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configuration and be prepared to move or elevate the antennas.
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
transmitting antenna, receiving antenna, or both 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 below. (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.
