Southport & District
Amateur Radio Club

Making friends around the world

 
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About propagation

Ionosphere layers

The ionosphere is right on the edge of the earth's atmosphere and exists between 75 kilometers and 400 kilometers above the earth's surface, depending on who you read.

During the day the sun ionises the particles in the ionosphere and these ionised particles affect radio waves.

There are four layers of the Ionosphere known as the D, E and F layers, plus Topside. the regions we are interested in are:

  • The D layer starts, depending on who you read at around 75 kilometers above the surface of the earth during the day and disappears at night. When ionised during daylight hours the D layer absorbs the lower frequency HF radio signals. As the D layer disappears at night, these lower frequency signals are allowed to reach the higher layers to be reflected back down to earth. This explains why there are few 160m and 80m contacts to be had during the day. Higher frequency radio signals are not affected by the D layer and pass straight through.
  • The E layer starts at around 95 kilometers above the earth's surface and remains ionised throughout the whole day, but to much lesser extend after sunset. When ionised, low to mid HF frequencies are refracted back to Earth in the much the same way as light bounces off a mirror at an angle. Higher frequency signals, and above, are not refracted as much and pass through to the F layer.
  • The F layer starts at around 150 and extends to about 200 - 300 kilometers during the winter and 400 kilometers in the summer. During daylight hours the F layer can split in to F1 and F2 layers, merging back in to a single layer at night. Unlike the other layers, however, the F layer can remain highly ionised right through the night and can effectively refract signals 24/7. Because it is higher than the E layer, it can refract signals over greater distances. Signals up to 50MHz are refracted by the F layer, higher frequency signals passing straight through in to space.

Levels of ionisation vary over a, roughly, eleven year cycle with the F layer rarely becoming ionised during the lows of these cycles. The cycles are synchronised with the sun spot cycle, maximum ionisation occuring during periods of maximum sunspot activity on the sun's surface.

Refraction of radio signals in the Ionosphere

Radio waves travel in straight lines, that's a given, but the world's surface is curved. So how can we communicate with Amateur Radio stations on the other side of the world? That's what propagation is all about.

The earth is surrounded by the Ionosphere and, as we discovered above, different parts of it absorb radio signals, whilst others refract the signal back to earth enabling us to communicate with other Amateur Radio stations around the world.

Since the ionosphere is a constantly changing medium the propagation of radio waves is constantly changing too. There are many other factors involved and it is not our intention to cover them here since there are many good resources already available on the Internet. Below are links to some of them.


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