ossible to rely on these modes for extending the range of a communications link. Where links are required a mode of propagation known as troposcatter can be used. This form of propagation relies on the fact that within the troposphere there are masses of air with a slightly different refractive index which are moving around randomly. These arise because of the continually moving nature of air, and the differing temperatures of different parts. These masses of air reflect and bend the signals and small amounts of the signal are returned to earth. In view of the small amounts of signal which are returned to earth using this mode, high transmitter powers, Radio waves and propagation 47 Main beam from Scattering transmitter volume Transmitter Receiver Figure 2.26 The mechanism behind troposcatter high gain antennas and sensitive receivers are required. Nevertheless this form of propagation can be very useful for links over distances around 1000 to 1500 km. Sporadic E Sometimes in summer it is possible for signals to be audible in the bands at the top end of the short wave spectrum at the bottom of the sunspot cycle. When the maximum usable frequency may be well below the frequencies in question signals from stations up to 2000 km distance may be heard in summer. This occurs as a result of a form of propagation known as sporadic E. Sporadic E used to be well known when television transmissions used frequencies around 50 MHz, and sometimes in summer reception would be disturbed by interference from distant stations. Even today reception of VHF FM signals can be disturbed when frequencies around 100 MHz are affected by it. The maximum frequencies it generally affects are up to around 150 MHz, although it has affected higher frequencies just over 200 MHz on very rare occasions. Sporadic E occurs as a result of highly ionized areas or clouds forming within the E layer. These clouds have a very patchy structure and may measure anywhere between 100 km and 1000 km across and less than a kilometre thick. This means that propa