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Troposcatter Propagation: from an article of G3YGF
The Mechanism of Troposcatter
Troposcatter uses the weak, but reliable, reflections that can be obtained from the dust particles, clouds and refractive index variations that occur in the atmosphere 1000 to 50,000 ft above sea level. This mechanism can be used for working DX reliably over distances of many hundreds of miles. A brief illustration of the relevant part of the atmosphere is shown in the picture.
Air density decreases with height, and reaches one-third of its sea level value at about 30,000 ft. The refractive index of the atmosphere depends on such properties as temperature, density (pressure), humidity or the presence of water. Variations in any of these properties can scatter the signals. The scattering process is more efficient at lower altitudes where the atmosphere is denser. Turbulence associated with the weather can have marked effects on the signal levels and characteristics. In practice, this mechanism is used by pointing both antennas along the great circle path between the two stations at as low an angle of elevation as possible. The two beams will intersect in a common volume of the atmosphere near the center of the path See the picture
Propagation will be line-of-sight to the common volume from the transmitter. A very small fraction of the Power passing through this volume will then be scattered in all directions by the irregularities in the atmosphere. This power then propagates by line-of-sight to the receiver. The height of the bottom of this scattering volume will depend on the path length, and to some extent on the horizons of the sites, but will be typically 2000 ft on a 60-mile path, and 30,000 ft on a 300-mile path. The loss in the scattering process is usually so large that the equipment is unlikely to have enough spare capability to overcome the extra losses introduced by any additional obstructions in the path. The path loss increases by about 10 dB for every degree of horizontal angle at each station. On paths over 60 miles the increase is about 9 dB for every extra 60 miles of path length. The choice of a site with a good horizon is vitally important; it can make a difference of several hundred miles in the obtainable range.