osphere where these effects occur and investigate its make-up. Layers above the earth The atmosphere above the earth consists of many layers as shown in Figure 2.12. Some of them have a considerable affect on radio waves whereas others do not. Closest to the surface is the troposphere. This region has very little effect on short wave frequencies below 30 MHz, although at frequencies above this it plays a major role. At certain times transmission distances may be increased from a few tens of kilometres to a few hundred kilometres. This is the area that governs the weather, and in view of this the weather and radio propagation at these frequencies is closely linked. F2 layer F1 layer E layer Meteors D layer Mt Everest Figure 2.12 Areas of the atmosphere Radio waves and propagation 31 Above the troposphere, the stratosphere is to be found. This has little effect on radio waves, but above it in the mesosphere and thermosphere the levels of ionization rise in what is collectively called the ionosphere. The ionosphere is formed as the result of a complicated process where the solar radiation together with solar and to a minor degree cosmic particles affect the atmosphere. This causes some of the air molecules to ionize, forming free electrons and positively charged ions. As the air in these areas is relatively sparse it takes some time for them to recombine. These free electrons affect radio waves causing them to be attenuated or bent back towards the earth. The level of ionization starts to rise above altitudes of 30 km, but there are areas where the density is higher, giving the appearance of layers when viewed by their effect on radio waves. These layers have been designated the letters D, E, and F to identify them. There is also a C layer below the D layer but its level of ionization is very low and it has no noticeable effect on radio waves. Night Electron density Figure 2.13 Approximate ionization levels above the earth The degree of ionization varies with time, and is dependent upon the amount of radiation recei