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Title: Using Adaptive Estimation to Minimize the Number of Samples Needed to Develop a Pattern to a Specified Uncertainty
Abstract: Obtaining far-field patterns in electromagnetics or acoustics, although generally not as computationally expensive as solving for the sources induced on an object, can none-the-less at times be a substantial fraction of the overall computer time associated with some problems. This can be especially the case in determining the monostatic radar cross section of large objects, since the current distribution must be computed for each incidence angle or when using physical optics to determine the radiation patterns of large reflector antennas. In addition, when employing the point sampling and linear interpolation of the far field that is most often used to develop such patterns, it can be necessary to sample very finely in angle to avoid missing fine details such as nulls. A procedure based on model-based parameter estimation is described here that offers the opportunity of reducing the number of samples needed while developing an easily computed and continuous representation of the pattern. It employs windowed, low-order, overlapping fitting models whose parameters are estimated from the sparsely sampled far-field values. The fitting models themselves employ either discrete-source approximations to the radiating currents or Fourier models of the far field. For the cases investigated, as few as 1.5 to 2 samples per far-field lobe are found to be sufficient to develop a radiation-pattern estimate that is accurate to 0.1 dB, and 2.5 samples per lobe for a simple scatterer. In general, however, the sampling density is not determined by the lobe count alone, but by the effective rank of the field over the observation window, which in turn is a function of both the aperture size and the spatial variation of the source distribution within that aperture.
Author(s): Edmund K. Miller
File Type: Journal Paper
Issue:Volume: 17      Number: 3      Year: 2002
Download Link:Click here to download PDF     File Size: 278 KB

Title: SOLUTION TO THE GENERAL HELMHOLTZ EQUATION STARTING FROM LAPLACE EQUATION
Abstract: In this paper we illustrate how to solve the general Helmholtz equation starting from Laplace’s equation. The interesting point is that the Helmholtz equation has a frequency term where as the Laplace’s equation is the static solution of the same boundary value problem. In his new formulation the frequency dependence is manifested in the form of an excitation. A new boundary integral method for solving the general Helmholtz equation is developed. This new formulation is developed for the two-dimensional Helmholtz equation. The new formulation is based on the method of moments Laplacian solution. The main feature of this new formulation is that the boundary conditions are satisfied independent of the region node discretizations. The numerical solution of the present method is compared with finite difference and finite element solutions of the same problem.
Author(s): Tapan K. Sarkar, Young-seek Chung, Magdalena Salazar Palma
File Type: Journal Paper
Issue:Volume: 17      Number: 3      Year: 2002
Download Link:Click here to download PDF     File Size: 333 KB

Title: Splitting of Material Cells and Averaging Properties to Improve Accuracy of the FDTD Method at Interfaces
Abstract: In this paper we present a simple modification to the traditional Finite Difference Time Domain (FDTD) method for treating material cells. The Yee cell is split into 8 smaller material subcells so that each E and H field point is considered to be located at the crosspoint of 8 cells with differing material properties. Thus there is no longer an overlap of the material cells associated with the components of the E and H fields. The 8 material properties are averaged at each crosspoint. Since the averaging is done outside the time marching loop there is little increase in the total computational time. Numerical results are shown for a sinusoidal plane wave scattering from a dielectric sphere. These results are compared with the exact Mie solution and the traditional material cell method along different cuts through the sphere. The splitting and averaging is shown to give improved amplitude accuracy in the vicinity of the sphere. Improvement is also observed at planar interfaces angled with respect to the grid. An additional benefit of this subcell formulation is that objects may be modeled with twice the geometrical resolution without increasing the size of the staggered grid.
Author(s): R. S. Schechter, M. Kragalott, M. S. Kluskens, W. P. Pala
File Type: Journal Paper
Issue:Volume: 17      Number: 3      Year: 2002
Download Link:Click here to download PDF     File Size: 277 KB

Title: Mathieu functions of complex arguments and thier applications to the scattering by lossy elliptic cylinders
Abstract: The aim of this paper is to outline the theory for calculating the angular and radial Mathieu functions of complex arguments. These functions are required for the computation of analytic solutions of electromagnetic scattering by lossy dielectric elliptic cylinders and waveguides. The backscattered echo width of a lossy dielectric elliptic cylinder is compared with the special case of lossy circular and weakly lossy elliptic cylinders and excellent agreement is obtained in all cases. Tabulated and plotted numerical results of Mathieu functions are presented.
Author(s): Abdul-kadir Hamid, M.I. Hussein, H. A. Ragheb, M. Hamid
File Type: Journal Paper
Issue:Volume: 17      Number: 3      Year: 2002
Download Link:Click here to download PDF     File Size: 391 KB

Title: Reducing Electromagnetic Coupling in Shielded Enclosures using a Genetic Algorithm -- Finite-Difference Time-Domain Solver
Abstract: Comprehensive shielding in modern electronic equipment may lead to resonant behaviour within the equipment enclosure. This paper presents a method for optimising the placement of sources of electromagnetic (EM) energy and susceptors to this EM energy within an enclosed resonant cavity. The source and susceptor are placed on a dielectric slab within a perfectly conducting enclosure to reduce the level of EM coupling between the two. Optimisation is facilitated using a genetic algorithm coupled with a finite-difference time-domain solver. Results are presented for single objective optimisation and multi-objective optimisation cases.
Author(s): Russell Iain Macpherson, N J Ryan, R. I. Macpherson, N. J. Ryan
File Type: Journal Paper
Issue:Volume: 17      Number: 3      Year: 2002
Download Link:Click here to download PDF     File Size: 1130 KB

Title: COUPLING 2D FINITE ELEMENT MODELS AND CIRCUIT EQUATIONS USING A BOTTOM-UP METHODOLOGY
Abstract: The aim of this paper is to present an approach, able to deal with all possible connections of voltage and current sources and impedances, combining conductors in which the skin effect is taken into account and conductors in which skin effect is neglected. This approach is obtained using a bottom-up methodology. In this way, the meaning of terms in the generalized approach is naturally inherited from some specific problems. This model is presented in a compact form, preserving sparse, symmetric and positive-definiteness matrices. The vectors and matrices, computed during the solution stage, are employed in engineering calculations as current and inductance computations providing compact expressions suitable for efficient algorithms. Finally, the proposed approach, implemented in a FEM software package developed by the authors, is applied to the study of a three-phase transformer, supplied with a balanced three-phase voltage (sinusoidal and nonsinusoidal) and loaded with an unbalanced three-phase RL impedance. The agreement between the computed and experimental results shows the validity of the proposed model and its implementation.
Author(s): Emilio Gomez Lazaro, Jose Roger-Folch, Antonio Gabaldon Marin, Angel Molina Garcia
File Type: Journal Paper
Issue:Volume: 17      Number: 3      Year: 2002
Download Link:Click here to download PDF     File Size: 331 KB

Title: Use of the Simultaneous Diagonalization Technique in the Ax=lBx Eigenproblem Applied to the Computation of the Characteristic Modes
Abstract: Characteristic modes developed by Garbacz, Harrington and Mautz have long been used in the analysis of radiation and scattering from conducting bodies and apertures. For their computation, it is necessary to solve an eigensystem of the form Ax=lBx. If the matrices (A,B) are Hermitian and B is positive definite, the generalized eigenvalue problem can be accurately solved using the simultaneous diagonalization technique (SDT). Because of numerical approximations and rounding sometimes it may happen that the matrices properties deteriorate and the SDT procedure becomes inapplicable. In this work a new technique, developed recently by Higham and Cheng is proposed as a method to solve these deteriorate cases. It is applied to the computation of the characteristic modes for some scattering problems. Results are analyzed and discussed.
Author(s): Giovanni Angiulli, Francesca Venneri
File Type: Journal Paper
Issue:Volume: 17      Number: 3      Year: 2002
Download Link:Click here to download PDF     File Size: 152 KB

Title: FDTD Analysis of ELF Wave Propagation in Inhomogeneous Subionospheric Waveguide Models
Abstract: The space formed by the ground and ionosphere is known to act as a resonator for extremely low frequency (ELF) waves. Lightning discharges trigger this global resonance, which is known as Schumann resonances at the frequencies of 8, 14, 21Hz etc. Even though the inhomogeneity (like day-night asymmetry, local perturbation etc.) is important for such subionospheric ELF propagation, the previous analyses have been always made by some approximations because the problem is too complicated to be analyzed by exact full-wave analysis. This paper presents the first application of the conventional FDTD method to such subionospheric ELF wave propagation, in which any kinds of inhomogeneities can be included in the analysis, to be compared with the observational results. We show the application of FDTD to our problem and present a few numerical computational results to be compared with those by the pre-existing analysis method.
Author(s): M. Hayakawa, T. Otsuyama
File Type: Journal Paper
Issue:Volume: 17      Number: 3      Year: 2002
Download Link:Click here to download PDF     File Size: 602 KB