Winter 2006 Issue

Calculating Tropospheric-Scatter Propagation Losses

This article was originally presented as a paper at GippsTech 2005, the Annual Australian Conference designed to encourage participation in VHF, UHF, and Microwave amateur radio operations. The name GippsTech is a hybrid name for the technical conference being held at the Gippsland campus of Monash University. Gippsland is a region within the state of Victoria.

By Rex Moncur, VK7MO

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

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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