Ionospheric Absorption of Radio Signals
- an overview of absorption or loss of radio signals in the ionosphere,
particularly by the D layer - an essential element when planning a radio
communications network and predicting HF propagation.
The behaviour of the ionosphere is one of the key areas to
consider when planning a radio communications network or system, or when
predicting HF propagation conditions.
The ionosphere is usually thought of as an area where radio
waves on the short wave bands are refracted or reflected back to Earth. However
it is also found that signals are reduced in strength or attenuated as they pass
through this area. In fact ionospheric absorption can be one of the major
contributors to the reduction in strength of signals.
Most of the attenuation occurs in the D layer. There is some
in the E and F layers, but the level is very much less than that experienced in
the D layer and it can generally be ignored.
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When signals enter the D layer they transfer energy to the
electrons and set them in motion, vibrating in line with the radio signal. As
the electrons vibrate in this manner they can collide with other molecule, ions,
or electrons. Each time a collision occurs a small amount of energy is
dissipated, and this is manifested as a loss in the strength of the signal.
The amount of energy that is lost depends primarily upon the
number of collisions that take place. In turn this also is dependent upon a
number of other factors. The first is the number of other molecules, electrons
and ions that are present. In the D layer the density of the air is relatively
high, and so there are a large number of other molecules around and the number
of collisions is high. The second factor is the frequency of the signal. As the
frequency is decreased, so the displacement of the vibrations increases and so
does the number of collisions. In fact it is found that the amount of
ionospheric absorption that occurs varies inversely as the square of the
frequency. In other words if the frequency is doubled, then the attenuation will
fall by a factor of four. This is one of the major reasons why when a number of
bands or frequencies will support HF propagation between two radio stations,
then the highest one will yield the better results. It is also found that the
level of attenuation is so high for signals on the medium wave radio broadcast
band that during the day when the D layer is present, no signals through it, and
signals are only propagated via the ground wave. At night when the D layer
disappears, signals are heard from much further afield.
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