Signal Fluctuations in Long-Range Overwater Propagation

Measurements of overwater propagation in the Pacific on 3 and 9 cm between airborne transmitters and fixed receivers are reported. Within the horizon, the direct and surface-reflected rays formed the usual interference patterns. Beyond the horizon, the 3-cm signal always behaved in general accord with normal-mode theory, the attenuation rate averaging 0.5 plus or minus 0.2 db per (nautical) mile. In roughly the first 30 miles past the horizon, the 9-cm signal had a heightgain and an exponential decay rate (0.9 to 1.9 db per mile), both reasonably dependent on measured duct strength. On 9 cm only, all greater ranges comprised a "turbulent region," where a new, lower attenuation rate prevailed, averaging 0.17 plus or minus 0.05 db per mile, and where there was no average height-gain, turbulent-region signals were approximately Rayleigh-distributed and the autocorrelation of their fluctuation vanished in at most 1.5 seconds. The turbulent-region signal undoubtedly was due to scattering by some highaltitude atmospheric mechanism. The surf in the foreground of the receiving tower, effectively forming an extension of the antennas, produced a periodicity in the records and other behavior suggesting the importance of foreground in diversity measurements.