The transmission data for 63 Mc and 170 Mc obtained by the U.S.N.E.L. on the San Diego coast on April 4, 1945, when a strong duct was formed near the ground, were analyzed by wave theory. The average observedM‐curve was fitted with a linear‐exponential term, and the electromagnetic field beyond the horizon was computed by the theory of normal modes. Good agreement between theory and experiment was obtained for both frequencies at a range of 32 miles (see Figs. 7 and 8). In the ranges between 32 and about 70 miles the observed variation of intensity with height agrees with theory, but the theoretical horizontal decrement is less than the observed by about 0.1 d.b./n. mile for 63 Mc, and by about 0.2 d.b./n. mile for 170 Mc. Beyond about 80 miles the observed field tends to assume a uniform distribution with height and shows little horizontal attenuation. Such an effect had been observed by the U.S.N.R.L. expedition to Antigua for 9‐cm waves under conditions of an oceanic surface duct. Its interpretation is still wanting.