Ionospheric propagation tutorial includes . . . .
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Understanding the way in which HF radio signals actually propagate can help the effects of ionospheric propagation to be used to its best.
Skywaves, skip zone and skip distances are three key concepts.
The skywave refers to the signal that travels away from the Earth’s surface towards the ionosphere. Unlike a ground wave it does not follow the contour of the ground, but instead it is directed towards the ionosphere.
The angle between the line of the skywave signal and the Earth’s surface at that point may be shallow or steep.
The skip distance is the distance over the Earth's surface between the point where a radio signal is transmitted, and the point where it is received having travelled to the ionosphere, and been refracted back by the ionosphere.
The skip distance is dependent upon a variety of factors:
- Frequency: The frequency of operation has a major influence on the skip distance that can be achieved. Typically as the frequency increases a lower angle of radiation is needed to return the signals to Earth in a shorter distance. Also higher frequencies tend to be reflected or refracted by higher layers or regions in the ionosphere. This will mean that higher frequencies tend to lead to longer skip distances.
- Ionospheric conditions: The ionospheric conditions play a major role in governing the skip distance. Under some circumstances when ionisation levels are high it may be possible for signals to achieve very short skip distances.
- Angle of radiation: The angle of radiation from the transmitting antenna will also have an impact on the skip distance. A lower angle of radiation will lead to longer skip distances as a result of the geometry.
The skip zone, which may also be called a silent zone or dead zone, is a region where a radio transmission can not be received. The skip zone is the region between the point where the ground wave signals can no longer be heard and the point where the skywave first returns to Earth.
The skip zone or dead zone depends upon a variety of factors:
- Ground wave coverage: The local coverage around the transmitter is governed by the ground wave. The extent of the coverage and the start of the skip zone will be dependent upon the frequency used. The ground wave coverage will be greater for lower frequencies. At MF it may extend as far as 100 miles or so, but for higher frequencies, e.g. 10MHz and above it may only extend for a mile or two.
- Skywave minimum skip distance: The minimum skip distance is influenced by a number of factors including the frequency of operation; the state of the ionosphere and the angle of radiation.
Skywaves & frequencies
To gain a better idea of the characteristics of HF propagation using the ionosphere, it is worth viewing what happens to a radio communications signal if the frequency is increased across the frequency spectrum. First it starts with a signal in the medium wave broadcast band. During the day signals on these frequencies only propagate using the ground wave. Any signals that reach the D region are absorbed. However at night as the D region disappears signals reach the other regions and may be heard over much greater distances.
If the frequency of the signal is increased, a point is reached where the signal starts to penetrate the D region and signals reach the E region. Here it is reflected and will pass back through the D region and return to earth a considerable distance away from the transmitter.
As the frequency is increased further the signal is refracted less and less by the E region and eventually it passes right through. It then reaches the F1 region and here it may be reflected passing back through the D and E regions to reach the earth again. As the F1 region is higher than the E region the distance reached will be greater than that for an E region reflection.
Finally as the frequency of the radio communications signal rises still further the it will eventually pass through the F1 region and onto the F2 region. This is the highest of the regions in the ionosphere and the distances reached using this are the greatest. As a rough guide the maximum skip distance for the E region is around 2500 km and 5000 km for the F2 region.
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