Part 5 - Propagation

The ability of the radio-wave to travel

We know that Radio-waves are electromagnetic waves so It's not too far fetched to imagine that they can be affected by electrical charges in the atmosphere.

If we look at what we have covered so far we would have radio-waves that leave the aerial and carry on their merry way in a straight line until picked up by a receiving aerial.

However, this is not the case and there are many things that can alter the properties of a radio-wave on its journey. The main properties that can be affected are:

• • • Strength

    • Polarisation

    • Direction

Strength

The strength of the radio-wave can only be reduced (attenuated) on its journey. This can be caused by travelling through any medium: air, rain, cloud. A metal sheet would attenuate the signal a lot, rain a bit, roof-tiles a bit etc. The amount of loss of strength is the attenuation of the signal over the path it travels and there is little we can do to improve it other than to change the effects immediately around the aerial.

Having the aerial outside to reduce the effects of brick, cabling, pipes, roof-tiles etc. is an obvious starting point. Then away from trees (they contain lots of water and get wet in rain!), fences, out-buildings. A large open field seems the best choice but don't despair. If these all made that much of a difference no-one would be on the air! We can just do what we can to improve our lot.

For example - my garden has a metal chain fence at the end (the neighbours). Two trees, a house at one end (obvious I suppose but worth mentioning) and a walloping great big mountain within 300 m that used to be mined for Tin. How bad can it get? Well I can still get out to the US and Eastern Europe but the South/South East is a no go it seems. I'll go mobile for those.

Polarisation

This is the way the magnetic and electric fields are orientated. For our purposes they are always at 90 degrees to each other; if the Electrical field is vertical then it is best received with a vertical aerial. If it is horizontal. it is best received with a horizontal aerial. The effects of polarisation are more pronounced at higher frequencies (VHF and above) but to reduce losses due to polarisation it makes sense to try to match the receiving aerials setup.

Directional beam aerials are invariably mounted horizontally (so they are not affected by the mast) and on 2m, SSB contacts are horizontally polarised. Try to listen to a SSB signal on 2m and change your aerial to be vertical (vertically polarized) and you will see a drop in signal strength. Or change your FM whip horizontal.

At HF, the polarisation is not so important , as the polarisation of the signal may be changed by the atmosphere the radio-wave travels through and we can't prevent that. When listening to HF on a pocket receiver change the whip between vertical and horizontal and you will probably find there are some signals stronger each way.

Direction - Satellite view:

This is the bit everyone concentrates on as there is no point worrying about strength and polarisation if the radio-wave isn't going where we want it!

If we look at the radio-wave direction from top-down, a satellite view from Google maps say, then it is important to know where it is radiating best. We can do better by entering our home coordinates into a great circle generation program like SM3GSJ's Great Circle Maps which generated this view...

We can see that from my QTH in the UK that North-West is to the US; South/South-East is to Europe and Africa; East to Eastern Europe and Australia and North to the pacific and far east.

Now if I want to talk to the US I need the radio-wave to go North-Eastwards (past my mountain!). An opening to Africa could do with a clear path to the South (bloody mountain!) and so on.

The aerial I choose and how I place it has an important role here. A dipole fires off to the sides best and my garden is East-West so it fires best North - South (did I mention I have a mountain to the south of me? pity please!). If I want a signal to the US I could try to get the dipole in across the garden then do something inventive with the ends or use a beam or a vertical, or a plethora of other options.

The map above shows the blockage (sorry - mountain) and it's possible affects. To fire over the top I did a quick drawing and I need a take--off angle of 40 degrees or more to clear it. With a low Dipole that should not be a prolem, with a vertical it might be harder. Any way my 40m dipole is currently running North/South so firing East/West. to the West we have Ireland and lots of water for the first 3,000 km. To the East we have Eastern Europe - my dipole is very low so I get a high angle of radiation/transmission and a short 'hop', no wonder most contacts are with Belgium or Germany. If I turn the dipole I should clear the mountain and be getting into Southern Europe - i'll try it sometime!

What I am trying to get across is that you should study your location, where you can reasonably expect to get-out to and see how you could possibly cover the other areas. If you can't ,then rejoice in going mobile or portable but you will be able to get out somewhere (hopefully with some Radio Amateurs at the other end!).

Direction - side on view:

Now this is the really fun bit! We talked about how a low angle of radiation is good for DX. Ok so let's imagine we are sending our radio-wave out at 20 degrees and it travels in a straight line - it will just keep getting higher and higher until it goes into space - no-one (on Earth) will ever receive it - not good!

This is indeed the case for frequencies above what is known as the Maximum Usable Frequency (MUF) that hits a layer of the sky at an angle greater than what is called the critical angle.

That sounds complicated so let's explain what the layer in the sky is and all will become clear.

This layer does to radio-waves what a mirror does to light waves - it reflects them (we'll see later it's not that simple but for now just believe me). The layer is more like a part mirror or sheet of glass - it reflects some radio-waves but lets other through. You know how sometimes you can't see through a pane of glass you just get reflections, and other times you can see through and see reflections - well the mechanics are different but the analogy isn't too far off!

To make things a little more challenging - there are several layers! not only that, they all behave differently and vary depending on the time of day, season, sunspot cycle and the suns activity and etc. etc. etc.!!!!

So how do we get to understand all these things? Well I can start you off with my take on the classical text book stuff and once you have a basic understanding then listening to the changes on the air and reading the propagation predictions (I like the Tomas Hood - NW7US (enjoy the music) articles in CQ Magazine) - I don't pretend to understand everything but like you I'm learning too.

Let's start with the names of the layers and a brief description of each form lowest to highest:

• • D Layer - Only exists during the day and absorbs low frequency radio waves (less than approximately 5 MHz) stopping them reaching the layers that could reflect them. limits 1.8 MHz (160 m) and 3.5 MHz (80 m) during the day but when it has gone at night you're in business. 7 Mhz (40 m) signals can get through the D layer but only if they take the shortest path which is straight through the layer almost vertically. If they enter at a shallower angle they have to travel through more of the D layer and get aborbed.

  • E Layer - Only during the day, absorbs a lot of radio waves but generally lets those that passed throught the D layer through. That is far too simple for the E layer as it also has some very exotic ways of affecting radio-waves, mostly on the upper HF and VHF bands where sporadic E and others occur, you can research those at your leisure!

  • F Layer - one during the night splitting into 2 (F1 and F2) during the day. These are the main layers responsible for round the world communications by reflecting the radio-waves.

That was as simple as I can make the layer descriptions.

How the layers work

One thing is very easy to understand - they are all dependant on the Sun.

Unfortunately the Sun is a very complicated and no-one can predict its output reliably but there are lots of measures you may see such as the A and K values- they are not as onerous as they may appear, but reading that the Ap index is 5 and the 10.7cm solar flux is 70; can put people off. Don't let it, they are not complicated if you know what they are and how they might help us - more on that later.

This is far too simple an explanation but it will do for now - The Sun sends us its solar energy across space and during the day it enters the atmosphere and hits the little molecules of gas high in the sky. It hits them with so much energy that they 'break-up' and become electrically charged (ionised). This electric charge on all these molecules forms a 'layer' of ionised gas - the F2 layer.. The Solar energy continues and hits more molecules to break them up into ions - the F1 layer. Shall we go on? Why not - then the solar energy does the same with the E and D layer.

You could say that the first layer the solar energy hits first is the most affected, the F2 layer, and why not - makes sense to me. Now ionised gases (our layers), if they are dense enough, will bend (refract) a radio wave. If they are dense enough they will bend it enough that the radio wave will come back to Earth as if it has been reflected back.

If the layer is not dense enough (ionised enough) then the radio wave might travel straight through it, or be bent just enough to travel in the layer before travelling through it or even returning to Earth (ducting).

But the D layer does not reflect - it absorbs - very much like the Ozone layer does to UV radiation, you could think of the D-Layer doing the same in reverse to radio-waves, it doesn't like to let them through unless they are above about 5 MHz.

So let's have a radio wave of 14 MHz (20 m) going at 20 degrees into space during the day. It flies through the D layer and the E layer, gets a little bent by the F1 Layer and then totally refracted (reflected) back to Earth by the F2 layer. Wow we have a radio-wave that has travelled into space (a little) and back again and might get received by someone.

Now Let's do the same with a radio wave of 21 MHz (15 m) going at 20 degrees into space during the day. It flies through the D layer and the E layer, unlike the 14 MHz signal this does not get bent by the F1 layer or the F2 layer but continues into space never to be heard by anyone - why?

Well the Sun's energy was just not enough to ionise the F1 and F2 layers enough to reflect that high a frequency. When the sunspot cycle is at its peak we hope it will send us lots of energy to ionise these layers enough so we can use the higher frequency bands, but in htis example we were at the low point.

The maximum frequency that the layers will return to Earth is known as the Maximum Useable Frequency (MUF) and we can go into more detail about it but have a look on Google for MUF to learn more, we just need to know what it is.

We said the D layer absorbed radio-waves. The lowest frequency that can punch its way through the D layer is known as the Lowest Usable Frequency (LUF).

Between the LUF and the MUF we have radio-waves that are getting through the D layer and being reflected by the E and F layers - cool. We know now what frequencies to use during the day.

Well there is another fly in the ointment - the MUF and LUF varies over the parts of the Earth in sunlight so we might have a high MUF to the states but a low one to Africa. This is where propagation becomes a scientific art-form - predicting which 'paths are open' to other areas of thee globe.

Night time.

Night time comes along and the Sun goes down. So its solar energy is not reaching the layers on the dark side of Earth. These Layers are very resilient and recover quickly to 'repair' the broken (ionised) molecules by re-combining the parts to become themselves again - non-ionised. (Again that was far too simplistic but do you get the drift?)

The D and E layers disappear totally so we have a very low LUF (no D layer to absorb low frequencies) hence 160 m and 80 m coming to life. The F1 layer does not repair itself totally and combines with the F2 Layer to just form one F layer.

This F layer is not as strongly ionised as it was in the day so it can't reflect as high a frequency and the MUF drops. 14 MHz might just fire off into space and often does unless we are at the peak of the sunspot cycle.

So now we have no LUF to speak of and a lower MUF just about 7 MHz will do! DX on 1.8, 3.5 and 7 MHz during the night is there for all to enjoy.

What happens between night and day? Well it's not like switching of the light! Did you know that twilight is classified as c3 seperate states civil, nautical and astronomical because the sun setting time is quite long? have a read on Google.

Well that was nothing to do with radio but just to demonstrate that as dusk and dawn take time so the same is true of the layers changing and some interesting effects can be seen when the D layer disappears but the F1 layer is still strong. Read about grey line propagation for more information.

So I think that does layers, ionisation, LUF, MUF, critical angle and a few others enough for you to start reading the standard texts with hopefully a new slant on what propagation is.

Now I like things being made simple but sometimes that hides the enormity of the achievements that we can make when working other countries or continents.

This simple model I've drawn of the various 'layers' of the Ionosphere (the upper part of the Atmosphere that is changed electrically (ionised) by the sun's rays) hopefully shows how radio-waves can be sent up from the earh and 'bounce' back again at a far-distant land. We can go into the details of the angles, hops etc. later. Let's just accept that a large majority of HF Radio waves will bounce around like this and we can see how the other parts of the world are reached.

I like that picture (mainly beacuse I drew it!) because it is simple; reasonably accurate and shows the difference beween day and night, but wait. How simplified is it? Well the layers aren't quite that high! To scale it looks like this.....

....quite a difference! Now those layers are pretty close to the surface of the Earth and the outer layer is shown at it's maximum (If your wondering if it is realy to scale I'll send you the scaled AutoCad file!). Now those radio waves have a pretty thin slice of Ionosphere to bounce around in don't you think?

Right - I'm Mr. DX and I have a super aerial that has its best radio gain at an elevation of 15 degrees, let's do some geometry - no maths just lines!

See the layers scaled to the curvature of the Earth - low eh? I've shown a Radio-wave (blue) at and angle of 15 degrees from the flat Earth and 'bouncing' (OK it's actually refracting) at the top of the F-layer at 400 km high. When it comes back to mother Earth it has travelled about 2,150 km and 19 degrees around Earth. If you read the text books you'll find this figure is not quite correct - the geometry is only correct for stright lines and reflections - in reality there are refractions between the layers and other amazing things happening, now it's your turn to read up why!

The Earth is 360 degrees round (like all circles) so 360/19 is about 20. To go all the way around the Earth and back will take 20 skips. Half way will take 10. The ARRL handbook reckons only 5 F layer hops possible and in reality the radio wave will be bent at the E layer and travel further and there are many other ways it might travel.

So that's it for a brief tour of propagation, as I have time and learn more I'll type it um but in the meantime have a Google search for Pederson Rays!