate the electromagnetic field scattered by an inhomogeneous object consisting of N+1 linear isotropic homogeneous regions. The procedure is based on surface integral equation (SIE) formulations and the method of moments. The method of moments (MM) is used to reduce the integral equations for each homogeneous dielectric region into individual matrices. These matrices are each solved for the equivalent electric current in terms of the equivalent magnetic current. A simple algebraic procedure is used to combine these solutions and to solve for the magnetic current on the outer dielectric surfaces of the scatterer. With the magnetic current determined, the electric current on the outer surface of the scatterer is calculated. Because the matrix corresponding to each dielectric region is solved separately, the authors call this procedure the region-by-region method. The procedure is simple and efficient. It requires less computer storage and less execution time than the conventional MM approach, in which all the unknown currents are solved for simultaneously. To illustrate the use of the procedure, the bistatic and monostatic radar cross sections (RCS) of several objects are computed. The computed results are verified by comparison with results obtained numerically using the conventional numerical procedure as well as via the series solution for circular cylindrical structures. The possibility of nonunique solutions has also been investigated     

Electromagnetic scattering from two adjacent objects

In the paper, the problem of electromagnetic scattering from two adjacent particles is considered and an iterative solution that accounts for multiple scattering up to second-order is proposed. The first-order solution can easily be obtained by calculating the scattered field of isolated particles when illuminated by a plane-wave. To get the second-order solution, the scattered field from one of the particles, with nonuniform phase, amplitude, and polarization, is considered as the illuminating wave for the other particle and vice versa. In the work, the second-order scattered field is derived analytically using a novel technique based on the reciprocity theorem. In specific, the analytical solution for bistatic specular scattering from a cylinder and sphere pair is discussed and the results are compared with numerical computations based on the method of moments     

Field feedback computation of scattering by 2-D penetrable objects

The field feedback formulation is applied to the solution of time-harmonic plane wave scattering by 2-D penetrable objects of arbitrary shape and composition. A conformal mesh, finite element algorithm is employed in the forward operator construction while a near-field Green's function integration is used in forming the feedback operator. Scattering validations for midresonance sized objects include a circular cylinder, a two-region bisected cylinder, a half-circular cylinder, a semiconductor shell and a thin lossy planar strip     

Inaccuracies in numerical calculation of scattering near naturalfrequencies of penetrable objects

Calculated scattering from a nonconducting cylindrical ring shows great variability with small changes in the material or geometric parameters within certain ranges of these parameters. The observations are explained by a resonance phenomenon in which the operating frequency is found to lie close to a complex natural frequency of the scattering object. This resonance is a real, observable effect predicted by analytical solutions and not the spurious numerical resonances which have been widely discussed and which one wishes to suppress. Attempts have been made to reproduce near-resonance scattering results using the method of moments and finite-difference-time-domain (FDTD) codes. These have failed despite the use of widely accepted discretization densities. Thus, the existence of such resonances requires additional care when interpreting computed results for scattering from similar nonconducting objects have electric and/or magnetic properties     

基于MT-SIE法求解电大介质目标电磁散射

介绍了一种用于均匀介质目标电磁散射求解的新型多区域表面积分方程(MT-SIE)方法。不同于传统的用于介质目标散射求解的积分方法,该方法将均匀介质目标分解为内、外2个独立的子区,通过在介质表面强加Robin传输条件来保证电流和磁流的连续性。由于介质目标被分解为内外2个独立的子区,不同的子区允许非共形剖分。相较于传统方法,该方法可以更高效地与多层快速多级子(MLFMA)相结合求解电大尺寸目标。为进一步加速矩阵的迭代求解,提出了一种高斯-赛德尔型预条件技术,可以有效改善矩阵的收敛,加快迭代求解速度。     

TM scattering by a general sheet impedance discontinuity

An integral equation and spectral-domain method of moments (MM) solution to the two-dimensional problem of transverse magnetic (TM) scattering by a general sheet impedance discontinuity in a plane multilayered medium is presented. The MM solution uses a combination of pulse and traveling-wave physical basis functions to efficiently analyze discontinuities that extend to infinity. Numerical results that illustrate the use and accuracy of the method are presented     

Transient response computation of spheroidal objects using impedance boundary conditions

The transient response of imperfectly conducting and permeable spheroidal objects is determined using the impedance boundary conditions. Since the impedance boundary conditions are not known in the time domain, the frequency domain analysis is used to formulate the problem. The use of impedance boundary conditions relates conveniently the solution to the object's physical parameters and simplifies the computations. For highly conducting objects a simplified method is used to determine numerically the frequency domain data, which utilizes the numerical code for perfectly conducting objects. The excitation is assumed to be due to a periodic pulse train and generated by a small circular loop antenna. The procedure for the computation of the transient response, at very low excitation frequencies, is presented and the response forms for different object parameters and orientations are computed.     

Electromagnetic scattering from a dielectric coated cylinder using OSRC-GMT

The electromagnetic scattering from a dielectric coated cylinder is analysed based on the generalised multipole technique (GMT) combined with the on-surface radiation condition (OSRC) for separating an open region of inhomogeneous media into the inside region of the homogeneous medium and the outside region of free space. The equivalent multipole sources inside which produce the outside scattered fields, are determined by the OSRC on the outer surface. Some numerical examples for calculating the RCS from the scatterer with concentrically circular or confocally elliptic cross-section are provided.<>     

Electromagnetic scattering by buried objects of low contrast

The Born approximation is used to derive the plane-wave scattering matrix for objects of low dielectric contrast. For general shapes a numerical integration over the volume of the scatterer is required, but analytical expressions are derived for a sphere, a circular cylinder and a rectangular box (parallelepiped). The plane-wave scattering-matrix theory is used to account for the air-Earth interface. Numerical results are presented for the scattered field and far field for plane-wave excitation. The scattered field are weak for low-contrast objects, but the near-field results have application to electromagnetic detection of buried objects     

Analysis of low frequency scattering from penetrable scatterers

A method is presented for solving the surface integral equation using the method of moments (MoM) at very low frequencies, which finds applications in geoscience. The nature of the Helmholtz decomposition leads the authors to choose loop-tree basis functions to represent the surface current. Careful analysis of the frequency scaling property of each operator allows them to introduce a frequency normalization scheme to reduce the condition number of the MoM matrix. After frequency normalization, the MoM matrix can be solved using LU decomposition. The poor spectral properties of the matrix, however, makes it ill-suited for an iterative solver. A basis rearrangement is used to improve this property of the MoM matrix. The basis function rearrangement (BFR), which involves inverting the connection matrix, can be viewed as a pre-conditioner. The complexity of BFR is reduced to O(N), allowing this method to be combined with iterative solvers. Both rectilinear and curvilinear patches have been used in the simulations. The use of curvilinear patches reduces the number of unknowns significantly, thereby making the algorithm more efficient. This method is capable of solving Maxwell's equations from quasistatic to electrodynamic frequency range. This capability is of great importance in geophysical applications because the sizes of the simulated objects can range from a small fraction of a wavelength to several wavelengths     

Electromagnetic scattering by a mixture of conducting and dielectric objects: analysis using method of moments

In this paper, the method of moments is employed to solve the combined field integral equation for characterizing electromagnetic scattering by large three-dimensional structures of arbitrary shape. Unlike those discussed in the literature, these structures consist of mixed conducting and homogeneous dielectric objects. To improve the matrix conditioning number, the basis functions used to represent magnetic currents are also chosen as the popular Rao-Wilton-Glisson functions, but are multiplied by a constant number. A Galerkin's procedure is implemented, i.e., the testing functions are chosen to be the same as the basis functions.     

Investigation of scattering by corrugated structures using impedance boundary conditions

The usefulness of impedance boundary conditions for the investigation of field problems concerning corrugated surfaces is studied by its application to the scattering by a partially corrugated conducting prism. The corrugated portion of the prism is represented by an impedance surface and the behaviour of the surface currents and the scattered fields are examined. The results are also compared with available data on a similar problem.     

Analysis of scattering from 2D penetrable object using iterativeFEM with novel mesh truncation

An iterative finite element method is proposed to analyse scattering from a two dimensional penetrable object. Conventionally, to solve this kind of problem, the whole penetrable object is discretised, which is unduly costly if the object contains a large homogeneous region. By introducing a fictitious boundary inside in addition to outside the scatterer, the problem region can be greatly reduced. Also, the typical finite element method is applied with iteratively improved radiation-type boundary conditions. To validate the proposed method, a numerical example is presented     

Reduction of forward scattering from cylindrical objects using hardsurfaces

We discuss how forward scattering can he characterized in terms of an equivalent blockage width, and a relation between this and the bistatic scattering width is derived. Then, we show how cylinders such as struts and masts can be constructed to reduce their blockage widths. Thereby, when the cylinders are mounted in front of an antenna, the sidelobes and losses caused by the blockage will be reduced. For thin metal cylinders the blockage width reduction is obtained by giving its cross section an oblong shape and, in addition (for the TM case), by coating the outer metal surface with dielectric material to obtain a hard boundary condition. For thick cylinders, the reduced scattering is obtained by designing them as dielectric-filled parallel plate waveguides with the outer surfaces of the plates coated in the same way as for the thin struts. Dual-polarized performance is obtained in both cases by strip loading the outer surfaces. The performance of both the thin and the thick struts have limited frequency bandwidth. Both computed and measured results are presented; the computations being done with the moment method. The designs are based on the concept of soft and hard surfaces in electromagnetics, and the results can be regarded as a proof of the existence of hard surfaces for electromagnetic waves. The study considers reduction of forward scattering which also will give a reduction of the total integrated power of the scattered field over all directions-even backward     

Electromagnetic scattering from a corrugated wedge

The scattering of a TE-polarized wave from a perfectly conducting wedge with one of its faces loaded with cavities is considered. The proposed formulation reduces the analysis of the original problem to two simpler configurations: one concerning the geometry of the corrugation (internal problem) and one concerning the geometry of the perfectly conducting wedge (external problem). By resorting to a suitable numerical technique, it is possible to account for arbitrarily shaped corrugations filled with inhomogeneous material. Numerical results demonstrate the validity of the technique and its applicability to general shape corrugations. The analysis of corrugated horns is considered and results are shown that demonstrate the flexibility of the procedure to analyze horns with arbitrarily shaped corrugations     

Controlled scattering from PEC surface using PSS boundary

The use of a phase-switched-screen boundary to actively control the electromagnetic scattering properties of an uncovered planar PEC surface is described. The structure incorporates a single active layer but is capable of tuning a reflectivity null of better than -20 dB over a significant bandwidth. Theoretical predictions are compared to measured data derived from an experimental system     

A hybrid wavelet expansion and boundary element analysis ofelectromagnetic scattering from conducting objects

A new wavelet approach is presented for treating the two-dimensional electromagnetic scattering problems over curved computation domains. The wavelet expansion method, in combination with the boundary element method (BEM), is applied to solve the integral equation for the unknown surface current. The unknown surface current is expanded in terms of a basis derived from a periodic, orthogonal wavelet in interval [0, 1]. The geometrical representation of the BEM is employed to establish the map between the curved computation domain and the interval [0, 1]. This technique exhibits the advantages of both the wavelet expansion method and the boundary element method: sparse matrix system and accurately modeling of curved surfaces. Comparisons of the results from the new technique with the moment method solutions, the analytical solutions or the data in previous publications are provided. Good agreements are observed