Meteor trails are not an uncommon sight. Most people have seen them by looking up into the sky on a clear night, especially during one of the meteor showers.
As already mentioned, meteor trails can be used for radio signal propagation. Although short-lived, they can be used to carry signals beyond the line of sight.
The design of the radio communications equipment, techniques and protocols or procedures has been built up to address this form of radio propagation. It addresses the special characteristics of the propagation resulting from the meteor trails and allows for reliable communications to be established.
Meteor trail formation
The meteor trails used by meteor scatter radio signal propagation form as the meteors enter the Earth's atmosphere. As the atmosphere becomes more dense, the meteors burn up as the friction from the rises. The meteors enter the atmosphere at speeds anywhere between about 10 and 80 kilometres a second and they normally burn up and form trails at altitudes ranging between 85 and 120 kilometres, dependent upon factors including the size, speed and angle of entry.
Meter trails and radio propagation
As the meteor enters the more dense areas of the atmosphere and heat starts to be generated as a result of the friction from the air, the meteor heats up to such a degree that the atoms vaporise, leaving a trail of positive ions and negative electrons. The trail that is formed is a very long thin parabola with the meteor at its head. Typically the trails are only a few metres wide, but they may be over 25 km long.
The level of ionisation in the meteor trail is very high. It is much higher than the level of ionisation generated by the Sun in the ionosphere. As a result the frequencies that can be affected are much higher than those normally experienced in the ionosphere. Often frequencies up to about 150 MHz can be reflected by these trails.
Meteor trails can be categorised into two categories according to the density of electrons. One type is termed "over dense", and the other "under dense". The point at which they change from one type to another is taken to be an electron density of 1 x 10^14 electrons per cubic meter. This actually corresponds to a critical frequency of 90 MHz. While the electron density is used to define the type of ionisation trail, it is actually the way in which a trail reacts that is of real importance.
The meteors that create the under dense trails are normally very small, often the size of a grain of sand. Those that generate the over dense trails are usually larger. Typically meteors have to have a mass larger than about 10^-3 grams with a radius of around 0.004 metres to create an over dense trail.
- Over dense trails: These trails provide relatively "strong" reflections. Having a high electron density, signals do not completely enter over dense trails and they are "reflected". These reflections have a slow rise to the peak strength and a slow decay. Their overall duration is generally a few seconds, but during the period of the reflection the signal undergoes multi-path related effects that affect their performance for the very high data rate transmissions normally used for professional applications. They are less common than under dense trials as they result from larger sized meteors.
- Under dense trails: These meteor trails are ones that act in a very much different way to the over dense trails. Having a lower electron density, the signal penetrates the trail and it is scattered rather than being refracting it. In this way some of the signal is returned to earth. Again the portion of the signal that is returned to earth is very small and very efficient radio systems are required to be able to make use of them. The reflected signal typically rises to a peak strength in a few hundred microseconds and then decays. This may take between a few hundred milliseconds to as long as a few seconds. This decay is attributed to the spreading and diffusion of the trail's electrons.
Of the two types of meteor ionisation trail, it is normally the under dense ones are normally used for commercial communications. Over-dense ones are used for ham radio operations. The reason for different types being used is that the requirements for the two types of communications are somewhat different.
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