Wave Theoretical Interpretation of Propagation of 10-Centimeter and 3-Centimeter Waves in Low-Level Ocean Ducts

The data on the transmission of 10- and 3-centimeter waves in low-level ocean ducts obtained by the Naval Research Laboratory expedition to Antigua, British West Indies, were analyzed by wave theory. The analysis was made for a distribution of modified index of refraction with height (shown by curve C′ in Fig. 1). Below the horizon, the 10-centimeter wave was found to propagate by the first normal mode with a theoretical decrement of 1 decibel per nautical mile, as against an observed value of about 0.8 decibel per nautical mile in the first 80 miles from transmitter. Theory vezified the observed constancy of decrement with height for this wavelength. Beyond 80 miles the observed rate of attenuation dropped to a low value of 0.2 decibel per nautical mile. This change of slope in the intensity curve is probably due to the emergence of scattered radiation after the direct diffracted beam had been depleted. With the exception of one point there is quantitative agreement between the observed and theoretical distribution of intensity with height (see Fig. 9) for the 10-centimeter wave. The 3-centimeter wave was found to propagate below the horizon by the first and second modes, with theoretical decrements of zero and 0.5 decibel per nautical mile, respectively. The latter agrees with the observed values at high elevations, but near the surface, where theoretically attenuation should be negligible, the observed rate of attenuation exceeds the theoretical value by about 0.3 decibel per nautical mile.