Propagation And The Layers Of The Atmosphere

Friday is propagation day but, once again, there is precious little to report in the way of propagation.

The sun is still blank and last week I commented that, if we don't have any more sunspots, then the sun will have been blank for over 30 days. Well, guess what? The sun has only been blank for 15 days but I was still right!

What has happened is that the information has been corrected because the Solar Influences Data Center in Belgium found a tiny sunspot on March 26th and this put the count back to zero. OK, I'm not going to argue with that (but it's still really over 30 days).

For the record, the solar flux was around 70 all week with no geomagnetic actively and the A and K index in really low numbers. We have had high pressure over the UK most of the week and that is now declining slowly with rain at the end of the week and the promise of a wet Easter holiday.

The Atmosphere
As there isn't a lot of propagation, I thought I'd post some general notes about propagation which, I hope, will help explain both why it is so variable and also why it is so fascinating.

Firstly, to begin at the bottom as it were, signals start life as a wave that has both magnetic and electrical properties and which arrives at the antenna. The wave oscillates along the antenna - it's rather like the way a wave that you make with your hand in the bath oscillates back and forth. These oscillations cause the wave to leave the antenna and propagate out into the air.

If there were no atmosphere, the wave would radiate out in all directions rather in the way that the rays of light radiate out from a light bulb and, indeed, this is what happens to radio waves in space.

It is the atmosphere surrounding our planet which causes changes in the way that waves move and in the directions that they take and this causes the variations in propagation. This is why a signal from a distant transmitter may suddenly become stronger at our receiving antenna and so produce an audible signal.

As a start, the heading image shows the various layers of the atmosphere in so far as they affect radio signals.

Bear in mind that, when talking about layers, not to think of them as the sort of layers that you find in liquids or in the strata of rocks. These are not well defined layers - although we speak of them as if they were - but variations in concentration so one layer runs into another and the layers are really just high points in the concentration of electrons.

Layers found in the Atmosphere that affect radio communication
The layer nearest to the surface of the Earth is the troposphere and it rises from ground level to about 10 km and mostly contains the clouds. It is here that the weather that we are to have is formed and this layer is important in VHF propagation.

Above this, from 10 and 50 km is the stratosphere, at the bottom of which is where you find the highest flying aircraft. So far as we are concerned, nothing much happens here.

Above the stratosphere from about 50km to 90km is the mesosphere extending into the thermosphere and then, at the very top of the atmosphere, a final layer called the exosphere.

Now, this is the bit that is of most interest to radio hams. Within these upper regions extends the area known as the ionosophere and it is in this region which is were most of the action takes place that bounces our HF radio signals around!

Next time
That's it for this post, next week - in the absence of any other news - we will look in more detail at the troposphere and the layers within the ionosphere.