Revisiting medium-wave ground-system requirements

The classic 1937 paper by Brown, Lewis, and Epstein [1] outlined the necessity for a proper "imaging screen" or "ground system" for efficient operation of a vertically polarized monopole antenna. While Brown et al. showed the effects of various configurations, they did not exhaustively examine the effects of successive reductions of the number of radials, particularly in systems with radial lengths less than 90". Furthermore, their measurements were made at 3 MHz, and were not corrected for the frequency scaling. Recent experimental work, undertaken for a US Department of the Navy laboratory to optimize the performance of temporary medium-wave antenna installations, showed that for surface-mounted radial systems with radials 90" or shorter, both radial length and the number of radials are significant. Property availability for siting of antennas at medium-wave frequencies often results in truncation or other restrictions on radial length. The results of this study provide a guide to the practical limits on such restrictions, as well as data on the resulting efficiency reductions.