Main page This is the most commonly experienced variation in ionospheric propagation. There are several causes of fading including:
Multipath
When a signal is propagated via the ionosphere it is possible for it to do so via a number of paths. When many signals arrive at the receiver, they may be slightly delayed as a result of slight differences in the path taken. These delays can be up to 10 milliseconds in duration. The impact of multipath is to superpose radio waves at the receiver's aerial; if two waves are displaced by an integer number of wavelengths they tend to add together and reinforce the signal, if they are a half-integer number of wavelengths apart they tend to cancel and produce a fade. This has a number effects on radio reception including frequency selective fading (which is often heard as intense distortion when the carrier on a double sideband AM transmission has faded); digital signals suffer inter-symbol interference; and WEFAX images have a smeared appearance.
Sudden ionospheric disturbances
Intense solar activity, such as from solar flares, can cause a marked increase in the level of ionisation present in the ionosphere. This is because a lot of particles and radiation can be released into space by the Sun at these times and it can cause increased ionisation in the D layer where a lot of radio frequency energy is absorbed. The result is a deep fade over a wide frequency range which can last from a few minutes to a few hours.
Proximity to LUF and MUF
Two commonly referred to terms are maximum usable frequency (MUF) and lowest usable frequency (LUF). These define the useful range of radio frequencies that will be supported between two locations for a given time. Above the MUF, signals are not refracted enough to return to Earth and the signal escapes into space. Below the LUF, radio frequencies are absorbed too much for propagation. If a signal is near either of these two limits it may be subject to fading as small fluctuations in the ionosphere cause the signal to be lost and regained by the receiver.