G.J. Burke

Numerical Electromagnetic Code (NEC)

The development and the application of the Numerical Electromagnetic Code (NEC) - Method of Moments are described. NEC is based on a previous method of moments code for thin wires, the Antenna Modeling Program (AMP), and yields improved estimates of the performance of antennas mounted on shore stations, ships, aircraft, and spacecraft. The NEC can model antennas in free space, over a perfectly conducting ground plane, and over finite conduction (lossy) earth.

Modeling antennas near to and penetrating a lossy interface

A technique for modeling wire objects interacting across or penetrating the planar interface which separates two half-spaces is described. The moment-method treatment is employed, based on the thin wire approximation to the electric-field integral equation, with the effect of the interface included via the Sommerfeld integrals. The computation time associated with evaluating the latter is substantially shortened by using an interpolation-based technique plus asymptotic field expressions. Although developed specifically for the wire problem, the procedure is also applicable, with slight modification, to modeling surface objects as well. Special care is taken to account for the charge discontinuity that occurs at the point a wire penetrates the interface. Example calculations are shown for a monopole antenna driven against ground stakes and simple ground screens, the fields of buried objects, and a simple electromagnetic pulse (EMP) simulator.

Accurate computation of wide-band response of electromagnetic systems utilizing narrow-band information

Cauchys technique for interpolating a rational function from samples of frequency responses and/or their derivatives is investigated. This technique can be used to speed up the numerical computations of parameters, including input impedance and RCS of any linear time-invariant electromagnetic system. This technique is utilized to find the far field of a slit conducting cylinder (TM incidence) over a bandwidth utilizing the information about the current and its derivatives at a few sample points. The numerical results are presented are in good agreement with exact computational data. This technique is a true interpolation/extrapolation technique as it provides the same defective result as the original electric field integral equation at a frequency which corresponds to the internal resonance of the closed structure. >

The Numerical Electromagnetics Code (NEC) - a brief history

The Numerical Electromagnetics Code, NEC as it is commonly known, continues to be one of the more widely used antenna modeling codes in existence. With several versions in use that reflect different levels of capability and availability, there are now 450 copies of NEC4 and 250 copies of NEC3 that have been distributed by Lawrence Livermore National Laboratory to a limited class of qualified recipients, and several hundred copies of NEC2 that had a recorded distribution by LLNL. These numbers do not account for numerous copies (perhaps 1000s) that were acquired through other means capitalizing on the open source code, the absence of distribution controls prior to NEC3 and the availability of versions on the Internet. We briefly review the history of the code. We show how it capitalized on the research of prominent contributors in the early days of computational electromagnetics, how a combination of events led to the tri-service-supported code development program that ultimately led to NEC and how it evolved to the present day product.

Low-frequency computational electromagnetics for antenna analysis

An overview of computational methods for modeling the low-frequency electromagnetic characteristics of antennas is given. Presented first is a brief analytical background that forms the basis for numerically solving low-frequency antenna problems using the method of moments. Next discussed are extensions to modeling perfectly conducting objects in free space, followed by a consideration of some computational issues that affect model accuracy, efficiency, and utility. A variety of representative applications is given to illustrate various aspects of modeling and capabilities that are currently available. A fairly extensive bibliography is included. >

Accurate computation of wideband response of electromagnetic systems utilizing narrowband information

Abstract The concept of analytical continuation is utilized to generate the electromagnetic response over a wide frequency band. The problem of interest is TM scattering from a conducting cylinder. A method of moment formulation is utilized to generate information about the current and its derivatives at a set of specific frequencies. This information is utilized to extrapolate the current at other frequencies. The numerical results presented are in good agreement with exact data. Two techniques, namely frequency derivative and Cauchys technique have been introduced for extrapolating and interpolating a rational function, respectively, from a few samples of frequency responses and their derivatives. This technique can be used to speed up the numerical computations of parameters like input impedance and RCS of any linear time-invariant system.

Use of Frequency-Derivative Information to Reconstruct the Scattered Electric Field of a Conducting Cylinder over a Wide Frequency Band

In this paper, the concept of analytical continuation is utilized to generate the electric field over a wide frequency band. The problem of interest is TM scattering from a conducting cylinder. A method of moments formulation is utilized to generate information about the current and its derivatives at a specific frequency. This information is utilized to extrapolate the current at other frequencies. The numerical results presented are in good agreement with exact data.

Recent improvements to the model for wire antennas in the code NEC

The author describes modifications of the numerical electromagnetics code (NEC) algorithm to improve the accuracy for wires with discontinuous radius and tightly coupled junctions and to avoid loss of precision in modeling electrically small antennas. To model wires with discontinuous radius, the current is now located on the wire surface with match points on the axis, reversing the previous convention. Openings in the surface at wire ends, voltage source gaps, and steps in radius are closed with radial caps so that the location of match points on the axis is justified under the extended boundary condition. A condition on the charge density at junctions is established by a small moment-method solution for the continuity of the scalar potential at each junction or change in radius. Steps to improve precision at low frequencies include changes in the evaluation of basis functions and field due to a segment, and a provision to use a constant basis and weighting function around selected small loops.<<ETX>>

Design of near-field irregular diffractive optical elements by use of a multiresolution direct binary search method

A multiresolution direct binary search iterative procedure is used to design small dielectric irregular diffractive optical elements that have subwavelength features and achieve near-field focusing below the diffraction limit. Designs with a single focus or with two foci, depending on wavelength or polarization, illustrate the possible functionalities available from the large number of degrees of freedom. These examples suggest that the concept of such elements may find applications in near-field lithography, wavelength-division multiplexing, spectral analysis, and polarization beam splitters.

Accuracy-modeling guidelines for integral-equation evaluation of thin-wire scattering structures

A numerical study to determine accuracy and modeling criteria for the integral-equation analysis of thin-wire scatterers is described. Results obtained using a collocation solution method with sinusoidal current interpolation show that 6-18 current samples per wavelength are sufficient to produce radar cross section results with absolute numerical convergence accuracies on the order of 10 percent or less ( \sim0.4 dB) depending upon structure complexity.