AM broadcast antennas with elevated radial ground systems

Results are presented of computer modeling studies indicating that an elevated vertical monopole antenna with four elevated horizontal radials produces more ground-wave field strength than does a conventional ground-mounted monopole with 120 buried radials. For the analysis, the length of the radials and the height of the monopole were set equal to 0.25 of the free-space wavelength, and the frequency of operation was fixed at 1.0 MHz. Three different sets of ground constants were used, simulating averaging, very good, and very bad soil electrical parameters in accordance with the Numerical Electrical Code (NEC-GS)     

Lawrence Livermore National Laboratory Electromagnetic Measurement Facility

Lawrence Livermore National Laboratory recently improved its electromagnetic measurement capability through the installation of a precision broad-band monocone antenna, shrouded by EM absorbers, at its Transient Range Facility. This paper provides a brief history of the facility, describes the current measurement systems, and discusses the improvements observed.     

Some computational aspects of Pocklington's electric field integralequation for thin wires

A procedure is presented for extracting the singularity from the kernel in Pocklington's electric field integral equation for thin wires. The modified kernel which results may be used to dramatically improve the computational efficiency of certain moment method formulations. A simple example is considered in which pulse basis functions and point matching are used. For this case, a convenient approximation of the self-impedance term is also derived     

A Simple Interior Decomposition Model: Its Development And Application

A simple interior decomposition model composed of the product of two terms, a shielding-effectiveness term and a coupling- effectiveness term, has been developed that bounds the measured wire responses within a simple generic test object. A variety of modest to strong perturbations to the test object geometry and composition changed the response trend downward by at most 12 dB and typically only by 3-6 dB. Thus the model, called shielding effectiveness × coupling effectiveness (SEXCE), serves as a reasonable characterization of a broad range of generic geometries. It is useful in understanding the interior coupling response behavior and can be used as a predictive tool.     

The modeling and measurement of HF antenna skywave radiationpatterns in irregular terrain

The method of moments (MoM) was used in conjunction with the geometric theory of diffraction (GTD) for predicting the elevation-plane radiation patterns of simple high-frequency (HF) vertical monopoles and horizontal dipoles situated in irregular terrain. The three-dimensional terrain was approximated by seven connected flat plates that were very wide relative to the largest wavelength of interest. The plate length along the terrain profile was the longest possible that still adequately followed the shape of the path on the azimuth of the elevation pattern of interest and no shorter than 1 wavelength at the lowest frequency of interest. The MoM model was used to determine the antenna currents under the assumption that the terrain was planar (i.e., locally flat) over the distance pertinent to establishing the input impedance. The currents thus derived were used as inputs to the GTD model to determine the gain versus elevation angle of the antennas for HF skywave when situated in the irregular terrain. The surface wave solution for groundwave was not included since this does not appreciably contribute any effect to the skywave far-field patterns at HF in this case. The model predictions were made using perfect electric conducting (PEC) plates and using thin plates made of lossy dielectric material with the same conductivity and relative permittivity as measured for the soil. These computed results were compared with experimental elevation-plane pattern data obtained using a single-frequency helicopter-borne beacon transmitter towed on a long dielectric rope in the far field on a linear path directly over the antennas. The monopoles and dipoles were situated in front of, on top of, and behind a hill whose elevation above the flat surrounding terrain was about 45 m. The patterns of all of the antenna types and sitings exhibited diffraction effects caused by the irregular terrain, with the largest effects being observed at the highest measurement frequency (27 MHz)     

Using elevated radials in conjunction with deterioratedburied-radial ground systems

Computer-modeling exercises performed indicate that it is possible to add a group of four elevated radials to a pre-existing conventional AM-broadcast vertical monopole antenna system in order to restore performance to its original level (or better) in situations where the buried-radial ground system has been damaged. A wide variety of different configurations for the elevated radials are examined, including variations in orientation, length, and height above ground. It is stressed that extensive outdoor testing should be performed in order to verify these computer predictions. The computer software used is the Numerical Electromagnetics Code     

An exact solution of the generalized exponential integral and itsapplication to moment method formulations

The generalized exponential integral is one of the most fundamental integrals in antenna theory and for many years exact solutions to this integral have been sought. This paper considers an exact solution to the generalized exponential integral which is completely general and independent of the usual restrictions involving the wavelength, field point distance and dipole length is considered. The exact series representation presented converges rapidly in the induction and near-field regions of the antenna, and therefore provides an alternative to numerical integration. Two method of moments formulations are considered. They use the exact expression for the generalized exponential integral in the computation of the impedance matrix elements. It is demonstrated that, for very thin straight-wire antennas, an asymptotic expansion can be used to obtain a numerically convenient form of the generalized exponential integral