3- and 9-Centimeter Propagation in Low Ocean Ducts

One-way radio propagation measurements on 9 and 3 centimeters between ship and shore, coupled with meteorological measurements on ship and ashore, were made in the Atlantic tradewind area off the east coast of Antigua, British West Indies, early in 1945. Persistent low-level ducts, averaging 20 to 50 feet in height, with an effective strength of 5 to 10 M units, were found to exist all the time. The height and strength of the duct appears to depend on the wind speed, low winds producing low ducts of moderate strength while higher winds result in higher but weaker ducts. Various antenna-height combinations were explored to determine the optimum heights for utilization of the duct. Very effective trapping was found on 3 centimeters, the optimum height being between 6 and 15 feet, depending on duct conditions. On 9 centimeters the degree of trapping was only partial, and the strongest signals were obtained with the highest heights available (46 feet transmitting and 94 feet receiving). Rates of decrease of signal averaged 0.85 decibel per nautical mile on 9 centimeters up to about 80 miles, and 0.45 decibel per nautical mile on 3 centimeters, for all ranges. Beyond 80 miles the rate of decrease of signal on 9 centimeters was much less, about 0.2 decibel per nautical mile. Rain squalls had no observable effect on the strength of received signals.