Afar AR24027, Operator'S Manual

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AR24027 Operator’s Manual
6-5
6.3.2 Radio Horizon (Maximum Line-of-Sight Range)
In visual terms, the horizon is the point in the distance where an object drops out of sight because it is
blocked by the earth’s curvature. If the observer or object is elevated, the visual horizon is extended,
that is, the object can be seen at a greater distance before it drops out of view.
The same concept applies to radio signals: The radio horizon is the point in the distance where the
path between two antennas is blocked by the curvature of the earth. Like the visual horizon, the radio
horizon can be extended by elevating the antennas to extend communication range.
The radio horizon can also be extended or shortened by certain phenomena such as refraction due to
atmospheric density and temperature inversions. Fog and rain, which reduce signal strength, can also
shorten the radio horizon although in the ISM band, this loss is negligible.
A reasonable approximation of the radio horizon based on antenna height can be obtained from the
graph in Figure 6.2 . (Note that this graph does not take atmospheric effects into account.) To use the
graph, set a straight edge so that it crosses the height of one of the antennas in the column on the left
and the height of the other antenna in the column on the right. The radio horizon in miles/km is
shown where the straight edge crosses the center column.
If the radio horizon is well within maximum communication range of the system, this graph provides
a reasonable guide for antenna height. However, as maximum range of the system is approached,
results are less reliable because of atmospheric effects and other unpredictable phenomena. In such
cases, the more thorough point-to-point path analysis described in the next section should provide
more reliable results.