Edmund K. Miller

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.

The admittance of the infinite cylindrical antenna immersed in a lossy, compressible plasma

Some numerical results obtained from an analysis of the admittance of an infinite cylindrical antenna excited at a circumferential gap of finite thickness and immersed in a lossy, compressible (warm) plasma are given. The linearized hydrodynamic equations are used for the electrons (ion motion is neglected). A free-space layer, or vacuum sheath, is used to approximate the ion sheath which forms about an object at floating potential in a nonzero temperature plasma. Values for the antenna admittance are obtained by a direct numerical integration of the Fourier integral for the current, and are presented as a function of frequency for plasma parameter values typical of the E region of the ionosphere. The admittance exhibits a maximum below the plasma frequency unless the electron temperature and sheath thickness are both zero; however, above the plasma frequency, the sheath and electron temperature have relatively little effect on the antenna admittance. The nonzero plasma temperature considerably enhances the antenna conductance below the plasma frequency compared with the zero-temperature case while at the same time reducing the dependence of the conductance on the electron collision frequency. A susceptance zero the location of which is not sensitive to the vacuum sheath thickness occurs near the plasma frequency.

Root-mean-square measure of nonlinear effects to the transient response of thin wires

A root-mean-square (rms) measure of effect of nonlinear loading on the transient response of thin wires is proposed. The transient behavior of nonlinearly loaded wires is analyzed directly in the time domain. The problem is formulated via the space-time Hallen integral equation. The equation is solved by the space-time Galerkin Bubnov boundary element procedure. Numerical results for the transient response of a thin wire computed by a time domain code based on this method are compared with results obtained from a frequency domain code. Some illustrative numerical results for the spatial distribution of the rms values of time varying currents are also presented.

An approximate formula for the admittance of a long, thin antenna

An approximate formula is presented for the admittance of a cylindricul antenna where the length is large and the diameter is small compared with the excited wavelength. A comparison of admittance results obtained from this formula is made with the more accurate results of Hurd and Wu.

Admittance of an Inhomogeneously Sheathed Infinite Cylindrical Antenna Immersed in an Isotropic, Compressible Plasma

The admittance of an infinite cylindrical antenna excited at a circumferential gap of nonzero thickness and immersed in a lossy, isotropic compressible plasma medium is obtained from numerical calculations, taking into account the inhomogeneous ion sheath which forms about a body at floating potential in a warm plasma. The admittance for the inhomogeneous sheath is found to be fairly similar to that for the sheathless case, with the exception of a rather sharp maximum or resonance in the admittance just below the plasma frequency which results from the sheath inhomogeneity.

Development of the Incremental IEMF Method to Validate FARS (Far-field Analysis of Radiation Sources)

FARS is reviewed here, and a procedure denoted as the Incremental Induced Electromotive Force (IIEMF) method is presented to prove the validity of FARS. An extension of the classical IEMF method, IIEMF involves determining the pairwise near-field power interaction between current samples on a PEC (perfect electrically conducting) object or a specified current. The IIEMF method not only validates FARS, but also provides an alternate method for determining the spatial distribution of radiation originating from such sources. The IIEMF formulation is based on the dyadic Greens function. Some representative numerical results are included, to demonstrate and compare the two techniques.

A fuzzy neural-network-model for aspect-independent target identification

A neural network is trained, using the fundamental properties of fuzzy-set theory, to achieve robust aspect-independent radar target identification. The radar cross section of two different aircraft are modeled using a thin-wire-time-domain (TWTD) code to compute their backscattered electric fields for twenty five different aspect angles. The scattered fields corresponding to a few aspect angles are then used to train the network and the rest of the scattered fields are used to test the performance of a neural network for target identification. A fuzzy neural network is found to provide superior performance for target identification compared with both a conventional neural network and a statistical Bayes classifier, especially in a noisy environment.<<ETX>>

On ionospheric composition measurements with an R.F. impedance probe.

Abstract Consideration is given to extending various methods used for determining electron number density in the ionosphere to ion composition measurements. A technique involving the impedance of an antenna operated in a frequency range on the order of the ion cyclotron frequency to the electron cyclotron frequency appears promising. Theoretical impedance results for some idealized combinations of ions as well as typical ion mixtures encountered in the ionosphere illustrate application of the technique. The advantages and limitations of the impedance probe are discussed.

Using Prony's Method To Synthesize Discrete Arrays for Prescribed Source Distributions and Exponentiated Patterns

Antenna pattern synthesis remains an important topic in electromagnetics. One goal of continuing interest has been that of developing an array of discrete sources that replicate the radiation pattern of continuous sources such as an aperture radiator or some current distribution. Another is to determine a discrete array that produces a prescribed radiation pattern. This paper outlines a procedure based on Pronys method for finding discrete-source arrays to accomplish these goals. A variety of problems are presented to demonstrate the performance of Prony pattern synthesis. In addition, discussed is the use of singular-value analysis to establish the rank of a given pattern, the synthesis of arrays that produce exponentiated patterns, and the effect of noisy source strengths on the realized patterns.

Infinite cylindrical antenna in a uniaxial compressible plasma

Numerical values are given for the admittance of an infinite cylindrical antenna in a uniaxial plasma, taking into account both plasma compressibility and a vacuum sheath. The susceptance is found to exhibit no significant sheath or compressibility dependence above the plasma frequency (ƒp) and only a slight dependence on these factors below ƒp. The conductance behavior shows no significant influence of compressibility or sheath in the range centered about ƒp, in which it is relatively constant in value except for a slight minimum at ƒp.