Winter 2006 Issue
Calculating Tropospheric-Scatter Propagation Losses
With the advent of Joe Taylor, K1JTís WSJT
software program it is possible to work distances of 700 km and more on a
regular basis on VHF and UHF using tropospheric scatter. Thus it is useful
to have an understanding of tropospheric-scatter losses so we can see what
is possible and understand the factors that affect these losses. A number
of methods of calculating tropospheric-scatter losses from the amateur
literature and Consultative Committee International Radio (CCIR) Report
238-51 have been applied, but they produce substantially different
answers. The CCIR methods are not easy to apply and this can lead to
errors. This paper aims to provide a better understanding of the
limitations of the various methods and concludes that CCIR method 1 is to
be preferred. Based on this method, look-up tables of propagation loss in
temperate climates are developed to make the method user friendly.
Tropospheric scatter arises from radio waves being scattered by small cells of different refractive index in the atmosphere. It allows signals to be detected at much greater range than line of sight or by diffraction around the Earth. Figure 1 shows the geometry of a tropospheric-scatter path.
Factors that affect the propagation loss are distance, frequency, and the scattering properties of the common scattering volume. The scattering properties vary with height and climate and the scattering angle. Both the scattering angle and the height of scattering vary as a function of distance due to Earth curvature and horizon obstructions. In addition, the effective Earth curvature is modified by the radio refractive index. All of these factors have an impact on the calculation of tropospheric-scatter losses. Simple methods of calculating tropospheric scatter take into account only distance and frequency, while others also include obstruction angles, climate, radio refractive index, and path reliability. There are also differences in the ways in which these factors are considered, with some being based on approximate empirical formulas and others on graphs derived from experimental results.
Figure 1. Geometry of a tropospheric scatter path where D (or d) is the distance between the stations, qS is the scattering angle, q1 and q2 are the horizon angles at each station, and a1 and a2 are the beamwidths of the antennas. (From ARRL UHF/Microwave Experimenterís Manual2)
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