Showing papers on "Computational electromagnetics published in 2000"

Modeling electromagnetic propagation in the Earth-ionosphere waveguide

Abstract: The ionosphere plays a role in radio propagation that varies strongly with frequency. At extremely low frequency (ELF: 3-3000 Hz) and very low frequency (VLF: 3-30 kHz), the ground and the ionosphere are good electrical conductors and form a spherical Earth-ionosphere waveguide. Many giants of the electromagnetics (EMs) community studied ELF-VLF propagation in the Earth-ionosphere waveguide, a topic which was critically important for long-range communication and navigation systems. James R. Wait was undoubtedly the most prolific publisher in this field, starting in the 1950s and continuing well into the 1990s. Although it is an old problem, there are new scientific and practical applications that rely on accurate modeling of ELF-VLF propagation, including ionospheric remote sensing, lightning remote sensing, global climate monitoring, and even earthquake precursor detection. The theory of ELF-VLP propagation in the Earth-ionosphere waveguide is mature, but there remain many ways of actually performing propagation calculations. Most techniques are based on waveguide mode theory with either numerical or approximate analytical formulations, but direct finite-difference time-domain (FDTD) modeling is now also feasible. Furthermore, in either mode theory or FDTD, the ionospheric upper boundary can be treated with varying degrees of approximation. While these approximations are understood in a qualitative sense, it is difficult to assess in advance their applicability to a given propagation problem. With a series of mode theory and FDTD simulations of propagation from lightning radiation in the Earth-ionosphere waveguide, we investigate the accuracy of these approximations. We also show that fields from post-discharge ionospheric currents and from evanescent modes become important at lower ELF (/spl lsim/500 Hz) over short distances (/spl lsim/500 km). These fields are not easily modeled with mode theory, but are inherent in the FDTD formulation of the problem. In this way, the FDTD solution bridges the gap between analytical solutions for fields close to and far from the source.

Acoustic and Electromagnetic Scattering Analysis Using Discrete Sources

Abstract: Preface. Acknowledgements. Elements of Functional Analysis. The Scalar Helmholtz Equation. Systems of Functions in Acoustic Theory. Discrete Sources Method in Acoustic Theory. Null-Field Method in Acoustic Theory. The Maxwell Equations. Systems of Functions in Electromagnetic Theory. Projection Methods in Electromagnetic Theory. Discrete Sources Method in Electromagnetic Theory. Null-Field Method in Electromagnetic Theory. Appendix. References. Index.

A special higher order finite-element method for scattering by deep cavities

Abstract: A special higher order finite-element method is presented for the analysis of electromagnetic scattering from a large, deep, and arbitrarily shaped open cavity. This method exploits the unique features of the finite-element equations and, more importantly, the unique features of the problem of scattering by a large and deep cavity. It is designed in such a manner that it uses minimal memory, which is proportional to the maximum cross section of the cavity and independent of the depth of the cavity, and its computation time increases only linearly with the depth of the cavity. Furthermore, it computes the scattered fields for all angles of incidence without requiring significant additional time. The technique is implemented with higher order tetrahedral and mixed-order prism elements, both having curved sides to allow for accurate modeling of arbitrary geometries. Numerical results show that higher order elements yield a remarkably more accurate and efficient solution for scattering by three-dimensional (3-D) cavities. Of the two kinds of element, the mixed-order prism is optimal for the proposed special solver.

Sparse pattern selection strategies for robust Frobenius-norm minimization preconditioners in electromagnetism

Abstract: We consider preconditioning strategies for the iterative solution of dense complex symmetric non-Hermitian systems arising in computational electromagnetics. We consider in particular sparse approximate inverse pre-conditioners that use static non-zero pattern selection. The novelty of our approach comes from using a different non-zero pattern selection procedure for the original matrix from that for the preconditioner and from exploiting geometric or topological information from the underlying meshes instead of using methods based on the magnitude of the entries. The numerical and computational efficiency of the proposed preconditioners are illustrated on a set of model problems arising both from academic and from industrial applications. The results of our numerical experiments suggest that the new strategies are viable approaches for the solution of large-scale electromagnetic problems using preconditioned Krylov methods. In particular, our strategies are applicable when fast multipole techniques are used for the matrix-vector product on parallel distributed memory computers. Copyright © 2000 John Wiley & Sons, Ltd.

2-D electromagnetic modeling by finite‐element method with a dipole source and topography

Abstract: I present a method for calculating frequency‐domain electromagnetic responses caused by a dipole source over a 2-D structure. In modeling controlled‐source electromagnetic data, it is usual to separate the electromagnetic field into a primary (background) and a secondary (scattered) field to avoid a source singularity, and only the secondary field caused by anomalous bodies is computed numerically. However, this conventional scheme is not effective for complex structures lacking a simple background structure. The present modeling method uses a pseudo‐delta function to distribute the dipole source current, and does not need the separation of the primary and the secondary field. In addition, the method employs an isoparametric finite‐element technique to represent realistic topography. Numerical experiments are used to validate the code. Finally, a simulation of a source overprint effect and the response of topography for the long‐offset transient electromagnetic and the controlled‐source magnetotelluric me...

Quasi-analytical approximations and series in electromagnetic modeling

Abstract: The quasi-linear approximation for electromagnetic forward modeling is based on the assumption that the anomalous electrical field within an inhomogeneous domain is linearly proportional to the background (normal) field through an electrical reflectivity tensor ![Graphic][1] . In the original formulation of the quasi-linear approximation, ![Graphic][2] was determined by solving a minimization problem based on an integral equation for the scattering currents. This approach is much less time-consuming than the full integral equation method; however, it still requires solution of the corresponding system of linear equations. In this paper, we present a new approach to the approximate solution of the integral equation using ![Graphic][3] through construction of quasi-analytical expressions for the anomalous electromagnetic field for 3-D and 2-D models. Quasi-analytical solutions reduce dramatically the computational effort related to forward electromagnetic modeling of inhomogeneous geoelectrical structures. In the last sections of this paper, we extend the quasi-analytical method using iterations and develop higher order approximations resulting in quasi-analytical series which provide improved accuracy. Computation of these series is based on repetitive application of the given integral contraction operator, which insures rapid convergence to the correct result. Numerical studies demonstrate that quasi-analytical series can be treated as a new powerful method of fast but rigorous forward modeling solution. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif

The Transmission Line Matrix Method

Abstract: The transmission line matrix (TLM) method [1–3], developed and first published in 1971 by Johns and Beurle has emerged as a powerful method for computer modeling of electromagnetic fields. In TLM the space is subdivided into cells. The electromagnetic field dynamics is modeled by wave pulses propagating between adjacent cells and scattered within the cells. The main advantage of the TLM simulation resides in the capability to model circuits of arbitrary geometry, and to compute and to display the time evolution of the fields. The TLM method exhibits an excellent numerical stability and is also suitable for modelling of lossy, dispersive and nonlinear media.

Wave Splitting in Direct and Inverse Scattering Problems

Abstract: The focus of this thesis is on the use of wave splitting in electromagnetic direct and inverse scattering problems. Wave splitting offers a decomposition of wave fields into appropriate input and output wave constituents. Several different wave splittings are studied including one-dimensional, multi-dimensional energy-flux, and multi-dimensional locally exact wave splittings. The Bremmer series is naturally connected to wave splitting as a method to decompose a complex scattering problem into a sequence of single scattering problems. The one-dimensional Bremmer series is reviewed and time-domain convergence is shown for the acoustic locally exact wave splitting. The emphasis of the inverse scattering problems is on the identification of the spatial structure of complex medium models in multi-dimensions from time-domain data. The parameter identification is determined in an iterative fashion with a conjugate-gradient algorithm where the least-squares error of the output field is minimized. The gradient is determined from the solution of an additional adjoint problem. The energy-flux split fields are shown to give a good representation of the boundary fields in the inverse scattering problem. Several multi-parameter identifications are performed in two spatial dimensions. A detailed analysis is included about electromagnetic modeling. The non-unique-ness of the instantaneous response and the long-time behavior is specially emphasized. Finally, time-reversal mirrors and time-reversal cavities are discussed. (Less)

An analysis of the scattering of high‐frequency electromagnetic radiation from rough surfaces with application to pulse radar operating in backscatter mode

Abstract: The scattering of high-frequency (HF) electromagnetic radiation from slightly rough, good conducting surfaces is presented. The analysis is based on a decomposition of the relevant space using generalized functions. The fundamental analysis incorporates a general source and involves all scattering orders for the normal component of the field. Subsequently, derivation of the scattered electric field (to third order in scatter) using a pulsed dipole source is effected. The first 2 orders are used to deduce an estimate of radar cross sections of bounded regions or targets when operation is carried out in the backscatter mode. Conditions of small height and small slope are imposed. Application is made to the determination of the first-order cross section of a perfectly conducting sphere (within the limits of the imposed contraints) and of an exponential boss. The results are shown to be consistent with Rayleigh scattering theory.

Propagation of electromagnetic dipole waves through dielectric interfaces.

Abstract: The problem of divergent electromagnetic dipole waves propagating through parallel dielectric interfaces is solved. The solution is obtained in an analytic form that can be readily evaluated numerically. The result is obtained as a solution to a boundary-value problem. Applications of the solution are described.

CTLSS-an advanced electromagnetic simulation tool for designing high-power microwave sources

Abstract: Simulation-based-design (SBD) techniques to achieve "first-pass design success" depend on the development of fast, accurate, realistic models that can handle material properties, geometry, and appropriate boundary conditions. This paper describes a new three-dimensional (3-D) electromagnetic and large-signal simulation tool. Cold-Test and Large-Signal Simulator (CTLSS), which has been developed as part of an SBD tool suite for vacuum electron devices. Computational electromagnetic codes are essential for applying the SBD methodology to the design of vacuum electron devices and components. CTLSS offers the unique advantage that its computational electromagnetics model is linked intimately with a large-signal simulation tool for computing the electron-wave interaction in the radiating structure. Currently, this link has been implemented for helix traveling-wave tubes (TWTs) only, using the CHRISTINE code as the large-signal model, but a new, general, large-signal model is under development and is described in this paper. The electromagnetic simulation engine in CTLSS has been designed and implemented as a volumetric frequency-domain model that can handle both resonant eigenvalue problems, using the Jacobi-Davidson algorithm, and nonresonant driven-frequency problems, using the quasi-minimal residual (QMR) technique to invert the non-Hermitian matrices that often occur in real problems. The features and advantages of this code relative to other models and results from the code for several classes of microwave devices are presented.

A Finite-Volume Method for the Maxwell Equations in the Time Domain

Abstract: A finite-volume scheme for the Maxwell equations is proposed using collocated nonorthogonal curvilinear grid arrangements, with the advantage that all components of the electric and magnetic field are stored at the same computational location and time. It is based on construction principles of high-resolution schemes for hyperbolic conservation laws and takes into account the local wave propagation. The implementation of boundary conditions based on the characteristic theory is described. A new divergence correction technique is proposed to preserve locally the charge conservation. This is important if the Maxwell solver is used within an electromagnetic particle-in-cell (PIC) code. The accuracy and efficiency of this finite-volume framework is demonstrated with problems of electromagnetic wave propagation and of the self-consistent motion of charged particles.

Accuracy of the method of moments for scattering by a cylinder

Abstract: We study the accuracy and convergence of the method of moments for numerical scattering computations for an important benchmark geometry: the finite circular cylinder. From the spectral decomposition of the electric-field integral equation for this scatterer, we determine the condition number of the moment matrix and the dependence of solution error on the choice of basis functions, discretization density, polarization of the incident field, and the numerical quadrature rule used to evaluate moment-matrix elements. The analysis is carried out for both the TM polarization (weakly singular kernel) and TE polarization (hypersingular kernel). These results provide insights into empirical observations of the convergence behavior of numerical methods in computational electromagnetics.

A fractal-based theoretical framework for retrieval of surface parameters from electromagnetic backscattering data

Abstract: A new theoretical framework for retrieval of dielectric and geometric surface parameters from electromagnetic backscattering data is presented. Use is made of fractal geometry for description of the surface. The suggested scheme leads to a two-step procedure that allows the authors to retrieve the surface complex permittivity and the fractal parameters. Limits of validity for the procedure applicability are presented. The robustness of the method is checked against input data measurement errors and electromagnetic model inaccuracies. Key points for application of the new framework to synthetic aperture radar (SAR) data are discussed.

3-D electromagnetic modeling and nonlinear inversion

Abstract: We describe a new algorithm for 3-D electromagnetic inversion that uses global integral and local differential equations for both the forward and inverse problems. The coupled integral and differential equations are discretized by the finite element method and solved on a parallel computer using domain decomposition. The structure of the algorithm allows efficient solution of large 3-D inverse problems. Tests on both synthetic and field data show that the algorithm converges reliably and efficiently and gives high-resolution conductivity images.

In-mold electromagnetic stirring in continuous casting

Abstract: To get the optimal operation of in-mold electromagnetic stirring through the fundamental characteristics, a three-dimensional magnetohydrodynamic calculation model was used, taking into consideration heat transfer and solidification as well as free surface. Comparison between measured velocity and calculated velocity is in agreement. A shadow method that calculates electromagnetic force in consideration of free surface transition is proposed to reduce computing time and maintain sufficient accuracy. A calculated free surface of mercury under a 200-Hz magnetic field shows an adequate agreement with the experimental one. Calculation results applied to the continuous casting process taking into consideration heat transfer and solidification are in good agreement with operation data. This model shows that electromagnetic stirring makes the solidified shell uniform and dynamic deviation of temperature stable.

A numerical model for stability considerations in HTS magnets

Abstract: We propose that in an HTS application, stability is lost more likely because of a global increase in temperature caused by heat generation distributed over the whole coil than because of a local normal zone which starts to propagate. For consideration of stability in HTS magnets, we present a computational model based on the heat conduction equation coupled with Maxwell's equations, whereby analysis can be performed by using commercial software packages for computational electromagnetics and thermodynamics. For temperature distribution inside the magnet, we derive the magnetic field dependent effective values of thermal conductivity, specific heat, and heat generated by electromagnetic phenomena for the composite structure of the magnet, while cooling conditions and external heat sources are described as boundary conditions. Our model enables the magnet designer to estimate a safe level of the operation current before a thermal runaway. Finally, as examples, we present some calculations of the HTS magnet with ac to review the effects of slanted electric field-current density E (J ) characteristics and high critical temperature of HTS materials.

Numerical solution of 2D and 3D induction heating problems with non-linear materml properties taken into account

Abstract: A numerical calculation model for the solution of 2D and 3D induction heating problems, which takes the nonlinearities of both the electromagnetic and thermal material properties into account, is described. The solution of a 2D-coupled field problem is found by traditional FEM. In a 3D analysis nonlinear surface impedances are utilized in the magnetic field problem and the power transfer to the workpiece is modeled using heat fluxes. The performance of the model was verified by comparing the calculated temperature profiles with the measurements.

Numerical Solution of the Time-Domain Maxwell Equations Using High-Accuracy Finite-Difference Methods

Abstract: High-accuracy finite-difference schemes are used to solve the two-dimensional time-domain Maxwell equations for electromagnetic wave propagation and scattering. The high-accuracy schemes consist of a seven-point spatial operator coupled with a six-stage Runge--Kutta time-marching method. Two methods are studied, one of which produces the maximum order of accuracy and one of which is optimized for propagation distances smaller than roughly 300 wavelengths. Boundary conditions are presented which preserve the accuracy of these schemes when modeling interfaces between different materials. Numerical experiments are performed which demonstrate the utility of the high-accuracy schemes in modeling waves incident on dielectric and perfect-conducting scatterers using Cartesian and curvilinear grids. The high-accuracy schemes are shown to be substantially more efficient, in both computing time and memory, than a second-order and a fourth-order method. The optimized scheme can lead to a reduction in error relative to the maximum-order scheme, with no additional expense, especially when the number of wavelengths of travel is large.

New power transformer model for the calculation of electromagnetic resonant transient phenomena including frequency-dependent losses

Abstract: A new power transformer model for the accurate predetermination of transient resonant processes is introduced in this paper. The accurate representation of the losses is the most important feature of the new model since it has been developed for the accurate determination of maximal stresses during resonance phenomena within transformers. The new equivalent circuit only contains constant lumped parameters. It represents very accurately the frequency variation of all impedances (real and imaginary parts) of the transformer, including mutual magnetic coupling. This allows carrying out transient calculations in the time domain directly using programs for the electromagnetic transients calculations like EMTP. Several laboratory measurements on transformer winding models have been performed in order to verify the agreement of measured and calculated values of resonant overvoltages.

Fast electromagnetic modeling of multilayer microstrip antennas and circuits

Abstract: An efficient electromagnetic modeling of multilayer microstrip antennas and circuits is presented The surface integral equation is solved by using the method of moments In this method, the multilayer Green's functions are efficiently evaluated by the discrete complex image method, and the higher-order basis functions defined on curvilinear triangular elements are employed to gain the fast convergence rate To achieve the fast frequency sweep, the asymptotic waveform evaluation is incorporated into the analysis To handle large-scale problems, two fast schemes are applied One is the fast Fourier transform accelerated scheme, the adaptive integral method The other is the multipole accelerated scheme, the multilevel fast multipole algorithm Both of the algorithms reduce the memory requirement and the CPU time

Modeling of arbitrary wire antennas above ground

Abstract: An efficient method for modeling wire antennas of arbitrary shapes and orientations above a lossy ground is presented by using Galerkin method. As opposed to the traditional point matching method, careful treatment is required for the electric-field integral equation (EFIE) and its numerical implementation. Comparing with the point matching method, only three simple Sommerfeld integrals are needed in the new method, and the convergence is much faster. To test the validity of this method, numerous numerical examples are given. Numerical results show that, for equivalent-accuracy solutions, the cell number used in this method is much fewer than that in the point matching method.

On the modeling of periodic structures using the finite-difference time-domain algorithm

Abstract: In this paper, we present a technique for modeling periodic structures, such as spatial filters and photonic bandgap materials, using the finite-difference time-domain (FDTD) algorithm, for both the normal and oblique incidence cases. A waveguide simulator technique is employed to model these periodic structures, and the issue of arbitrary angles of incidence as well as the problem of sweeping frequency of the illuminating wave, that are difficult to handle in the FDTD waveguide simulator approach, are addressed. Comparisons with results derived using other techniques are presented, and good agreement is demonstrated. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 24: 151–155, 2000.

Automatic generation of subdomain models in 2D FDTD using reduced order modeling

Abstract: A new method combining a finite difference method and a reduced order model (ROM) algorithm is presented for two-dimensional (2-D) electromagnetic problems. The problem space is subdivided into subdomains of a generic type. By discretizing the spatial derivatives in a way similar to the finite-difference in time-domain technique (FDTD), the state equations are written down in each subdomain. From that, an FDTD-subdomain model is derived. Finally, the different subdomains are reconnected and the original problem is solved by a leapfrog time-stepping algorithm. Some numerical results are presented to illustrate the new approach.

Electromagnetic Scattering by Multilayered Chiral-Media Structures: a Scattering-to-Radiation Transform

Abstract: For many years, the electromagnetic radiation and the plane wave scattering by various dielectric objects have been considered as two different subjects and theoretical formulations for analyzing the radiation and scattering problems also differ. This paper establishes a bridge between the radiation and scattering theories. Instead of an isotropic medium, chiral media have been considered as dielectric materials used for fabricating the planar, cylindrical, and spherical objects if multilayers. In the analysis, the electromagnetic plane waves of parallel and perpendicular polarizations are equivalent to radiated waves due to two differently polarized electric sources located at infinity. The volumetric current distributions of the two sources at infinity are thus obtained. Instead of considering the electromagnetic plane wave scattering, we can analyze the electromagnetic radiation due to the electric sources at infinity for the field distribution elsewhere outside of the scattering objects. To verify the...

Electromagnetic scattering by nonplanar junctions of resistive sheets

Abstract: Approximate uniform asymptotic expressions are provided to determine the field scattered by a penetrable wedge illuminated at normal incidence. The wedge is formed by two resistive sheets or two thin dielectric slabs definable as resistive sheets having identical geometric and electromagnetic characteristics. The solution is limited to wedge angles and source positions where internal reflections cannot occur. It is obtained by using a geometrical optics (GO) approximation for the field internal to the slabs and by performing a uniform asymptotic evaluation of the physical optics (PO) radiation integral in the hypothesis that a resistive sheet condition is valid. Samples of numerical results so obtained are presented and compared with other methods to demonstrate the effectiveness of the proposed technique.

Interconnect electromagnetic modeling using conduction modes as global basis functions

Abstract: A new method is formulated for modeling current distributions inside conductors for a quasi-static or a full-wave electromagnetic field simulator. In our method, we model current distributions inside interconnects using a small number of conduction modes as global basis functions for discretization of the mixed potential integral equation. A very simple example is presented to illustrate the potential of our method.

Conductive masses in a half-space Earth in the diffusive regime: fast hybrid modeling of a low-contrast ellipsoid

Abstract: Electromagnetic three-component magnetic probes at diffusion frequencies are now available for use in slim mineral-exploration boreholes. When a source is operated at or below the surface of the Earth in the vicinity of a conductive orebody, these probes provide, after appropriate processing, the secondary vector magnetic field attributed to this body. Proper inversion of the resulting datasets requires as a first step a clear understanding of the electromagnetic interaction of model signals with model bodies. In this paper, the response of a conductive ellipsoid buried at shallow depth in a half-space Earth is investigate by a novel hybrid approach combining the localized nonlinear approximation and the low frequency scattering theory. The ellipsoidal shape indeed fits a large class of scatterers and yet is amenable to analytical calculations in the intricate world of ellipsoidal harmonics, while the localized nonlinear approximation is known to provide fairly accurate results at least for low contrasts of conductivity between a scattering body and its host medium. In addition, weak coupling of the body to the interface is assumed. The primary field accounts for the presence of the interface, but multiple reflection of the secondary field on this interface is neglected. After analyzing the theoretical bases of the approach, numerical simulations in several geometrical and electrical configurations illustrate how estimators of the secondary magnetic field along a nearby borehole behave with respect to a general-purpose method-of-moments (MoM) code. Perspectives of the investigation and extensions, in particular, to two-body systems, strong coupling to the interface, and high contrast cases, are discussed.