l as the night progresses, reaching a minimum around sunrise. At this time levels of ionization start to rise again. Often it is easy to consider the ionosphere as a number of fixed layers. However, it should be remembered that it is not a perfect 'reflector'. The various layers do not have defined boundaries and the overall state of the ionosphere is always changing. This means that it is not easy to state exact hard and fast rules for many of its attributes. Ground wave The signal can propagate over the reception area in a number of ways. The ground wave is the way by which signals in the long and medium wave bands are generally heard. When a signal is transmitted from an antenna it spreads out, and can be picked up by receivers that are in the line of sight. Signals on frequencies in the long and medium wave bands (i.e. LF and MF bands) can be received over greater distances than this. This happens because the signals tend to follow the earth's curvature using what is termed the ground wave. It occurs because currents are induced in the earth's surface. This slows the wave front down nearest to the earth causing it to Signals spreading out from the transmitter Transmitter antenna Close to the earth's surface the wave fronts are angled downwards allowing them to follow the earth's surface Figure 2.16 A ground wave Radio waves and propagation 35 tilt downwards, and enabling it to follow the curvature, travelling over distances that are well beyond the horizon. The ground wave is generally only used for signals below about MHz. It is found that as the frequency increases the attenuation of the whole signal increases and the coverage is considerably reduced. Obviously the exact range will depend on many factors. Typically a high power medium wave station may be heard over distances of a 150 km and more. There are also many low power broadcast stations running 100 watts or SO. These might have a coverage area extending to 15 or 20 miles. As the affects of attenuation increase with frequency, even very high power sh