Radio-wave propagation to the antipode

Abstract

A two-year (1960–1961) experiment was undertaken between Perth, Australia, and Bermuda to test the hypothesis of an antipodal focus for high-frequency radio waves.The transmitting and receiving sites were antipodal to within one degree in latitude and longitude. Three frequencies: 5, 16, and 30 MHz were utilized. Results indicate a definite intensity enhancement at the antipode, with best reception found for the center frequency. At the 48 percentile level, reception persisted for 24 h in December and 8 h in June. Reception at 5 and 30 MHz was mainly constrained to two periods a day of 3–5 h each, centered near ground sunrise and sunset. Reception at the antipode was superior to that at two other stations, each located about 1700 km closer to the transmitter. Characteristic changes in the signal intensity were marked around sunrise and evident around sunset. These included a presunrise minimum in percent reception (30–70 min prior to ground sunrise), followed by a steep ascent to a postsunrise maximum within 60 min after ground sunrise. Azimuths of arrival at 16 MHz avoided the sector ± 90° around the bearing of the subsolar point but favored those ± 90° around the bearing of the subsolar nadir point. The predominant sector of arrival rotated through 360° in 24 h. Reception was poorest over paths intersecting the polar caps and best over paths having the greatest extent over water. Few of the results could have been derived from the standard predictions for the most usable frequency.Because of the high intensities involved, it is believed that ionospheric ducting and superrefraction, in addition to some multihop propagation, are probable mechanisms.