FIELD TEST OF AN/GRA-93() HF ANTENNA KIT FOR SHORT-PATH SKYWAVE COMMUNICATION IN THAILAND FOREST ENVIRONMENT.

Abstract : The results of a one-month field test of the AN/GRA-93() HF antenna kit in a Thailand forest and in an adjacent clearing for use on short (0 to 50 km) skywave paths are presented. Both relative gain and impedance were measured, and gain relative to an isotropic radiator was calculated. The kit configuration in the 2-6 MHz band having the greatest gain relative to an isotropic radiator was the inverted L (+0.6 dB) in the clearing at 6 MHz; the kit element having the least gain was the slant wire (-13 dB) in the forest at 4 MHz. The monopole, designed for ground-wave use, was down 20 to 35 dB from an isotropic radiator when employed on a short HF skywave path. None of the configurations in the kit exhibited as much gain at the zenith as the resonant center-fed, half-wave horizontal dipoles located in the clearing, which were used as reference antennas. A test at 4 MHz in the forest indicated that a resonant, center-fed, horizontal dipole may need to be raised to only half the height of the best kit element to launch an equivalent skywave at vertical incidence. The kit antennas were difficult to set up, requiring about one hour for a two-man team in the forest and over 30 minutes in the clearing.