gation is quite selective when the clouds are small with signals coming from a particular area. However, their electron density is much greater than that normally found in the E layer and as a result signals with much higher frequencies are reflected. It is also found that there are irregularities in the structure of the clouds and this makes them opaque to lower frequency signals. 48 Newnes Guide to Radio and Communications Technology At the onset of propagation via sporadic E the level of ionization starts to build up. At first only the less high frequencies are affected. Those at the top end of the HF part of the spectrum are affected first. As the levels of ionization increase further, frequencies into the VHF region are reflected. In temperate regions, sporadic E normally occurs in the summer, reaching a peak broadly around mid summer. Even SO frequencies at the top of the short waveband may be affected on some days at least a couple of months either side of this. Frequencies well up into the VHF portion of the spectrum are normally affected closer to the centre of the season because much high ionization levels are required. It is also found that the very high frequencies are not affected for as long. Sometimes signals may only be heard for a few minutes before propagation is no longer supported. The sporadic nature of this form of propagation means that it is very difficult to predict when it will occur. Even when propagation is supported by this mode it is very variable. The ionized clouds are blown about in the upper atmosphere by the swiftly moving air currents. This means that the area from which stations are heard can change. Accordingly sporadic E is not a mode normally used for commercial communications. Meteor scatter Meteor scatter or meteor burst communication is a useful form of propagation for distances of up to about 2000 km. It is generally used for data links and for applications where real time communications are not required for which it provides a cost effective method of communication. Me