A boundary-element solution of the Leontovitch problem

A boundary-element method is introduced for solving electromagnetic scattering problems in the frequency domain relative to an impedance boundary condition (IBC) on an obstacle of arbitrary shape. The formulation is based on the field approach; namely, it is obtained by enforcing the total electromagnetic field, expressed by means of the incident field and the equivalent electric and magnetic currents and charges on the scatterer surface, to satisfy the boundary condition. As a result, this formulation is well-posed at any frequency for an absorbing scatterer. Both of the equivalent currents are discretized by a boundary-element method over a triangular mesh of the surface scatterer. The magnetic currents are then eliminated at the element level during the assembly process. The final linear system to be solved keeps all of the desirable properties provided by the application of this method to the usual perfectly conducting scatterer; that is, its unknowns are the fluxes of the electric currents across the edges of the mesh and its coefficient matrix is symmetric     

Shape Reconstruction of a Perfectly Conducting Scatterer Using Differential Evolution and Particle Swarm Optimization

The shape reconstruction of a perfectly conducting 2-D scatterer by inverting transverse magnetic scattered field measurements is investigated. The reconstruction is based on evolutionary algorithms that minimize the discrepancy between measured and estimated scattered field data. A closed cubic B-spline expansion is adopted to represent the scatterer contour. Two algorithms have been examined the differential-evolution (DE) algorithm and the particle swarm optimization (PSO). Numerical results indicate that the DE algorithm outperforms the PSO in terms of reconstruction accuracy and convergence speed. Both techniques have been tested in the case of simulated measurements contaminated by additive white Gaussian noise.     

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 anisotropic materials, part II: Computer code and numerical results in two dimensions

The two-dimensional problem of oblique scattering by penetrable cylinders of arbitrary cross section made of materials which are linear, lossy, anisotropic and possibly inhomogeneous is considered. The materials are characterized by arbitrary tensor susceptibilitiesbar{x}_{ec}andbar{x}_{m}. The frequency-domain volume integrodifferential equations satisfied by the electric and magnetic fields and obtained in a previous paper (Part 1) are analyzed numerically. Optimal ordering of the unknowns and transverse electric-transverse magnetic (TE-TM) decomposition in the matrix formulation of the problem are discussed. The cross section of the scatterer is broken down into a triangular mesh. The field components at the vertices of the triangles are the unknowns; within each triangle, each field component is a linear combination of its values at the vertices. Computed field distributions inside the scatterer are found to be in excellent agreement with results obtained by other methods.     

距离扩展目标的双门限恒虚警检测器及性能分析

针对普通积累检测器检测稀疏散射点目标存在坍塌损失以及依赖于散射点密度的二元积累(SDD-BI)检测器检测密集散射点目标存在信杂比损失的问题,该文充分利用目标的散射点密度及幅度信息,提出了一种距离扩展目标的双门限恒虚警检测器(DT-CFAR),在高斯杂波背景下推导了DT-CFAR虚警概率和检测概率与检测门限关系的解析表达式,并给出了最优第1门限的确定方法。仿真结果表明,DT-CFAR对不同散射点密度目标都具有较优的检测性能,并且对散射点密度估计失配具有鲁棒性。     

Exterior moment method analysis of conducting scatterers by using the interior Green's function

A new method using a Green's function in the interior region of a conducting scatterer is proposed to obtain a mutual admittance matrix in an exterior moment method analysis. A numerical example of a two-dimensional magnetic strip source located on an exterior surface of a perfectly conducting rectangular cylinder shows the validity of the method.<>     

FFT-based algorithm for scattering from a circular, cylindrical shell in the presence of an inhomogeneous cylinder

An FFT-based algorithm is proposed to solve the problem of scattering from a homogeneous, thin, circular, dielectric cylindrical shell in the presence of an inhomogenous, cylindrical scatterer. The incident wave is taken to be transverse magnetic (TM); the problem considered is one of two-dimensional, scalar type. Results of computer simulations are provided illustrating the method proposed.<>     

软组织超声散射子频率特性的研究

由于生物软组织对超声波的衰减、吸收等作用,组织超声背散射射频(RF)信号的中心频率会发生偏移,利用超声背散射RF信号中心频率的偏移量来估计软组织超声特征散射子的粒径等特征成为可能.该文在软组织超声背散射RF信号功率谱理论分析的基础上,制作了4种不同散射子粒径的实验仿体,并对实验仿体进行了信号采集和数据处理,研究其频率特性.研究结果表明,生物软组织超声背散射RF信号的中心频率随超声特征散射子粒径的增大而向低频方向移动,且越来越接近发射超声信号的中心频率.     

Boundary element method for electromagnetic scattering from cylinders

The computation of low frequency scattering of electromagnetic fields by solid/hollow dielectric or conducting cylinders using the boundary element method (BEM) is considered. A general computer program has been developed for both transverse electric and magnetic cases. Numerical examples are given for conducting circular cylinders, and solid and hollow dielectric cylinders. The computational accuracy is checked by comparing the results with the analytic solution or computing an error defined from the optical theorem. In addition some problems at an interior resonance of the scatterer are discussed. The method can be directly applied to more complicated geometries.     

Robotic aided dense medium target fabrication

A method for fabricating dense medium targets with known distributions and scatterer locations has been developed. The technique involves the use of a robot for accurate scatterer positioning. Performing measurements on these targets will give a better understanding of dense medium scattering mechanisms     

Study of backscattering from a complex scatterer at 3mm wave band

The backscattering characteristics of complex scatterer has been studied theoretically at 3 mm wavelength for the first time. RCS of the practial complex scatterer was calculated with GTD and the physical optics. The constrict model was also measured. The experimental results are in agreement with the theoretical calculations.     

A new formulation of electromagnetic wave scattering using an on-surface radiation boundary condition approach

A new formulation of electromagnetic wave scattering by convex, two-dimensional conducting bodies is reported. This formulation, called the on-surface radiation condition (OSRC) approach, is based upon an expansion of the radiation condition applied directly on the surface of a scatterer. Past approaches involved applying a radiation condition at some distance from the scatterer in order to achieve a nearly reflection-free truncation of a finite-difference time-domain lattice. However, it is now shown that application of a suitable radiation condition directly on the surface of a convex conducting scatterer can lead to substantial simplification of the frequency-domain integral equation for the scattered field, which is reduced to just a line integral. For the transverse magnetic (TM) case, the integrand is known explicitly. For the transverse electric (TE) case, the integrand can be easily constructed by solving an ordinary differential equation around the scatterer surface contour. Examples are provided which show that OSRC yields computed near and far fields which approach the exact results for canonical shapes such as the circular cylinder, square cylinder, and strip. Electrical sizes for the examples areka = 5andka = 10. The new OSRC formulation of scattering may present a useful alternative to present integral equation and uniform high-frequency approaches for convex cylinders larger thanka = 1. Structures with edges or corners can also be analyzed, although more work is needed to incorporate the physics of singular currents at these discontinuities. Convex dielectric structures can also be treated using OSRC. These will be the subject of a forthcoming paper.     

Matrix interpretation of the spectral iteration technique[electromagnetic scattering]

A matrix interpretation of the spectral iteration technique is presented to illustrate improvements in accuracy and convergence for both transverse magnetic and transverse electric waves incident on two-dimensional homogeneous scatterers producing solutions identical to a method of moments scheme. In this scheme, it is possible to improve the convergence rate of the technique by the use of nonphysical Green's function terms in the extended matrix. These terms result in the generation of a nonphysical field outside the scatterer, while still maintaining the correct solution of the current. Although the problem of nonconvergence has not been entirely overcome, a particular taper method used in the examples provided shows an improvement over their nontapered counterparts     

Backscatter RCS for TE and TM excitations of dielectric-filledcavity-backed apertures in two-dimensional bodies

Transverse electric (TE) and transverse magnetic (TM) scattering from dielectric-filled, cavity-backed apertures in two-dimensional bodies are treated using the method of moments technique to solve a set of combined-field integral equations for the equivalent induced electric and magnetic currents on the exterior of the scattering body and on the associated aperture. Results are presented for the backscatter radar cross section (RCS) versus the electrical size of the scatterer for two different dielectric-filled cavity-backed geometries. The first geometry is a circular cylinder of infinite length which has an infinite length slot aperture along one side. The cavity inside the cylinder is dielectric filled and is also of circular cross section. The two cylinders (external and internal) are of different radii and their respective longitudinal axes are parallel but not collocated. The second is a square cylinder of infinite length which has an infinite length slot aperture along one side. The cavity inside the square cylinder is dielectric-filled and is also of square cross section     

Radiation and scattering from electrically small conducting bodies of arbitrary shape

A simple moment solution is given for low frequency electromagnetic scattering and radiation problems. The problem is reduced to the corresponding electrostatic and magnetostatic problems. Each static problem is solved using the Method of Moments. The surface of the perfectly conducting scatterer is modeled by a set of planar triangular patches. Pulse expansion functions and point matching testing are used to compute the charge density in the electrostatic problem. For the magnetostatic current a set of charge-free vector expansion functions is used. The problem is formulated assuming the scatterer to be in an unbounded homogeneous region. Scatterers of various shapes, such as the circular disc, the sphere, and the cube are studied. Special attention is paid to a conducting box with a narrow slot. The computed results are the scattered fields, the induced charge and current distributions, and the induced electric and magnetic dipole moments. These are in close agreement with whatever published data are available.     

Dipole configurations with strongly improved radiation efficiencyfor hand-held transceivers

In this paper the design criteria for antenna structures with improved radiation efficiency while operating in close vicinity to a biological scatterer are investigated. The study is performed using a simple λ/2 dipole combined with a directive or reflective element. The optimization criteria are the effective radiation efficiency, the spatial peak specific absorption rate (SAR), and the sensitivity of the input resistance to the distance from the scatterer. It is demonstrated that the primary design criterion to improve the radiation efficiency is not directivity but the reduction of the maximum incident magnetic field strength in the exposed skin area of the user's head. For the reflectively coupled dipole, all performance parameters are improved by several decibels compared to a standard, λ/2 dipole, whereas for some other directive structures, the performance is impaired. The study is conducted with the generalized multipole technique (GMT) numerical simulation method, the results of which are validated by measurement methods     

Hybrid solutions at internal resonances

The use of hybrid solutions for integral equation (IE) formulations in electromagnetics is illustrated at frequencies where a perfectly conducting scatterer exhibits internal resonances. Hybrid solutions, incorporating the Fock theory and physical optics Ansatzes, and the Galerkin representation, are compared with the method of moments (MM) solutions of the electric, magnetic, and combined field formulations at such frequencies. Numerical results are presented for spheres and a right circular cylinder.     

On combined source solutions for bodies with impedance boundary conditions

Numerical solutions to the impedance boundary condition (IBC) combined source integral equation (CSIE) for scattering from impedance spheres are presented. The CSIE formulation is a well-posed alternative to the IBC electric and magnetic field integral equations which can be contaminated by spurious resonant modes. Compared with the IBC combined field integral equation (CFIE), CSIE solutions have the same accuracy when the combined source coupling admittance is chosen to be the same value as the combined field coupling admittance. However, the CSIE formulation is better suited than the CFIE for creating a general purpose computer code capable of handling aperture radiation problems and/or a scatterer which has a spatially varying surface impedance.     

Conservative Polarimetric Scatterers and Their Role in Incorrect Extensions of the Cameron Decomposition

The properties of conservative symmetric polarimetric scatterer scattering matrices are examined. It is shown that the unambiguous rotation angle for "conservative" symmetric polarimetric scatterers is the interval (-pi/4,+pi/4] as compared with (-pi/2,+pi/2] for nonconservative symmetric scatterers. Errors relating to the Cameron decomposition, which are recently introduced into the literature, are discussed. It is demonstrated that these errors arise from a faulty symmetric scatterer scattering matrix distance measure. This, in turn, leads to an improper mapping of the symmetric scatterer unit disk to a hemisphere. The correct mapping of the symmetric scatterer unit disk to the symmetric scatterer unit sphere is described