Showing papers on "Computational electromagnetics published in 1999"

A universal model for accurate calculation of electromagnetic transients on overhead lines and underground cables

Abstract: This paper presents a transmission line model for the simulation of electromagnetic transients in power systems. The model can be applied to both overhead lines and cables, even in the presence of a strongly frequency dependent transformation matrix and widely different modal time delays. This has been achieved through a phase domain formulation where the modal characteristics have been utilized in the approximation for the propagation matrix. High computational efficiency is achieved by grouping modes with nearly equal velocities and by columnwise realization of the matrices for propagation and characteristic admittance.

Lattice electromagnetic theory from a topological viewpoint

Abstract: The language of differential forms and topological concepts are applied to study classical electromagnetic theory on a lattice. It is shown that differential forms and their discrete counterparts (cochains) provide a natural bridge between the continuum and the lattice versions of the theory, allowing for a natural factorization of the field equations into topological field equations (i.e., invariant under homeomorphisms) and metric field equations. The various potential sources of inconsistency in the discretization process are identified, distinguished, and discussed. A rationale for a consistent extension of the lattice theory to more general situations, such as to irregular lattices, is considered.

Modeling and estimation of mutual coupling in a uniform linear array of dipoles

Abstract: The mutual coupling in a uniform linear array (ULA) of dipoles is calculated using basic electromagnetic concepts. Since the coupling often is unknown and needs to be estimated, a simpler model is proposed based on the electromagnetic analysis. The parameterization of this model is shown to be locally unambiguous. A necessary condition for the joint solution of directions and coupling parameters to be unique is also derived. Finally, the directions and coupling parameters are estimated using a maximum likelihood method. It is found that the simpler coupling model with just a few parameters well describes the full electromagnetic model.

Multidisciplinary shape optimization in aerodynamics and electromagnetics using genetic algorithms

Abstract: SUMMARY A multiobjective multidisciplinary design optimization (MDO) of two-dimensional airfoil is presented. In this paper, an approximation for the Pareto set of optimal solutions is obtained by using a genetic algorithm (GA). The first objective function is the drag coefficient. As a constraint it is required that the lift coefficient is above a given value. The CFD analysis solver is based on the finite volume discretization of the inviscid Euler equations. The second objective function is equivalent to the integral of the transverse magnetic radar cross section (RCS) over a given sector. The computational electromagnetics (CEM) wave field analysis requires the solution of a two-dimensional Helmholtz equation which is obtained using a fictitious domain method. Numerical experiments illustrate the above evolutionary methodology on an IBM SP2 parallel computer. Copyright © 1999 John Wiley & Sons, Ltd.

Electromagnetic modeling of composite metallic and dielectric structures

Abstract: A new, general, and very efficient method for analysis of arbitrary composite metallic and dielectric structures, based on the PMCHW formulation and Galerkin method, is presented in this paper. Flexible geometrical modeling is performed by isoparametric surfaces (i.e., by bilinear surfaces in the particular case). Efficient approximation of currents is achieved by using polynomial entire-domain expansions (i.e., rooftop subdomain expansions in the particular case) that automatically satisfy the continuity equation, assuming that there are no line charges along surface edges. Special care is devoted to the treatment of arbitrary multiple metallic and/or dielectric junctions. Numerical results for different structures, obtained by using an extremely small numbers of unknowns, show very good agreement with other available data.

Electromagnetic model order reduction for system-level modeling

Abstract: Reduced-order modeling of an electromagnetic system is understood as the approximation of a continuous or discrete model of the system by one of substantially lower order, yet capable of capturing the electromagnetic behavior of the original one with sufficient engineering accuracy. Specific methodologies for model order reduction of distributed electromagnetic systems are discussed in this paper. It is shown that electromagnetic model order reduction enhances computational efficiency and, thus, facilitates system-level modeling and computer simulation of multifunctional systems. The proposed methodologies are demonstrated through applications to the reduced-order modeling of high-speed interconnects, electromagnetic waveguides, and microstrip antennas.

Post-modern Electromagnetics: Using Intelligent MaXwell Solvers

Abstract: Geometry, differential and integral forms function analysis optimization generating matrix equations Maxwell theory computational electromagnetics generalizing finite differences MMP - a general boundary method implementation applications.

The inverse source problem of electromagnetics: linear inversion formulation and minimum energy solution

Abstract: We address the inverse source problem of finding the time-harmonic current distribution (source) with minimum L/sup 2/ norm (minimum energy) that generates a prescribed electromagnetic field outside the source's region of support. Using the well-known multipole expansion of the electromagnetic field we compute (via a linear operator formalism) the sought-after minimum L/sup 2/ norm-current distribution consistent with the data.

Development and application of a novel class of hierarchical tangential vector finite elements for electromagnetics

Abstract: Tangential vector finite elements (TVFEs) overcome most of the shortcomings of node-based finite elements for electromagnetic simulations. For a triangular element, this paper proposes a class of hierarchical TVPEs that differ from traditional TVFEs. The hierarchical nature of the proposed TVFEs makes them ideally suited for employing an efficient selective field expansion (the lowest order TVFE employed within part of the computational domain and a higher order TVFE employed within the remaining part of the computational domain). This is an attractive feature not shared by nonhierarchical TVFEs for which a more traditional approach (the same TVFE employed throughout the computational domain) must be applied. For determining the scattering by composite cylinders, this paper argues that the performance (in terms of accuracy, memory, and, in most cases, CPU time) of the proposed class of hierarchical TVFEs when applying selective field expansion is superior to that of the lowest order TVFE and a traditional nonhierarchical TVFE. This is the case when an artificial absorber as well as a boundary integral is used for truncating the computational domain. A guideline is given as to how lowest and higher order TVFEs shall be combined for optimal performance of the proposed class of hierarchical TVFEs.

Simulations of GPR in dispersive media using a frequency-dependent PSTD algorithm

Abstract: Recently an efficient pseudospectral time-domain (PSTD) algorithm has been developed to solve partial differential equations in computational electromagnetics and acoustics. It uses the fast Fourier transform (FFT) algorithm to approximate spatial derivatives, and the perfectly matched layer (PML) to eliminate the wraparound effect. Due to its high accuracy in the spatial derivatives, this method requires a significantly smaller number of unknowns than a conventional finite-difference time-domain (FDTD) method when solving large-scale problems. In this work, the authors further extend the PSTD algorithm to frequency-dependent media and apply the algorithm to simulate ground-penetrating radar (GPR) measurements in a dispersive Earth. The dispersion of the soil is treated by the recursive convolution approaches. The convergence property of the PSTD algorithm is investigated for the scattering of a dispersive cylinder. Multidimensional large-scale problems in GPR measurements are presented to demonstrate the efficiency of this frequency-dependent PSTD algorithm.

Efficient electromagnetic modeling of microstrip structures in multilayer media

Abstract: This paper presents an efficient method-of-moments solution of the mixed-potential integral equation for a general microstrip structure in multilayer media. In this method, the general forms of the spectral-domain Green's functions for multilayer media are derived first. The spatial-domain Green's functions are then obtained by the discrete complex-image method, which obviates the time-consuming numerical evaluation of the Sommerfeld integral. The Rao-Wilton-Glisson basis functions are employed to provide necessary flexibility to model arbitrary shapes. To expedite the computation of frequency response over a broad band, a reduced-order model is presented using asymptotic waveform evaluation. Numerical results of multilayer circuits and antennas are presented to show the efficiency and accuracy of this method.

Development and numerical solution of integral equations for electromagnetic scattering from a trough in a ground plane

Abstract: We develop a set of scalar integral equations that govern the electromagnetic scattering from a two-dimensional (2-D) trough in an infinite perfectly conducting ground plane. We obtain accurate and efficient numerical solution to these equations via the method of moments (MoM). Our numerical implementation compares favorably to popular methods such as the finite element/boundary integral (FE/BI) method, generalized network formulation (GNF), and electric field integral equation (EFIE) techniques.

A finite-difference time-domain method for solving electromagnetic problems with bandpass-limited sources

Abstract: The complex-envelope representation of bandpass-limited signals is used to formulate a bandpass-limited vector wave equation and a new finite-difference time-domain (FDTD) scheme that solves the bandpass-limited vector wave equation is presented. For narrow-band electromagnetic systems, this new method allows the time step to be several orders of magnitude larger than current FDTD formulations while maintaining an amplification factor equal to one. Example results obtained by this method are presented and compared with analytic solutions.

Electromagnetic Scattering from an Orthotropic Medium

Abstract: We investigate electromagnetic wave propagation in an inhomogeneous anisotropic medium. For the case of an orthotropic medium we derive the Lippmann-Schwinger equation, which is equivalent to a system of strongly singular integral equations. Uniqueness and existence of a solution is shown and we examine the regularity of the solution by means of integral equations. We prove the innnite Fr echet diierentiability of the scattered eld in its dependence on the refractive index of the anisotropic medium and we derive a characterization of the Fr echet derivatives as a solution of an anisotropic scattering problem.

A new approach to FET model scaling and MMIC design based on electromagnetic analysis

Abstract: A new approach, using electromagnetic analysis, is proposed for field-effect transistor model scaling and monolithic-microwave integrated-circuit (MMIC) design. It is based on an empirical distributed modeling technique where the active device is described in terms of an external passive structure connected to a suitable number of internal active sections. On this basis, an equivalent admittance matrix per gate unit width is obtained which, as confirmed by experimental results provided in this paper, is consistent with simple scaling rules. The same technique can also be adopted for a "global approach" to MMIC design where complex electromagnetic phenomena are also taken into account. An example of application concerning this aspect is presented.

Differences between the narrow-angle and wide-angle propagators in the split-step Fourier solution of the parabolic wave equation

Abstract: For tropospheric electromagnetic propagation, Maxwell's equations can be reduced to a parabolic wave equation, which is solved by marching over range steps. In each step, the solution is split into a product of three operators. The first and third account for atmospheric and surface variation, while the center operator propagates the field as though in vacuum. This center operator is the object of interest here. Older versions of the method used the narrow-angle propagator, while more recent versions use the wide-angle propagator. It was thought that the wide-angle propagator was entirely superior to the narrow-angle propagator, but some artifacts observed in experiments have led to the present investigation. The two propagators are examined numerically and analytically and found to exhibit subtle differences at large angles from the horizontal. This has required modifications to the way in which sources are created for starting the split-step solution. The narrow- and wide-angle propagators are also compared on two problems with analytic solutions to quantify the improvement of the wide-angle over the narrow-angle.

On the choice of basis functions to model surface electric current densities in computational electromagnetics

Abstract: Basis functions that are used to model surface electric current densities in the electric field integral equations of computational electromagnetics are analyzed with respect to how well they model the charge distribution, in addition to the current. This analysis is carried out with the help of the topological properties of open and closed surfaces meshed into networks of triangles and quadrangles. The need for current basis functions to properly model the charge distribution is demonstrated by several examples. In some of these examples, the basis functions seem to be perfectly legitimate when only the current distribution is considered, but they fail to deliver a correct solution of the electromagnetic problem, since they are not capable of properly modeling the charge distribution on some surfaces. Although the idea of proper modeling of the charge distribution by the current basis functions is easy to accept and can even be claimed well known, the contrary uses encountered in the literature have been the motivation behind the investigation reported in this paper.

Far-field contribution of evanescent modes to the electromagnetic Green tensor

Abstract: Understanding the behavior of the evanescent part of the electromagnetic field has important implications in many branches of modern physics, such as near-field optics. Motivated by recent disagreement in the literature, we derive an expression for the far-field asymptotic behavior of the free-space electromagnetic Green tensor that is due to the evanescent modes.

Fast algorithm for electromagnetic scattering by buried conducting plates of large size

Abstract: This letter presents a fast algorithm for electromagnetic scattering by buried conducting plates of large size and arbitrary shape using the conjugate gradient (CG) method combined with the fast Fourier transform (FFT). Due to the use of FFT in handling the cyclic convolutions related to Toeplitz matrices, the Sommerfeld integrals' evaluation for the buried scattering problem, which is usually time consuming, has been reduced to a minimum. The memory required for this algorithm is of the order N-the number of unknowns-and the computational complexity is of order N/sub iter/NlogN (N/sub iter/ is the iteration number N/sub iter//spl Lt/N for large problems).

Asymptotic methods in electromagnetics

Abstract: All in all, the book is well written, and provides numerous entries for the CCIR (ITU-R) reports available and ready for use. Every chapter starts of with an introduction, and a summary of the chapter is given at its end. Except for the overlooked subjects (site shielding, deterministic modeling of propagation) the subject matter is well covered. The book provides what its title says. It is a practical and pragmatic introduction to radio propagation for fixed and mobile communications.

Higher-order MoM implementation to solve integral equations

Abstract: Higher-order basis functions have received intensive attention for solving electromagnetic problems with the finite element and Galerkin's methods. The advantage of using higher-order basis functions lies in their ability to model the fields and sources, as well as geometries, more accurately than conventional low-order methods. We investigate the convergence properties of divergence conforming interpolatory higher-order basis functions for evaluating the Galerkin's solution of integral equations. Both the electric field integral equation (EFIE) and the magnetic field integral equation (MFIE) are used to obtain the scattered field from perfectly conducting objects. Our solution is first validated by comparing the radar-cross section (RCS) with the corresponding Mie series solution for conducting spheres. Next, we calculate the error convergence of the RCS from objects such as spheres and plates for several orders. In the case of objects with no analytical solution such as plates, over discretized solution is taken as the reference solution and the error results are then calculated.

An electromagnetic scattering model for multiple tree trunks above a tilted rough ground plane

Abstract: An efficient and complete electromagnetic scattering model for multiple tree trunks above the ground is presented. The individual trunks are modeled by finite-length stratified cylinders, with an external corrugated bark layer. The ground is modeled by a dielectric half space underneath the cylinders, with a slightly rough interface. The spatial distribution of the cylinders is random over an elliptic illuminated area, and their lengths follow a Gaussian distribution. Moreover, cylinders are randomly oriented with a variable degree of noncolinearity, and the ground can have an arbitrary orientation. The scattering results provided in this paper consider the summation of all scattered fields from the trunk-ground sets as a response to the incident field. Results have been computed for the mean square error of the bistatic scattering pattern, by means of a Monte Carlo simulation. The computed patterns are highly dependent on the geometry of the problem (lengths and radii of the trunks, tilting, surface roughness, incidence, and observation angles). The variability of the involved random parameters produces softening effects and an increased cross-polarized signal. Moreover, results are very sensitive to the dielectric characteristics of the trunks. As a result, the method can be useful in the interpretation of remote-sensing measurements from vegetation and in the development of classification schemes.

Implementing tri-potential approach in EMAP3D

Abstract: The Institute for Advanced Technology, The University of Texas at Austin has developed the finite element code EMAP3D which is capable of modeling coupled mechanical, thermal and electromagnetic diffusive processes with moving conductors. The Lagrangian dual potential formulation (magnetic vector potential and electrical scalar potential) was used in the earlier version of EMAP3D because it has the desirable property of being single-valued in multiply-connected regions. Using the magnetic vector potential for nonconducting regions requires solving for three unknowns at each node and results in large storage requirements and high computing cost. An alternative is to use the magnetic scalar potential, the third potential in the tri-potential formulation, in nonconducting regions. This requires solving for only one unknown at each node in nonconducting regions; however, the magnetic scalar potential is not single-valued in multiply-connected regions and the user is required to define appropriate branch cuts. A detailed Lagrangian tri-potential formulation is presented and compared to the dual potential formulation through the simulation of a typical railgun problem. The tri-potential approach shows distinct advantages in terms of model size, computing cost, and solution accuracy.

Three-Dimensional Quasi-linear Electromagnetic Modeling and Inversion

Abstract: The quasi-linear (QL) approximation replaces the (unknown) total field in the integral equation of electromagnetic (EM) scattering with a linear transformation of the primary field. This transformation involves the product of the primary field with a reflectivity tensor, which is assumed to vary slowly inside inhomogeneous regions and therefore can be determined numerically on a coarse grid by a simple optimization. The QL approximation predicts EM responses accurately over a wide range of fre­ quencies for conductivity contrasts of more than 100 to I between the scatterer and the background medium. It also provides a fast-forward model for 3-D EM inversion. The inversion equation is linear with respect to a modified material property tensor, which is the product of the reflectivity tensor and the anomalous conductivity. We call the (regularized) solution of this equation a quasi-Born inversion. The material property tensor (obtained by inversion of the data) then is used to estimate the reflectivity tensor inside the inhomogeneous region and, in tum , the anomalous conductivity. Solution of the nonlinear inverse problem thus proceeds through a set of linear equations. In prac ­ tice, we accomplish this inversion through gradient minimization of a cost function that measures the error in the equations and includes a regularization term. We use synthetic experiments with plane-wave and controlled sources to demonstrate the accuracy and speed of the method.

Electromagnetic Scattering Problems in Chiral Media: A Review

Abstract: Time-harmonic electromagnetic waves are scattered by a homogeneous chiral obstacle. It is shown that the corresponding transmission problem has a unique solution. This transmission problem is reduced to a pair of uniquely solvable coupled integral equations over the interface between the obstacle and the surrounding medium. Some scattering relations are established, and the spectrum of the far-field operator is studied and related to that of the T-matrix. If the far-field pattern is known for all incident waves with a fixed wave number, uniqueness of the obstacle and its material parameters is established.