 for a given path there is a maximum frequency that can be used. This is known as the maximum usable frequency or MUF. Generally the MUF is three to five times the critical frequency depending upon which layer is being used and the angle of incidence. For optimum operation a frequency about 20 per cent below the MUF is normally used. It is also found that the MUF varies greatly depending Radio waves and propagation 41 upon the state of the ionosphere. Accordingly it changes with the time of day, season, position in an 11-year sunspot cycle, and the general state of the ionosphere. When the frequency of a signal is reduced, further reflections are required and losses increase. As a result there is a frequency below which the signal cannot be heard. This is known as the lowest usable frequency or Skip zone When a signal travels towards the ionosphere and is reflected back towards the earth, the distance over which it travels is called the skip distance as shown in Figure 2.23. It is also found that there is an area over which the signal cannot be heard. This occurs between the position where the signals start to return to earth and where the ground wave cannot be heard. The area where no signal is heard is called the skip or dead zone. Sky wave Ground wave skip or dead distance Figure 2.23 Skip zone and skip distance State of the ionosphere Radio propagation conditions are of great importance to a vast number of users of the short wave bands. Broadcasters, for example, are very interested in them, as are other professional users. To detect the state of the ionosphere an instrument called an ionosonde is used. This is 42 Newnes Guide to Radio and Communications Technology basically a form of radar system that transmits pulses of energy up into the ionosphere. The reflections are then monitored and from them the height of the various layers can be judged. Also, by varying the frequency of the pulses, the critical frequencies of the various layers can be judged. Fading One of the characteristics of listening to short 