Propagation through an elevated duct: Tradewinds III

In July and August of 1960, a joint Naval Research Laboratory-Naval Electronics Laboratory radio-meteorological program (Project Tradewinds III) was carried out between San Diego, Calif., and Opana, Hawaii. The purpose of this project was to evaluate the propagation characteristics of the duct formed by the tradewinds inversion which is generally found in this region. Two frequencies, 220 Mc and 445 Mc, were used and data concerning signal strength vs range from each terminal, height gain data at several points in the path, as well as the gross meteorological conditions which prevailed over the path were taken. This report attempts both to correlate the radio propagation data with the observed meteorology and to formulate a mechanism to explain the presence in the duct of signal power transmitted from a point well below the duct. In general, cloud heights and types serve as indicators of the stability of the tradewinds inversion and can be correlated with the altitude and strength of the inversion and consequently of the duct. By using both ray and scatter theories, a relation is developed which may be used to determine the signal losses expected in transmission through an elevated duct in which either or both terminals are well below the duct.