Beyond-the-horizon point-to-point UHF radio systems

Propagation measurements made in recent years have indicated that the attenuation of UHF signals transmitted over beyond-the-horizon distances is substantially less than that predicted by diffraction theory. The observed signal levels are generally attributed to scattering or partial reflection from non-uniformities in the troposphere or lower atmosphere, Average losses in excess of free space of the order of 70 db are observed at distances of 200 miles beyond the horizon. Due to changes in the troposphere, the median received signals undergo variations about the average value of as great as db. In addition, severe rapid fading is usually present. Apparently modulation bandwidths of as great as one megacycle can be transmitted using antenna beamwidths of a few degrees. In order to overcome the large propagation losses, high gain antennas, high power transmitters, and sensitive receivers are used. Parabolic reflector antennas as large as 60 feet in diameter are employed together with high power klystron transmitters of up to 10 kw power output. Frequency modulation techniques similar to those employed in line-of-sight systems are common. The receivers use low noise-figure pre-amplifiers and wide-band fast acting limiter-discriminator circuits for operation with the weak received signals. To reduce the effects of the rapid fading, space diversity reception with either switching or combining of the receiver output signals is usually employed. Using these techniques, multichannel UHF communication systems have been designed for operation with as many as 60 voice channels over distances as great as 200 miles beyond the horizon with long-term reliabilities of 99.9% or more. The research reported in this paper was supported jointly by the Department of the Army, the Department of the Navy, and the Department of the Air Force under Air Force Contract No. AF 19