wave stations is that some signals appear to fade in and out all the time. These alterations are taken as a matter of course by listeners who are generally very tolerant to the imperfections in the quality of the signal received from the ionosphere. There are a number of different causes for fading but they all result from the fact that the state of the ionosphere is constantly changing. The most common cause of fading occurs as a result of multipath interference. This occurs because the signal leaves the antenna at a variety of different angles and reaches the ionosphere over a wide area. As the ionosphere is very irregular the signal takes a number of different paths as shown in Figure 2.24. The changes in the ionosphere cause the lengths of these different paths to vary. This means that when the signals come together at the receiving antenna they pass in and out of phase with one another. Sometimes they reinforce one another, and then at other times they cancel each other out. This results in the signal level changing significantly over periods of even a few minutes. Another reason for signal fading arises out of changes in polarization. It is found that when the ionosphere reflects signals back to earth they can be in any polarization. For the best reception, signals should have the same polarization as the receiving antenna. As the polarization of the reflected wave will change dependent upon the ionosphere, the signal strength will vary according to the variations in polarization. In some instances the receiver may be on the edge of the skip zone for a particular signal. When this happens any slight variations in the state of Figure 2.24 Signals can reach the receiver via several paths Radio waves and propagation the ionosphere will cause the receiver to pass into or out of the skip zone, giving rise to signal strength variations. On other occasions severe distortion can be heard particularly on amplitude modulated signals. This can occur when different sideband frequencies are affected differently by the i