The effect of the oceanic duct on microwave propagation

A body of experimental radio and meteorological data taken over Cardigan Bay, on the British west coast, is examined to ascertain how radar propagation is affected by the microwave duct lying next to the sea surface. The boundary‐layer meteorological profiles are analyzed in terms of a semi‐empirical relation due to E. L. Deacon, and propagation is predicted according to a method of S. A. Schelkunoff. At three‐cm wave‐lengths, it is found that the agreement between observation and theory becomes steadily better as wind speed increases, until at speeds above 15 mi/hr (7 m/s) the consistency is truly striking. At lower wind speeds there is a definite tendency to follow the theoretical pattern, but a great deal of scatter is noted. Possible explanations for this scatter are offered. More scatter is observed at nine cm than at three cm, and the transition to trapping conditions is less definite. The nine‐cm waves appear to “feel” the effects of the duct some time before trapping occurs. An interpretation is offered of the fact noted by McPetrie and Starnecki that, in general, high signals are associated with large positive air‐sea temperature differences, while low signals are accompanied by large negative differences. Also, Jones' observation that, under unstable conditions, high winds are accompanied by high signals is nicely explained.