Ionospheric Radio Propagation Signal Fading
Fading is present on all signals that use ionospheric radio propagation and results from a numebr of causes.
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Fading and signal variations are a major feature associated with ionospheric propagation. The signal variations may be fairly shallow with the signal changing in level between 10 and 20 dB or it may result in the signal completely disappearing. There are a number of reasons for this but they all result from the ever changing state of the ionosphere.
One of the major causes results from multi-path interference. Even with directive antennas the signal will illuminate a wide area of the ionosphere. As it is very irregular the signal will reach the receiving station via a number of paths each with different path lengths and the overall received signal is the summation of tem all. As the ionosphere changes, the signals will fall in and out of phase with one another, resulting in the strength varying by a considerable degree.
This may also be noticed on MF signals at night. Normally signals are audible via the ground-wave during the day, but at night the sky-wave may also be audible. As the state of the ionosphere changes, so will the path length for the reflected signal, and accordingly the phase will vary. This will give rise to fading of the overall signal as the ground-wave and sky-wave signals interfere,
Irregularities in the ionosphere may also cause the path lengths of closely spaced frequencies to be different. For signals such as amplitude modulation (AM) and single sideband (SSB) this may result in a reduction in some frequencies in the audio range, while others are intensified. When this "selective fading" occurs serious distortion of an AM signal may occur if the carrier suffers the selective fading and a reduction in level. Since SSB signals do not depend on the transmission of a carrier, this mode is less affected by this form of fading. Additional synchronous detection of an AM signal also makes significant improvements in the signal quality under these conditions.
The fact that there are significant changes in density of free electrons in the ionosphere gives rise to other forms of fading apart from path length changes. In one form of fading arising from this the ability of a region to reflect a signal at a given frequency may change. When operating near the MUF the signal may fade as the signal starts to pass through the region.
There may also be regions or clouds of high electron density in the D region. Accordingly as the clouds move signals, especially those lower in frequency will fade as the cloud passes through the signal path. Fading of this type normally takes place over a period of half an hour to an hour and may reduce the signal by a figure between 5 to 10 dB.
A further type of fading occurs when the ionosphere causes changes in the polarisation of the signal. As the polarisation of the incoming signal changes so will the signal strength of the signal picked up by the antennas as it falls more closely with the antenna polarisation and then away from it.
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