Result of Bandwidth Tests on the 185-Mile Florida-Cuba Tropospheric Scatter Radio System

Some proposed tropospheric scatter paths are several hundred miles long, and their geometry suggests the possibility that the increased relative delays of signal components traveling the higher-altitude paths could result in significant interchannel modulation interference in an FM system. Therefore, a program of experimental investigation has been undertaken by Bell Telephone Laboratories to determine the extent of the intermodulation problem. Tone amplitude stability tests have been included as part of the study. The first in a planned series of tests was conducted during March, 1960, over the 185-mile FM system between Florida City, Fla. and Guanabo, Cuba. The system was loaded with random noise of various bandwidths up to 2.5 Mc, and peak deviations up to ±8 Mc were used. Tests were made with no diversity and with dual diversity. It was found that intermodulation, resulting from multipath propagation can _limit system performance. Both deviation and base bandwidth are important and have an influence on intermodulation and tone stability. Diversity has little effect on intermodulation but improves tone stability appreciably. The paper presents numerical results of the measurements of average intermodulation and some information on cumulative probability distributions for various combinations of base bandwidth, deviation, and diversity.