有耗平面和三维目标复合散射的FDTD分析

采用FDTD方法计算了有耗地面与三维目标的复合散射,对吸收边界条件,连接边界条件和近远场变换作了细致讨论.吸收边界使用了广义PML吸收层,它对电磁波有较好的吸收效果.连接边界处则利用解析入射波和三波迭加技术,上半平面用入射波和反射波、下半平面用透射波作为对目标的外加场进行计算.得到近场数据后,为避免出现复杂的Sommerfeld积分,用互易原理简化了外推过程.FDTD算法与矩量法和快速多层多极子相比,具有节省内存,计算时间短等优点.通过地上物体和地下物体的计算验证了FDTD方法的精确性,讨论了散射体离地高度对后向RCS的影响.     

Plane wave scattering from an infinite planar dipole array buried in a lossy half-space

The plane wave scattering from an infinite planar impedance-loaded dipole array buried in a lossy half-space is investigated theoretically and experimentally. Formulations for the scattering field and the input impedance of the dipole are derived by using Floquet's theorem and the induced electromotive force (EMF) method. In order to demonstrate the validity of these formulations, the numerical examples for a concrete as the lossy medium at frequency 9.32 (GHz) are compared with the experimental data. The results show that this array, even when loaded by zero impedance, can greatly suppress the radio wave reflection from the concrete surface by choosing the array dimensions properly and the depth of the array surface.     

Wire antennas over a lossy half-space

A recently developed technique for approximate but accurate evaluation of the various vector potential components associated with a current element radiating over a lossy ground is used to study the problem of antennas radiating over a lossy ground. A general integral equation for an arbitrarily shaped thin-wire antenna over a lossy half-space is derived, and the method of moments is employed to process this equation numerically. Illustrative numerical results are presented to demonstrate the effect of the lossy half-space on a number of antenna configurations.     

Impulse response of a lossy magnetoplasma half-space

An exact solution for the transient reflected wave from a lossy magnetoplasma half-space is derived for the case of a vertically incident impulsive plane wave. The solution is expressed as an infnite series of modified Bessel functions of the first kind with complex argument. For small values of the collision and magnetic parameters, the exact solution is approximated in terms of a single integer-order Bessel function.     

Scattering from a metallic object embedded near the randomly roughsurface of a lossy dielectric

Two-dimensional electromagnetic scattering from a perfectly conducting target embedded near the randomly rough surface of an isotropic lossy dielectric is investigated. The randomly rough surface is illuminated by a finite width beam from an antenna in the free space above the surface, with off-normal incidence. Standard integral equation methods are applied and include all subsurface interactions between the object and rough surface. For a chosen embedded target, Monte Carlo simulations are performed for a selection of ensembles of rough surface types intended to be suggestive of natural ground. Far field scattering coefficient distributions and corresponding synthetic images suggest when the buried object should be discernible. Sensitivities are explored in terms of surface type, polarization of the incident field, depth and orientation of target, soil characteristics, incidence angle, and beamwidth. Many of the scattering features identified should also apply in 3D     

Wide-band electromagnetic scattering from a dielectric BOR buriedin a layered lossy dispersive medium

A method of moments (MoM) analysis is developed for electromagnetic scattering from a dielectric body of revolution (BOR) embedded in a layered medium (the half-space problem constituting a special case). The layered-medium parameters can be lossy and dispersive, of interest for simulating soil. To make such an analysis tractable for the wide-band (short-pulse) applications of interest here, we have employed the method of complex images to evaluate the Sommerfeld integrals characteristic of the dyadic layered-medium Green's function. Example wide-band scattering results are presented, wherein fundamental wave phenomenology is elucidated. Of particular interest, we consider wide-band scattering from a model plastic mine, buried in soil, with the soil covered by a layer of snow     

Time-domain modeling of electromagnetic field coupling to finite-length wires embedded in a dielectric half-space

The transient behavior of a single straight line embedded in a dielectric half-space and illuminated by the electromagnetic pulse (EMP) is analyzed directly in the time domain using the wire antenna approach. The formulation is based on the corresponding space-time Hallen integral equation. The effects of a two-media configuration are taken into account via the Fresnel reflection and transmission coefficient, respectively. The space-time Hallen integral equation is handled by the time-domain version of the indirect Galerkin-Bubnov boundary element method. The transient response obtained using the direct time-domain approach is compared with the results obtained via an indirect frequency-domain analysis method. Some illustrative numerical results are presented in the paper. Numerical results obtained via the different approaches agree satisfactorily, i.e., the maximum deviation between the results is around 6%.     

MMW scattering by rough lossy dielectric cylinders and tree trunks

The millimeter-wave (MMW) radar backscatter response of a rough, lossy, dielectric cylinder is examined both analytically and experimentally. Models for both the coherent and incoherent components of the scattered field are developed based on the geometric-optics approximation. The accuracy of the analytical models and their applicability in predicting the backscatter response of tree trunks are determined experimentally by measuring the radar backscatter at 95 GHz from a rough conducting cylinder and from a section of a tree trunk. In both cases, very good agreement was achieved between the model predictions and the measurements     

High-frequency scattering by a conducting circular cylinder coatedwith a lossy dielectric of nonuniform thickness

A closed-form expression for the field produced by a plane wave incident on an infinitely long conducting cylinder, coated with a lossy dielectric of nonuniform thickness, is obtained using perturbation theory. This approximate series solution is later evaluated asymptotically for electrically large cylinder sizes. The scattered fields are interpreted using geometric optics and creeping waves. The fields are calculated using the exact series, the approximate perturbation series, and the high-frequency asymptotic solutions, and compared for different angles of incidence     

Electromagnetic scattering by arbitrary shaped three-dimensional homogeneous lossy dielectric objects

The recent development and extension of the method of moments technique for analyzing electromagnetic scattering by arbitrary shaped three-dimensional homogeneous lossy dielectric objects is presented based on the combined field integral equations. The surfaces of the homogeneous three-dimensional arbitrary geometrical shapes are modeled using surface triangular patches, similar to the case of arbitrary shaped conducting objects. Further, the development and extensions required to treat efficiently three-dimensional lossy dielectric objects are reported. Numerical results and their comparisons are also presented for two canonical dielectric scatterers-a sphere and a finite circular cylinder.     

Moment-method modeling of short-pulse scattering from and theresonances of a wire buried inside a lossy, dispersive half-space

A frequency-domain method-of-moments (MoM) algorithm is used to model short-pulse plane-wave scattering from a wire buried inside a lossy, dispersive half-space with the time-domain scattered fields computed via the Fourier transform. Further, the complex resonant frequencies of such targets are also calculated via the MoM. The phenomenology associated with scattering from a buried wire is investigated in detail, and it is demonstrated that the time-domain scattered fields and the target resonances depend on the buried-wire orientation as well as the electrical properties of the half-space     

基于FGMRES-PBTG算法的介质粗糙面散射特性的模拟

针对具有大介电常数介质粗糙面的电磁散射问题,提出了灵活的广义最小余量法(FGMRES)和基于物理意义的双网格法(PBTG)的混合算法,并对介质粗糙面的电磁散射系数和发射率进行了仿真.在PBTG算法的基础上,应用了FGMRES迭代算法求解矩阵方程.通过PBTG算法加速矩阵向量积和FGMRES算法加快迭代收敛速度,达到减少计算量和加快计算速度的目的.通过计算实例可知,FGMRES相对其他迭代法具有很好的收敛性,耗时较少,在满足精度要求的前提下,较其他迭代法明显加快了计算速度,并有效地模拟了介质粗糙面的电磁散射特性.     

THE INVERSE ELECTROMAGNETIC SCATTERING OF INHOMOGENEOUS DIELECTRIC THIN TUBES

By using the method of Fourier series expansion, the inverse electromagnetic scattering problemof a 2- dimensional lossy dielectric body, of which the cross section is a narrow annulus, is studied. Theanalytic expressions of the electric parameters of a dielectric body are given in terms of the outsidesca,ttering field, and the simulative results are satisfactory.     

Scattering by a lossy dielectric cylinder in a rectangularwaveguide

Electromagnetic fields in a rectangular waveguide containing a lossy dielectric cylinder are investigated by means of the orthogonal expansion method. The calculated results are provided by measurement. Resonance effects become visible through the frequency responses of the scattering parameters and understandable by patterns due to magnetic fields and Poynting vectors. The lowest resonance is nonsymmetric and can be used to realize tunable bandstop filters with a relative 3-dB bandwidth of about 0.04 and an attenuation of more than 40 dB     

On a linearized inverse scattering model for a three-dimensionalflaw embedded in anisotropic advanced composite materials

A three-dimensional linearized model to reconstruct the flaw in advanced anisotropic composite materials is developed. The inversion scheme takes the form of an integral equation, which is discretized by means of the method of moments. The measured data are then inverted by means of the conjugate gradient (CG) fast Fourier transform algorithm, which incorporates Levenberg-Marquardt regularization and adaptive preconditioning schemes. The reconstruction of the flaw based on simulated as well as measured data, along with some of the salient features of the newly developed CG algorithm are presented     

An improved MFIE formulation for TE-wave scattering from lossy,inhomogeneous dielectric cylinders

A method-of-moments formulation involving the magnetic-field integral equation (MFIE) is presented for the analysis of nonhomogeneous electric (TE) source. The approach involves the use of triangular subdomain cells, a piecewise linear expansion for the total magnetic field, and point matching at the nodal points of the triangular-cell model. Results for internal fields and radar cross section are presented and shown to be stable and convergent, even for dielectrics characterized by a large-magnitude complex relative permittivity. It is noted that the complementary problem of TM-wave scattering from ferrite cylinders characterized by a permeability function can be treated using the dual to the approach presented     

A new procedure of point-matching method for calculating theabsorption and scattering of lossy dielectric objects

A method of numerically calculating the electromagnetic scattering and absorption by dielectric objects of high aspect ratios and composite geometrics has been developed. The solution procedure is based on expanding the scattered and internal fields in terms of multiple spherical vector wave functions, using point-matching to satisfy the boundary conditions and the least-square method to solve the resulting system of equations. The unique feature of this technique is that it utilizes multiple spherical expansions to describe the fields both inside and outside the object. The various parameters used to examine the convergence of the solution are discussed; they include the number of subdomain expansions interior and exterior to the object, the number of terms in each expansion and the advantages of the least-square method of solution. The new method was found suitable for making calculations for objects with aspect ratios as high as nine, and even for objects with composite geometries, including a capped cylinder and an object that consists of a spherical and a prolate spheroidal section. Numerical results were compared with results based on volume integral equations and method of moments, and excellent agreement was found     

Direct solution for dielectric properties of a lossy two-layeredmaterial

A technique for calculating the dielectric constants of both layers, the loss tangent, and the upper-layer thickness of a lossy two-layered stratified dielectric material is presented. The method requires measurement of the complex voltage reflection coefficient over the frequency range between a maximum and adjacent minimum of the cyclic frequency response. A noniterative solution for dielectric properties is given in terms of a function of the reflection coefficient at the air-media interface. The application of this procedure to measure the moisture content of stratified soils is discussed     

Analysis of a microstrip crossover embedded in a multilayeredanisotropic and lossy media

The equivalent circuit and the scattering parameters of the orthogonal microstrip crossover discontinuity are determined by assuming that the conducting strips are embedded in a multilayered substrate which may contain both anisotropic dielectrics and materials with a nonnegligible conductivity. The equivalent circuit of the crossover is obtained in terms of the complex excess charge densities on the strips. These excess charge densities are computed by means of the Galerkin method in the spectral domain. Comparison is carried out with previously existing results for microstrip crossovers on lossless isotropic substrates and original results are presented for crossovers on anisotropic and lossy substrates