A moment method solution for TMz and TEzwaves illuminating two-dimensional objects above a lossy half space

A moment method (MM) solution for analyzing the electromagnetic shielding and scattering properties of two-dimensional (2-D) objects over a lossy half space is presented. The materials of the objects can be metal, dielectric, or magnetic. Also, the lossy half space is included to simulate the effects of the earth ground or any flat homogeneous lossy surface. An MM based on a volume formulation and a special Green's function in the spectral domain is developed. Both TM _z and TE_z waves incident upon 2-D metal or lossy material structures are demonstrated for the shielding effects of those bodies in the presence of the lossy ground. Besides, the echo widths of a composite object either in free space or above the lossy half space are determined by using the MM. Some of the results are compared with those by other methods, and good agreements are obtained. The MM solution can be used to study the shielding and scattering problems for cylindrical structures located over a lossy ground     

FDTDSIBC混合方法计算近地导线表面电流

通过在FDTD方法中引进时域表面阻抗界条件来研究瞬态电磁脉冲对有耗地面附近电缆线耦合问题。由频域表面阻抗出发通过拉氏变换得到时域表面阻抗边界条件（SIBC）,并将这种边界条件应用于FDTD方法中来模拟有耗地面的反射。计算结果表明了些方法的有效性。     

Diffraction tomographic algorithm for the detection ofthree-dimensional objects buried in a lossy half-space

A diffraction tomographic (DT) algorithm has been proposed for detecting three-dimensional (3-D) dielectric objects buried in a lossy ground, using electric dipoles or magnetic dipoles as transmitter and receiver, where the air-earth interface has been taken into account and the background is lossy. To derive closed-form reconstruction formulas, an approximate generalized Fourier transform is introduced. Using this algorithm, the locations, shapes, and dielectric properties of buried objects can be well reconstructed under the low-contrast condition, and the objects can be well detected even when the contrast is high. Due to the use of fast Fourier transforms to implement the problem, the proposed algorithm is fast and quite tolerant to the error of measurement data, making it possible to solve realistic problems. Reconstruction examples are given to show the validity of the algorithm     

用统一的时域近场—远场变换求解目标瞬态散射特性

利用拉氏变换和卷积的方法,导出了求解3-D和2-D时域散射远场的形式上统一的公式,使得2-D情况成为3-D的一个特例,这给编制求解时域散射远场的通用程序带来方便.同时这种3-D的近场-远场变换算法和原来的算法比较并没有增加计算量.对简单形体的计算验证了算法的可行性.     

FD-TD法分析无载频脉冲探地雷达特性

本文首先利用三维ＦＤＴＤ法对集中电阻加载圆柱形偶极天线的近地面辐射特性进行了计算,给出了天线上电流波形并分析了加载电阻和有耗媒质参数等因素对电流波形的影响;分析并计算了天线“方向图”及其随天线高度和媒质参数变化的规律。其次,利用色散媒质中２．５维ＦＤＴＤ法迭代公式,模拟计算了地下目标雷达回波电平图,并与实际探测结果进行了对比,二者具有较好的一致性;分析了色散媒质参数对雷达探测深度的影响。     

A General Three-Dimensional Tensor FDTD-Formulation for Electrically Inhomogeneous Lossy Media Using the Z-Transform

A general three-dimensional tensor finite-difference time-domain (TFDTD) formulation is derived to model electrically inhomogeneous lossy media of arbitrary shapes. The time domain representation of electric losses is achieved using Z-transforms. The regular cubical grid structure is maintained everywhere in the calculation domain by defining a 3-D face-fraction based 3 x 3 permittivity tensor on the interfaces that describes the relationship between the (known) average flux density vector and the (unknown) local electric field vector. For electrically lossy media, this tensor is complex in the frequency domain. However, it can be modified for use with the Z-transform. Only this modified real form is inverted, then transformed from the frequency into the Z-domain, and finally into the time domain. Furthermore, a local interface matrix is used to describe the relationship between the local electric field in the grid node and its counterpart on the other side of the interface. This matrix is complex in the frequency domain for lossy media. By applying the Z-transform, this matrix can also be transformed into the time domain using only real modified matrix elements. The accuracy of the method is confirmed by comparisons with analytical solutions.     

Coupled Field Inside Shielding Enclosure With an Aperture Due to Nearby Lightning

Due to a nearby lightning return stroke, the coupled electromagnetic (EM) fields inside a rectangular shielding enclosure, on the finitely conducting ground, with an aperture in one wall are calculated numerically by using the finite-difference time-domain (FDTD) method. First, the near fields generated by the return stroke are obtained in two-dimensional (2-D) cylindrical coordinates by the FDTD method. Then, the coupled fields inside a rectangular shielding enclosure are calculated in three-dimensional (3-D) rectangular coordinates through the total field-scattered field connecting boundary, with the sources obtained by coordinates transformations of the return stroke near fields     

Multidomain pseudospectral time-domain simulations of scattering by objects buried in lossy media

A multidomain pseudospectral time-domain (PSTD) method with a newly developed well-posed PML is introduced as an accurate and flexible tool for the modeling of electromagnetic scattering by 2-D objects buried in an inhomogeneous lossy medium. Compared with the previous single-domain Fourier PSTD method, this approach allows for an accurate treatment of curved geometries with subdomains, curvilinear mapping, and high-order Chebyshev polynomials. The effectiveness of the algorithm is confirmed by an excellent agreement between the numerical results and analytical solutions for perfectly conducting as well as permeable dielectric cylinders. The algorithm has been applied to model various ground-penetrating radar (GPR) applications involving curved objects in a lossy half space with an undulating surface. This multidomain PSTD algorithm is potentially a very useful tool for simulating antennas near complex objects and inhomogeneous media.     

Numerical modeling of an enhanced very early time electromagnetic(VETEM) prototype system

In this paper, two numerical models are presented to simulate an enhanced very early time electromagnetic (VETEM) prototype system, which is used for buried-object detection and environmental problems. Usually, the VETEM system contains a transmitting loop antenna and a receiving loop antenna, which run on lossy ground to detect buried objects. In the first numerical model, the loop antennas are accurately analyzed using the method of moments (MoM) for wire antennas above or buried in lossy ground. Then, the conjugate gradient (CG) methods, with the use of the fast Fourier transform (FFT) or MoM, are applied to investigate the scattering from buried objects. Reflected and scattered magnetic fields are evaluated at the receiving loop to calculate the output electric current. However, the working frequency for the VETEM system is usually low and, hence, two magnetic dipoles are used to replace the transmitter and receiver in the second numerical model. Comparing these two models, the second one is simple, but only valid for low frequency or small loops, while the first modeling is more general. In this paper, all computations are performed in the frequency domain, and the FFT is used to obtain the time-domain responses. Numerical examples show that simulation results from these two models fit very well when the frequency ranges from 10 kHz to 10 MHz, and both results are close to the measured data     

A galerkin moment method for the analysis of insulated wires above a lossy half-space

In this paper, an efficient method is proposed for the analysis of insulated wires above a lossy half-space. The reaction concept is used to formulate the electromagnetic boundary value problem. The solution is a Galerkin moment method solution with piecewise sinusoids as basis and testing functions. The influence of lossy half-space is taken into account via equivalent images, and the currents of images are related to the source current. The dielectric coating is modeled by equivalent volume polarization currents, which are simply related to the conduction current distribution. In this way, the dielectric-coating and lossy half-space cause no new unknowns to the problem solution, the size of the impedance matrix for insulated wires above lossy ground is the same as that of bare wires in free space. The insulation is accounted for entirely through a modification of the symmetric impedance matrix. The results obtained exhibit excellent agreement with those by rigorous method or experiments.     

FDTD分析探地雷达天线的辐射特性

探地雷达系统一般采用超宽带短脉冲信号,因而其天线系统必须具有较好的宽带性能。只有几种类型的宽带天线能够用于探地雷达系统中,如电阻加载的蝶形天线、TEM喇叭天线及其变形形式。本文将给出一种新型的探地雷达天线,该天线为置于镜像面上且具有离散指数电阻加载的单偶极子。文中将采用FDTD计算和分析该天线在自由空间和有耗媒质上方时的辐射特性。结果表明,通过选择一定的电阻加载形式,可使天线具有较好的辐射波形,从而能够满足实际探地雷达的需要。最后,通过地下目标散射场的理论结果和实验结果说明了本文所采用方法的正确性。     

Joint source and channel coding for 3-D scene databases using vector quantization and embedded parity objects.

Three-dimensional graphic scenes contain various mesh objects in one geometric space where different objects have potentially unequal importance regarding display. This paper proposes an object-oriented system for efficiently coding and streaming 3-D scene databases in lossy and rate-constrained environments. Vector quantization (VQ) is exploited to code 3-D scene databases into multiresolution hierarchies. For the best distortion-rate performance, adaptive quantization precisions are allocated to different objects and different layers of each object based on a weighted distortion model. Upon transmission, scalably coded objects are delivered in respective packet sequences to preserve their manipulation independency. For packet loss resilience, a plurality of FEC codes are generated as "parity objects" parallel to graphic objects, which protect the graphic objects concurrently and also preferentially in regard to their unequal decoding importance. A rate-distortion optimization framework is then developed, which performs rate allocation between graphic objects and parity objects and generates the parity data properly. We show that, by treating graphic objects jointly and preferentially in source and channel coding while preserving their independencies in transport, the proposed system reduces the receiving distortion of the 3-D database significantly compared to conventional methods.     

A Three-Dimensional Semi-Implicit FDTD Scheme for Calculation of Shielding Effectiveness of Enclosure With Thin Slots

The hybrid implicit-explicit finite-difference time-domain (HIE-FDTD) scheme for a 2-D transverse electric (TE) wave is extended to a full 3-D electromagnetic wave in this paper. With the weakly conditional stability, this approach simulates shielding effectiveness of an enclosure with high computation efficiency. Numerical formulations of the 3-D HIE-FDTD scheme are presented, and simulation results are compared to those obtained by using the conventional 3-D FDTD and alternating-direction implicit (ADI) FDTD methods. The accuracy and efficiency of the 3-D HIE-FDTD for prediction of shielding effectiveness are validated by numerical simulation results.     

Electromagnetic scattering interference between two shallow objects buried under 2-D random rough surfaces

A rigorous electromagnetic model has been used to analyze the scattering from two dielectric shallow objects buried under the two-dimensional (2-D) random rough ground (3-D scattering problem) as a means of predicting false alarms. The method of moments (MoM) accelerated by the steepest descent fast multipole method (SDFMM) is used to compute the equivalent electric and magnetic surface currents on all scatterers (i.e., the rough ground and the two buried objects). The roughness parameters influence the scattering interference mechanism of the two objects, however, a large separation distance (e.g., several correlation lengths) showed stronger effect for small ground roughness.     

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     

Maxwellian material-based absorbing boundary conditions for lossymedia in 3-D

A two time-derivative Lorentz material (2TDLM), which has been shown previously to be the correct Maxwellian medium choice to match an absorbing layer to a lossy region, is extended here to a complete absorbing boundary condition (ABC) for three-dimensional (3-D) finite-difference time-domain (FDTD) simulators. The implementation of the lossy 2TDLM (L2TDLM) ABC is presented. It is shown that in contrast to the one-dimensional (1-D) and two-dimensional (2-D) versions, the full 3-D ABC requires a three time-derivative Lorentz material in the edge and corner regions to achieve a rigorous matching of the resulting Maxwellian absorbing layer to the lossy medium. The 3-D ABC implementation thus requires the introduction of an auxiliary field to handle the edge and corner regions to achieve a state-space form of the update equations in the ABC layers. Fully 3-D examples including pulsed dipole radiation and pulsed Gaussian beam propagation in lossless and lossy materials as well as pulse propagation along a microstrip over lossless and lossy materials are included to illustrate the effectiveness of the L2TDLM ABC     

Ground loss effects in power line reradiation at standard broadcastfrequencies

Reradiation of a standard broadcast antenna's signal from a high-voltage power line is often analyzed by computer modeling. The resonant behavior of a power line operating over lossy ground is investigated using the numerical electromagnetics code (NEC) which uses the Sommerfeld-Norton (SN) ground model, for a highly accurate computation of the interaction of a power line with lossy ground. It is shown that lossy ground damps the resonant response of the power line and so, substantially reduces the reradiated field. Since the SN ground option is costly in CPU time, the effects of lossy ground were approximated using the tower footing impedance concept. A systematic evaluation of the accuracy of the approximation is made. An approximate technique for incorporating the approximate techniques presented are sufficiently accurate that the economical method-of-images solution can be used in most cases     

Three-dimensional electromagnetic scattering from inhomogeneousobjects by the hybrid moment and finite element method

The hybrid moment and finite element methods are used to obtain 3-D scattering and/or absorption from inhomogeneous, arbitrarily shaped objects. The surface of the object is approximated by triangles and the volume of the object is approximated by tetrahedrons. The electrical parameters are assumed constant in each tetrahedron. The Galerkin testing procedure is used. To avoid contaminations of spurious mode, a divergenceless vector basis function is used in finite elements. The calculated internal field and scattered field for a homogeneous sphere, a layered sphere, and a lossy prolate spheroid are compared with Mie series solutions and other numerical techniques. The accuracy and rate of convergence of the solution are discussed     

Aperture excitation of electrically large, lossy cavities

We present a theory based on power balance for aperture excitation of electrically large, lossy cavities. The theory yields expressions for shielding effectiveness, cavity Q, and cavity time constant. In shielding effectiveness calculations, the incident field can be either a single plane wave or a uniformly random field to model reverberation chamber or random field illumination. The Q theory includes wall loss, absorption by lossy objects within the cavity, aperture leakage, and power received by antennas within the cavity. Extensive measurements of shielding effectiveness, cavity Q, and cavity time constant were made on a rectangular cavity, and good agreement with theory was obtained for frequencies from 1 to 18 GHz     

Application of a point-matching MoM reduced scheme to scatteringfrom finite cylinders

One of the most common methods for the solution of three-dimensional (3-D) scattering problems is the electric-field volume integral equation numerically solved by the application of the method of moments (MoM)-usually the point-matching version. Although simple to formulate, it shows inherent difficulty and complexity because of the 3-D integrals appearing in the interaction matrix elements and of the singularity of the dyadic Green's function (DGF) present in the computation of the self-cell elements. In this paper, a transformation method is presented, which in the case of the point-matching MoM, both reduces the 3-D integrals to two-dimensional (2-D) ones, and also eliminates the need of separate treatment of the singularity while maintaining the same degree of approximation. Comparison to published results is made for the case of scattering by a finite dielectric cylinder. Further examples are presented for scattering by layered dielectric cylinders and lossy cylindrical shells excited by uniform plane waves