Scattering center analysis via Prony's method

High-frequency scattering can often be described in terms of scattering centers, and an understanding of the geometries which give rise to these centers is important in the area of radar cross section modification. Certain canonical geometries have been treated theoretically with asymptotic methods, but, in general, one must study the behavior of scattering centers empirically. Prony's method provides a way of extracting the locations (projected on the path of propagation) and weighting coefficients of scattering centers from the backscattered field as a fuction of frequency. It has been found to be superior to the conventional Fourier transform technique in resolution and dynamic range.     

过孔转换信号完整性分析的区域分解有限元法

提出了一种基于区域分解的二维有限元法分析多层印制电路板电源/地平面中过孔转换结构的信号完整性.过孔电流产生的电磁场呈三维结构,其中,一部分电磁波沿过孔轴向传输,另一部分电磁波在电源/地平面间沿径向传播.采用一虚拟柱面将求解区域分割为过孔区和电源/地平面区.将过孔区建模为以周向磁场为主分量的二维轴对称问题,而将电源/地平面区建为以垂直电场为主分量的二维模型.首先求解电源/地平面区的二维边值问题获得分割边界上节点的波阻抗,然后将该波阻抗代入过孔区模型中分割边界节点的边界条件,从而计算出过孔信号传输的S参数.所提方法通过模型缩减可实现对微细过孔结构信号完整性的精确快速计算,且采用全波电磁场分析软件对算法的有效性和准确性进行了验证.     

A hybrid finite-element method for axisymmetric waveguide-fed horns

A new method for finding radiation patterns and the reflection coefficients associated with an axisymmetric waveguide fed horn is presented. The approach is based on a hybrid finite element method (FEM) wherein the electromagnetic fields in the FEM region are coupled to the fields outside by two surface integral equations. Because of the local nature of the FEM, this formalism allows for the presence of inhomogeneities to be included in the problem domain. The matrix equation which results from the application of this method is shown to be complex-symmetric. Comparisons of calculated and measured data for two different horns show good agreement     

Time-domain finite-element simulation of three-dimensional scattering and radiation problems using perfectly matched layers

An effective algorithm to construct perfectly matched layers (PMLs) for truncating time-domain finite-element meshes used in the simulation of three-dimensional (3-D) open-region electromagnetic scattering and radiation problems is presented. Both total- and scattered-field formulations are described. The proposed algorithm is based on the time-domain finite-element solution of the vector wave equation in an anisotropic and dispersive medium. The algorithm allows for the variation of the PML parameters within each element, which facilitates the efficient use of higher order vector basis functions. The stability of the resultant numerical procedure is analyzed, and it is shown that unconditionally stable schemes can be obtained. Numerical simulations of radiation and scattering problems based on both the zeroth- and higher order vector bases are presented to validate the proposed PML scheme.     

腔基微带天线的矢量有限元--边界积分方法分析

该文将矢量有限元——边界积分(Edge-Based FE-BI)混合方法用于腔基微带贴片线的辐射特性分析。分别计算了在无阻抗负载和加有阻抗负载两种情况下的输入阻抗，以此验证了该混合方法的正确性；然后计算了贴片天线表面缝隙部分的场分布，验证微带天线分析模型——传输线模型的合理性；最后计算了E面、H面方向图以及相应的交叉极化方向图。     

Full-vector finite-element beam propagation method for three-dimensional nonlinear optical waveguides

A full-vector finite-element beam propagation method (VFE-BPM) in terms of all the components of slowly varying electric fields is described for the analysis of three-dimensional (3-D) nonlinear optical waveguides. Electric fields obtained with this approach can be directly utilized for evaluating nonlinear refractive index distributions. To eliminate nonphysical, spurious solutions, hybrid edge/nodal elements are introduced. Furthermore, to avoid spurious reflections from the computational window edges, anisotropic perfectly matched layer boundary conditions are implemented, and to reduce computational effort for the nonlinear optical waveguide analysis, an iterative algorithm is also introduced. The effectiveness of the present approach is verified by way of numerical examples: nonlinear directional couplers, spatial soliton emission phenomena, and soliton couplers.     

Two-dimensional semiconductor analysis using finite-element method

Two-dimensional simulation of semiconductor devices using a finite-element formulation is described. In the present analysis, Poisson's equation is solved by a finite-element method, based on the variational principle, and current continuity equations are solved by a method of weighted residuals. The advantage of this method is mentioned. In order to demonstrate the validity of this method, a bipolar n-p-n transistor is analyzed, considering the generation-recombination term. Not only voltage-current characteristic, but also junction capacitance and cutoff frequency are calculated. Then transistor behavior under inverse mode by using the n-type buried layer as a common emitter is discussed.     

Scattering from a large body with cracks and cavities by the fast and accurate finite-element boundary-integral method

A large body with cracks and cavities is a typical structure widely existing in realistic targets. In this paper, a newly developed fast and accurate finite-element boundary-integral (FA-FE-BI) method is applied to compute scattering by this kind of scatterer. A thorough analysis on this FA-FE-BI numerical technique is presented, clearly demonstrating that this technique has computational complexity O(N log N) and memory requirement O(N) (N is the total number of surface unknowns). An inward-looking approach is employed as a preconditioner to speed up the rate of convergence of iterative solvers for this structure. Under these techniques, a powerful code is developed for this kind of scatterer whose accuracy, efficiency, and capability is well confirmed by various numerical results.     

Microwave imaging using the finite-element method and a sensitivity analysis approach

A method for reconstructing the constitutive parameters of two-dimensional (2-D) penetrable scatterers from scattered field measurements is presented. This method is based on the differential formulation of the forward scattering problem, which is solved by applying the finite-element method (FEM). Given a set of scattered field measurements, the objective is to minimize a cost function which consists of two terms. The first is the standard error term, which is related to the measurements and their estimates, while the second term, which is related to the Tikhonov regularization, is used to heal the ill posedness of the inverse problem. The iterative Polak-Ribière nonlinear conjugate gradient algorithm is applied to the minimization of the cost function. During each iteration of the algorithm, the direction of correction is computed by using a sensitivity analysis approach, which is carried out by an elaborate finite-element scheme. The adoption of the finite-element method results in sparse systems of equations, while the computational burden is further reduced by applying the adjoint state vector methodology. Finally, a microwave medical imaging application, which is related to the detection of proliferated bone marrow, is examined, while the robustness of the proposed technique in the presence of noise and for different regularization levels is investigated.     

Holey fiber analysis through the finite-element method

A holey fiber (HF), having very complex hole geometry, is studied by means of a numerical simulator for modal analysis based on the finite-element method (FEM). Polarization and dispersion properties as well as the full vector field distribution of the fundamental mode are investigated. The obtained numerical results show a good agreement with experimental ones reported in literature     

Capacitance computation for CPW discontinuities with finitemetallization thickness by hybrid finite-element method

A variational equation is derived for the capacitances of coplanar waveguide (CPW) structures with finite metallization thickness. The equation is expressed in terms of the static potential in the slot region and is solved by applying the hybrid finite-element method (FEM). In the case of small metallization thickness, it is reduced to a perturbation formula for the incremental capacitances. Numerical results for the equivalent capacitances of various discontinuities with finite metallization thickness are presented and compared with measured data. The reasonable agreement between the measured data and the theoretical results validates the present approach. Being simple and computationally efficient, the method is suitable for dealing with extensive CPW discontinuity problems where the metallization thickness is not negligible     

Design optimization of conformal antennas by integrating stochasticalgorithms with the hybrid finite-element method

Previous work in antenna optimization has primarily focused on applications of optimization algorithms in conjunction with problem-specific or semi-analytic tools. However, previous developments in fast algorithms now offer the possibility of designs and moreover allow for full flexibility in material specification across three dimensions. As an example, this paper combines genetic algorithms (GA) and simulated annealing (SA) with fast hybrid finite-element boundary integral simulations to develop full three-dimensional (3-D) antenna designs using shape, topology, as well as material optimization. To illustrate these optimization methods as well as compare between GA and SA, three different antenna designs are considered. First, a folded-slot antenna is optimized for broad-band performance, followed by an irregular-shaped dual-band antenna design. As a third design, which combines shape and material optimization, a bandgap substrate is designed to substantially increase the bandwidth of a patch residing on the optimized substrate     

Susceptibility analysis of arbitrarily shaped 2-D slotted screensusing a hybrid generalized scattering matrix finite-element technique

The electromagnetic susceptibility (EMS) inside slotted screens has been studied using a hybrid technique. The screen is characterized by a generalized admittance or impedance matrix, computed using the finite element method (FEM), which is then combined with a modal solution in free-space. The scattering matrix for the screen can then be easily computed. As a practical application, the electrical performance of a slotted square envelope has been studied. In general, it is shown that coupling to the interior of slotted screens is maximized at frequencies corresponding to resonances of the shorted screen, provided that the fields do not vanish near the aperture     

Scattering and radiation from planar structures containing smallfeatures using a finite element-fast integral method

Fast integral equation algorithms such as the adaptive integral method (AIM) have been demonstrated to reduce memory and execution time associated with moment method solutions for computing electromagnetic scattering and radiation from arbitrarily shaped three-dimensional geometries. The authors examine the efficiency of AIM in modelling planar structures that contain small and intricate details as is the case with spirals and slot antennas. Such geometries require high tessellation due to the inclusion of very small features resulting in a large number of unknowns. The AIM, with its ability to translate the original grid into an equivalent sparser uniform grid, is uniquely suited to the analysis of such geometries. The application of the AIM in connection with finite element-boundary integral formulation for cavity-backed antennas is also presented     

Full-vector finite-element beam propagation method for anisotropicoptical device analysis

A finite-element full-vectorial beam-propagation method is presented, for the first time, for the analysis of 3-D anisotropic optical waveguides. Full 3×3 permittivity and permeability tensors are considered. The formulation takes into account the polarization dependence and the component coupling due to the waveguide geometry and the medium anisotropy without any analytical approximations. The perfectly matched anisotropic absorber is introduced to eliminate the influence of the computational border on the numerical solutions. The correctness of the proposed approach is verified by analyzing several kinds of anisotropies. For the first time, a full vectorial finite-element propagation analysis is presented for diffuse waveguides and magnetooptic devices     

Complex finite-element method applied to the analysis of opticalwaveguide amplifiers

A complex finite-element method and a three-level model for erbium ions are applied to obtain gain and propagation constants for erbium-doped waveguide amplifiers (EDWA's). The complex refractive index profile includes the effect of the dopant polarization induced by the pump field. The method allows to consider arbitrary dopant density profile as well as the modal structure of the pump field. For different waveguide geometries we obtain gain curves as function of pump intensity as well as slight variations in the modal propagation constants. The threshold pump power is shown to be a function of the waveguide geometry, which agrees qualitatively with experimental results,     

Quasi-static finite-element analysis of a skewed microstripcrossover

In this work, we present a quasistatic analysis of a microstrip crossover on a dielectric substrate. The microstrips are located at different planes and may cross at an arbitrary angle. Capacitances and inductances are calculated from scalar potentials. For magnetostatic formulation, the boundary conditions for scalar potential are introduced by means of partitioning surfaces. The use of the adaptive finite element method provides the required flexibility with respect to the analyzed geometry, optimal discretization and good efficiency     

Scattering from periodic strip gratings via the secant-spectraliterative method

The secant method is used in an iterative algorithm for calculating the electromagnetic scattering from planar, periodic gratings. Results are compared with the moment method and the contraction-corrector spectral-iteration technique (SIT) methods. The secant approach does not depend on the evaluation of numerical derivatives to achieve convergence like the contraction-corrector SIT method. Suggestions for applying this method to more complicated structures are included     

复杂媒质电磁散射特性的有限元—边界元方法分析

通过有限元-边界元方法分析具有复杂媒质特性目标的电磁散射特性,如各向异性、双各向同性、双各向异性等.该混合方法能够利用有限元灵活地处理散射体内部的复杂材料,利用边界积分方法分析物体的非闭合区域,避免了使用吸收边界条件截断开放区域.通过推导基于有限元法的双各向异性媒质的泛函表达式,实现铁氧体、等离子体、手征等各类复杂媒质的统一建模,使该方法有很强的通用性.数值结果证明:本文发展的有限元-边界元方法对复杂媒质电磁散射问题分析的准确性和有效性.     

Analysis of leaky modes by a modified finite-element method

The finite-element method with integral boundary conditions is used to investigate leaky modes (quasi-eigenmodes) of open dielectric waveguides. Dispersion characteristics and field distributions in double-layer and multilayer circular waveguides are calculated. The influence of uniform deformations of the waveguide cross section on the properties of these modes is studied.