ort wave stations are only heard over relatively short distances using ground wave. Instead these stations use reflections from layers high up in the atmosphere to achieve coverage to areas all over the world. Skywaves Radio signals travelling away from the earth's surface are called skywaves and they reach the layers of the ionosphere. Here they may be absorbed, reflected back to earth or they may pass straight through into outer space. If they are reflected the signals will be heard over distances which are many times the line of sight. An exact explanation of the way in which the ionization in the atmosphere affects radio waves is very complicated. However, it is possible to gain an understanding of the basic concepts from a simpler explanation. Basically the radio waves enter the layer of increasing ionization, and as they do SO the ionization acts on the signal, bending it or refracting it back towards the area of lesser ionization. To the observer it appears that the radio wave has been reflected by the ionosphere. When the signal reaches the ionization, it sets the free electrons in motion and they act as if they formed millions of minute antennas. The electrons retransmit the signal, but with a slightly different phase. This has the result that the signal is made to bend away from the area of higher electron density. As the density of electrons increases as the signal enters the layer, the signal is bent back towards the surface of the earth, SO that it can often be received many thousands of kilometres away from where it was transmitted. The effect is very dependent upon the electron density and the frequency. As frequencies increase much higher electron densities are required to give the same degree of refraction. The way in which radio waves travel through the ionosphere, are absorbed, reflected or pass straight through is dependent upon the 36 Newnes Guide to Radio and Communications Technology Layer in the ionosphere Transmitter Receiver The earth Figure 2.17 Signals reflected and returned to earth b