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As electromagnetic waves, and in this case, radio signals travel, they interact with objects and the media in which they travel. As they do this the radio signals can be reflected, refracted or diffracted. These interactions cause the radio signals to change direction, and to reach areas which would not be possible if the radio signals travelled in a direct line.
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.
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.