y the ionosphere frequency in use. Low frequency signals are affected in totally different ways to those at the top end of the short wave spectrum. This is borne out by the fact that medium wave signals are heard over relatively short distances, and at higher frequencies signals from much further afield can be heard. It may also be found that on frequencies at the top end of the short wave spectrum, no signals may be heard on some days. To explain how the effects change with frequency take the example of a low frequency signal transmitting in the medium wave band at a frequency of f1. The signal spreads out in all directions along the earth's surface as a ground wave that is picked up over the service area. Some radiation also travels up to the ionosphere. However, because of the frequency in use the D layer absorbs the signal. At night the D layer disappears and the signals can then pass on being reflected by the higher layers. Signals pass into outer space F21ayer Dlayer Figure 2.18 Radio wave propagation at different frequencies Radio waves and propagation 37 Signals higher in frequency at f2 pass straight through the D layer. When they reach the E layer they can be affected by it being reflected back to earth. The frequency at which signals start to penetrate the D layer in the day is difficult to define as it changes with a variety of factors including the level of ionization and angle of incidence. However, it is often in the region of 2 MHz or 3 MHz. Also as the frequency increases the ground wave coverage decreases. Medium wave broadcast stations can be heard over distances of many tens of miles. At frequencies in the short wave bands this is much smaller. Above 10 MHz signals may only be heard over a few kilometres, dependent upon the power and antennas being used. The E layer only tends to reflect signals in the lower part of the short wave spectrum to earth. As the frequency increases, signals penetrate further into the layer, eventually passing right through it. Once through this layer they travel on 