The modeling and measurement of HF antenna skywave radiation patterns 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).<>