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Title: Characterization, Comparison, and Validation of Electromagnetic Modeling Software
Abstract: The continuously increasing number of electromagnetic computer models (codes) and applications thereof is one result of a rapidly expanding computing resources base of exponentially growing capability. While the growing use of computers in electromagnetics attests to the value of computer modeling for solving problems of practical interest, the proliferation of codes and results being produced increases the need for their validation with respect ot both electromagnetic formulation and software implementation. But validation is perhaps the most difficult step in code development, espeically for those models intended for general-purpose application where they may be used in unpredictable or inappropriate ways. A procedure or protocol for validating codes both internally, where necessary but not always sufficient checks of a valid computation can be made, and externally, where independent results are used for this purpose, is needed. Ways of comparing differing computer models with respect to their efficiency and utility to make more relevant intercode comparisons and thereby provide a basis for code selection by users having particular problems to model are also needed. These issues are discussed in this article and some proposals are presented for characterizing, comparing, and validating EM modeling codes in ways most relevant to the end user.
Author(s): E. K. Miller
File Type: Journal Paper
Issue:Volume: 4      Number: 3      Year: 1989
Download Link:Click here to download PDF     File Size: 1546 KB

Title: Difficulties Encountered When Attempting to Validate Thin-Wire Formulations for Linear Dipole Antennas
Abstract: When analyzing wire antennas, the "thin-wire" kernel is often used as a convenient approximation to the exact singular kernel in the electric-field integral equation. In this paper, it is shown that the thin-wire kernel is a poor approximation to the true kernel, but its use does yield good values for input impedance over a significant range of parameters (e.g., wire radii, size of subsectional cells). The validity of the thin-wire kernel when used within the electric field integral equation appears to be due to the fact that the approach is often a good approximation to the Extended Boundary Condition (EBC) formulation. Although it is often implied in the literature that use of the thin-wire kernel will produce "convergent" values for input impedance, in actuality there is no guarantee that results improve as more cells are taken along the wire. Despite widespread use of the "thin-wire" kernel, there are inherent difficulties in the validation of codes based on this approximation.
Author(s): A. F. Peterson
File Type: Journal Paper
Issue:Volume: 4      Number: 3      Year: 1989
Download Link:Click here to download PDF     File Size: 913 KB

Title: Evaluation and Validation of the Method of Moments Code NEC2 in the Design of Log Periodic Dipole Arrays With Low Sidelobe Levels for Broadcast Applications
Abstract: The evaluation and validation of the Method of Moments Numerical Electromagnetics Code (NEC2) is undertaken by the comparison of measured and computed radiation patterns of Log Periodic Dipole Arrays. Emphasis is placed on the correspondence of the sidelobe levels -30 to -40 dB down from the major lobe. This paper is concerned with the evaluation of a single antenna in free space.
Author(s): D. C. Baker, T. G. Reuss
File Type: Journal Paper
Issue:Volume: 4      Number: 3      Year: 1989
Download Link:Click here to download PDF     File Size: 504 KB

Title: Moment Method Analysis of Radiation and Scattering by Thin Wires in an Infinite Dissipative Medium
Abstract: In this report, a modified version of AMP (Antenna Modelling Program), a forerunner of NEC (Numerical Electromagnetics Code), is validated for applications involving thin wire structures immersed in an infinite dissipative medium. A Pascal derivative of MiniNEC2, the Mini Numerical Electromagnetics Code, is also used to provide numerical comparison as needed when other data are not available. Computations from both programs are compared with analytical and pre-existing experimental results. Drive point impedances, currents and near fields for a range of thin wire radiators and scatterers are presented. Useful results are obtained for the highest conductivity case tested and for less conductive media, agreement of computed parameters with available comparative data of approximately 10% or better is achieved.
Author(s): A.H.J. Fleming, K.H. Joyner
File Type: Journal Paper
Issue:Volume: 4      Number: 3      Year: 1989
Download Link:Click here to download PDF     File Size: 907 KB

Title: A Comparison of Two Eurpean Reflector Antenna Codes
Abstract: Two computer programs for the analysis of reflector antenna are evaluated for the purposes of establishing their general capabilities and, more specifically, accuracy and computational efficiency. Both programs were used to analyse a number of test cases and the results obtained are compared.
Author(s): I.M. Roberts, R. Jorgensen, S.D. Hayward
File Type: Journal Paper
Issue:Volume: 4      Number: 3      Year: 1989
Download Link:Click here to download PDF     File Size: 840 KB

Title: Comparison of Electromagnetic Scattering Solutions Using the Disk as a Target
Abstract: Results are compared for scattering from a perfectly conducting circular disk using three solutions: A closed form physical optics (PO) solution, a Physical Theory of Diffraction (PTD) solution, and a computer implementation of an exact eigenfunction solution. The far field patterns are compared for various ka values as well and incidence angles for both vertically and horizontally polarized electric fields. For normal incidence, the solutions agree very well. For small ka the agreement is remarkable. As ka increases, the agreement is still good as long as the observation angle, theta, does not exceed 45 degrees. The applicability of using the disk as a target to calibrate an RCS measurement range is discussed. The disk calibration curve is a plot of the specular return as a function of bistatic angle. This curve is computed for both horizontal and vertical polarizations and various ka values using the PO and eigenfunction solutions. The specular bistatic RCS (calibration curve) is relatively invariant; hence, the disk is an ideal candidate as a calibration standard for bistatic RCS measurements. Agreement between the PO and eigenfunction solutions for large ka means there is an easy method of generating the applicable calibration curve for a particular disk as long as one remains within the valid angular region for a proper size disk.
Author(s): K.D. Trott
File Type: Journal Paper
Issue:Volume: 4      Number: 3      Year: 1989
Download Link:Click here to download PDF     File Size: 735 KB

Title: Validation of a Z-Matrix Finite Element Program for Analyzing Electrostatic Fields
Abstract: Modern electrical equipment designs demand more compact, complex geometries and require more strigent analyses of the associated three-dimensional electrostatic fields. Analytical solutions of such fields are limited to relatively simple geometries. Numerical solutions using the finite element method are limited by available computer storage and processing time. Lauber posed an economical method for solving three-dimensional finite element electrostatic fields by forming the finite element system matrix using Brown's Z-matrix techniques. (Lauber, 1982) (Brown, 1975) This method was implemented by a FORTRAN computer program. (Barber and Lauber, 1986) (Barber, 1988) The purpose of this paper is to briefly describe the present version of the program called FEWZ (Finite Elements using a Windown and the Z-matrix) and to show how it was validated.
Author(s): P.F. Barber, T.S. Lauber, E.D. Patterson
File Type: Journal Paper
Issue:Volume: 4      Number: 3      Year: 1989
Download Link:Click here to download PDF     File Size: 945 KB

Title: The Comparison of a Time-Domain Numerical Code (DOTIG1) with Several Frequency-Domain Codes Applied to the Case of Scattering From a Thin-Wire Cross
Abstract: In this paper we compare, via Fourier transform, results obtained using the DOTG1 time-domain numerical code with those obtained using several frequency-domain thin-wire codes [1] and with the experimental measurements obtained by Burton [2], specifically applied to the scattering from a cross.
Author(s): A.R. Bretones, A. Salinas, R. Gomez Martin, A. Perez
File Type: Journal Paper
Issue:Volume: 4      Number: 3      Year: 1989
Download Link:Click here to download PDF     File Size: 307 KB

Title: The Validationof EM Modelling Codes. A User Viewpoint
Abstract: Computer codes for solving radiation and scattering problems have becomed powerful and widely available. The user of such a code must initially convince himself that his copy of the code executes correctly on his specific computer with his particular compiler. He must then build up expertise in constructing models for solution by the code which obey the restrictions imposed by the "modelling guildelines" accompanying the code in a user's manual. Users often want to solve "real" problems that do not conform to the problem geometry envisaged by the code writer. The user replaces the "real" structure with a computer model solvable by the code, and which obeys the modelling guidelines. The user must then carry out a "model validation" in which the computer model is tested against full-scale or scale model measurements. A successful "model validation" contributes to the user community's "experience base" and lends confidence to both the computer code and to the modelling process. Sometimes an unsuccessful attempt at modeling exposes a genuine limitation of the code. Then a new "modelling guideline" can be formulated to aid other users in avoiding the same difficulties. This paper reviews code development to highlight the origin of "modelling guidelines", and how they are extended by the user community. From the user's point of view, the "experience base" is augmented whenever a successful "model validation" is carried out and reported. Several examples are presented of the difficulties that may be encountered in computer modelling, and how such difficulties lead to further "modelling guidelines" aimed at aiding others in solving similar problems.
Author(s): S.J. Kubina, C.W. Trueman
File Type: Journal Paper
Issue:Volume: 4      Number: 3      Year: 1989
Download Link:Click here to download PDF     File Size: 731 KB

Title: Wire Grid Moment Method (NEC) Models of a Patch Antenna
Abstract: Wire grid models of an air-loaded resonant circular patch antenna are used to calculate several observable quantities. The resonant frequency, input reactance, and bandwidth are accurately calculated, but the input resistance is underestimated by the grid models due to the existence of separate currents on both the top and bottom of the measured solid patch. The Lawrence Livermore (NEC3) Method of Moments computer program was used to generage the grid models. Averge wire lengths of about 0.03 wavelengths are required for accurate results. A comparison with other studies suggests that the 0.03 wavelength segmentation may be required because the patch is close to the ground plane (0.0175 wavelengths), and/or because errors in the near field calculation are compounded by the large fraction of energy stored in the near field since the patch is a high Q resonator.
Author(s): P.G. Elliot, S.T. McBride
File Type: Journal Paper
Issue:Volume: 4      Number: 3      Year: 1989
Download Link:Click here to download PDF     File Size: 703 KB

Title: What Do You Mean By a Solution to an Operator Equation?
Abstract: The main thrust of this presentation is to illustrate that for many electromagnetic field problems the quality of the solution is very subjective and not objective at all. Therefore to classify solutions on a subjective criteria which is not scientific will create more problems than it would solve. The underlying feature to all this is what do "we mean" by a solution.
Author(s): T.K. Sarkar, E. Arvas
File Type: Journal Paper
Issue:Volume: 4      Number: 3      Year: 1989
Download Link:Click here to download PDF     File Size: 333 KB