Cullen M. Crain

Radar and communications antenna-siting for low-angle radiation at high frequencies

It is sometimes assumed that, in order to obtain good low-angle radio transmission and reception at HF, one must use vertical polarization and have an extensive horizontal highly conducting foreground, such as an ocean surface or an enormous ground screen. It is shown that, in the absence of such a surface, use may be made of inland sites having a sloping foreground combined with distant level terrain. The latter can be a fresh water surface. The angle of slope of inland sites should be about four times the minimum angle of elevation for which coverage is required. Coverage down to 1\deg therefore requires a slope of about 1 in 14. At a wavelength of 15 m, an antenna with a phase center about 30 m above the local terrain must be erected about 1.5 km up the slope from the location where the terrain becomes level. The sloping and level terrains must be fiat to an accuracy of better than about 20 m, and the level terrain, or fresh-water surface, must extend to a distance of about 15 km. All linear dimensions scale proportionally to the wavelength.