Investigation of Long-Distance Overwater Tropospheric Propagation at 400 MC

The results of an investigation of overwater tropospheric propagation under both summer (July, 1955) and winter (February, 1956) conditions are presented. Transmissions were from a point on the south shore of Massachusetts, near New Bedford, to a ship traveling along great circle courses to a maximum distance of 630 nautical miles (724 statute miles) from the transmitter. A 10-kw, 385.5-mc transmitter feeding a 28-foot paraboloid antenna was used for the summer phase. For the winter investigation, this same facility was used, supplemented by a 40-kw transmitter feeding a 60-foot paraboloid for use at the greater distances. The frequency used for the winter was 412.85 mc. The receiving antenna aboard the ship was a 17-foot paraboloid for both series of tests. All antennas were horizontally polarized and approximately 100 feet above sea level. The data obtained are presented to show the median path loss vs distance. The strip chart recordings of the received signal levels were analyzed with respect to fading characteristics in an effort to separate out those transmissions which were enhanced by superrefractive conditions. The fast fading signals, which were well represented by the Rayleigh distribution, were assumed to be unaffected by superrefractive conditions. The data for the fast-fading Rayleigh type signals appear to show a cyclic variation of the attenuation rate with distance although there is no substantial deviation from a linear rate of between 0.16 and 0.18 db per nautical mile.