Sporadic E

Sporadic E is a form of propagation that can arise with little warning, and enable frequencies of 100 MHz and more to travel over distances of a thousand kilometres and more. Many people experienced it in the days of the old VHF television transmissions. When sporadic E propagation arose, it would result in severe interference to the signals. Even now VHF FM broadcasts in the 88 - 108 MHz band can be affected. In many instances the arrival of sporadic E can cause unwanted interference as signals that are normally too distant to be heard appear. As a result it is of great interest when planning a radio system or network and for predicting achievable distances..

What is sporadic E?
Sporadic E arises when clouds of intense ionisation form in the region of the E layer. These clouds can have very high levels of ionisation, allowing frequencies up to about 150 MHz to be reflected on some occasions. The clouds are usually comparatively small, measuring only about 50 to 150 kilometres in diameter. Their shape is irregular. Sometimes they may be almost circular, whereas others may be long and thin. They are also surprisingly thin, often only measuring a few hundred metres in depth.

The clouds appear almost at random, although there are times when they are more likely to occur. They form in the day, and dissipate within a few hours. They are also far more common in summer, peaking approximately in mid summer. As they form the level of ionisation gradually builds up, affecting first the lower frequencies, and later higher frequencies as the level of ionisation increases. 

Propagation via sporadic E occurs in the same way as normal ionospheric propagation. Signals from the transmitter leave the earth as a sky-wave, travelling towards the ionosphere. Here they are reflected (or more correctly refracted) back to earth where they are heard at a considerable distance from the transmitter. Like normal ionospheric propagation it is the free electrons that affect the signals, causing them to bend back towards the earth. In view of the fact that the sporadic E clouds occur at around the same height as the E layer, similar distances are achieved. Typically the maximum distances are about 2000 km.

It is found that the sporadic E ionisation clouds move. Being in the upper atmosphere they are blown by the winds in these areas and can drift at speeds of up to 300 kilometres per hour. This means that when sporadic E is being experienced, the area from which stations are heard will change over the life of the cloud.

Theories
There are many theories about sporadic E and how it occurs. Some believe that it may be related to thunderstorms, others think it results from the winds in the upper atmosphere. None of these theories have been established, leaving the reasons behind sporadic E a mystery, and predictions of when it will occur have to be left to statistics. However even though the mechanism behind the formation of sporadic E is not fully known it is still possible for radio amateurs to utilise them to enable them to make contacts over long distances.