# UK Amateur Radio ## Foundation Licence 2002
### Propagation
### Radio propagation basics
Q. How does light travel? Q. How do Radio Waves travel? | A. In a straight line. A. In a straight line.
Well that is not strictly true for either. Each can be bent by reflection and diffraction - but it is a good starting point to think of the light and radio waves going out in a straight line UNTIL they meet something.
If a hill gets in the way then radio waves can be "diffracted" over the hill but will be much attenuated. If the hill is very high then they will not bent sufficiently to get reach down the other side and a station at the bottom of the hill would be said to be in "radio shadow". So how can say HF radio waves reach the other side of a hill well they could be reflected off the ionosphere. The ionosphere is a series of conducting or reflection layers of gas at heights between 70 and 400 km above the surface of the earth. But is only the HF radio waves that get reflected VHF and UHF normally pass through and disappear into space. We will come back to reflection later. TIP: Remember the height of the ionosphere 70 to 400 kms you might just need it in the written assessment!!
Imagine a powerful torch with a good beam. If you shine the beam out of a window eventually the beam fades out and you cannot make the beam light up an object. The same is true for radio waves. The further they have to travel the weaker they get. Also if you measured the width of the beam at the torch and then at distance away where you could still see the beam it would be wider. Also at that point it would be less intense and the light fades as it gets further away and it is spreading out. Alternatively the beam could be focused into a narrow beam just like a yagi focuses the beam of radio signal. As a generalization radio waves spread out as they leave the aerial. Let's consider VHF(about 144mhz) and UHF(about 433mhz) frequencies. These frequencies act more like the torch beam than those at HF because if you shine a torch at a pillow and have the beam just going over the top the back of the pillow is in shadow as the light from the beam cannot reach it but a little of the beam can be seen still at a distance. If this was your radio signal the pillow would have severely reduced the signal just as it has reduced the torch beam. ### Can radio waves go through a wall? Now consider shining a torch at a solid wall. Nothing comes out the other side, but the beam would penetrate a little way into the wall even is only less than 1mm. This again is much the same as a radio wave but they are a little better at getting through a wall but the received signal would be much reduced and some of the signal would be reflected back. ### Can radio waves go through a window? Yes a torch beam travels right through. What you cannot detect is how much the beam is reduced. It is much less than when shone at the wall. And so with radio waves they too can go through glass much more easily. Thus VHF and UHF can go through windows but the HF might have a bit of trouble because it wave length is bigger it might have to penetrate the window and part of an adjacent wall. We can see from this that a clear path from one point to another is much easier to get a clear signal than having a signal say 10 or 100 times stronger and trying to go through a dense object. This is why satellite communication is so good. There is a clear path (for all practical purposes) between the aerial and the satellite and therefore very low power can be used at a very great distance. Here though the Foundation Licence will hold you back for as far as I can see you are not allowed to operate through satellites !!! A hand held (with you located at ground level) in the clear can have a range of about 3kms, but with the same output power, a mast mounted aerial will give the same hand held a range of many many kms. In fact the higher the mast the better and this is also better than higher power as the height helps both the transmitted signal and the received signal performance. If you try using a hand held indoors then your success rate will be much poorer than using the same unit out doors. So what are the limits of range for a simple hand held. Well on both VHF and UHF it is considered that what you can see you can normally reach. There will be some diffraction/refraction over the horizon but this will not greatly increase the range under normal circumstances. If you have building near by then the signal can be bounced off the building and reflected but this also causes losses in signal strength and therefore cannot be relied upon to provide access to areas not line of sight. Building will have a screening effect on radio waves due to their construction. Those with high metal content will have a great propensity to screen the signals.
### Contacting amateurs around the world This section is all about the HF frequencies from about 1.8MHz to 30MHz. Imagine a large ball, the diagram above is showing one part of the balls surface, and then imagine you have a zig-zig line around the ball from its surface to an imaginary ionosphere reflecting layer. The radio signals would take the path of the zig-zag line and where they touch the ball then the signal could be heard on the earth but between the touching points the area would be said to be in the "skip" zone. Thus to get a radio signal around the world it would have to bounce up and down many times each time getting weaker. Signals are heard around the world at times when the bands are said to be "open" for communication. Whether the signal is heard or disappears into space or is absorbed in the other layers of the ionosphere depends upon the frequency being used, the time of day and the season. It all depends on how well the ionosphere bends/reflects the signals back to earth. So whilst you can sometimes, when condition are right, reach Australia on HF bands, you cannot get there on the VHF or UHF as the signal pass through or are absorbed by the ionosphere.

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