THE MODELING AND MEASUREMENT OF HF ANTENNA SKYWAVE RADIATION-PATTERNSIN IRREGULAR TERRAIN

The Method of Moments (MoM) was used in conjunction with the Geometric Theory of Diffraction (GTD) for predicting the elevation-plane radiat ion patterns of simple high-frequency (HF) vertical monopoles and hori zontal dipoles situated in irregular terrain. The three-dimensional te rrain was approximated by seven connected flat plates that were very w ide relative to the largest wavelength of interest. The plate length a long the terrain profile was the longest possible that still adequatel y followed the shape of the path on the azimuth of the elevation patte rn of interest and no shorter than 1 wavelength at the lowest frequenc y of interest. The MoM model was used to determine the antenna current s 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 determi ne the gain versus elevation angle of the antennas for HF skywave when situated in the irregular terrain. The surface wave solution for grou ndwave was not included since this does not appreciably contribute any effect to the skywave far-field patterns at HF in this case. The mode l predictions were made using perfect electric conducting (PEC) plates and using thin plates made, of lossy dielectric material with the sam e conductivity and relative permittivity as measured for the soil. The se computed results were compared with experimental elevation-plane pa ttern data obtained using a single-frequency helicopter-borne beacon t ransmitter towed on a long dielectric rope in the far field on a linea r path directly over the antennas. The monopoles and dipoles were situ ated in front of, on top of, and behind a hill whose elevation above t he 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 hig hest measurement frequency (27 MHz). The results for the PEC plates an d the lossy dielectric plates were essentially identical for the horiz ontal dipoles, whereas the lossy dielectric plates were required to pr operly match the measured results for the vertical monopoles. The gain of the antennas in irregular terrain and the gain of the same antenna s situated in flat, open terrain differed by up to 20 dB at the lower elevation angles (eg., 3-degrees-5-degrees). This difference in gain i s significant for most HF systems.