Last Updated on November 16, 2023 by John Berry
Meteors are the phenomenon. Meteor trails (of ionisation) are the effect. Radio systems are the technology which exploits the trails. Momentary communications is the outcome.
Here’s how phenomenon, effect and outcome are linked.
Trail duration
Meteor trails exist for a short time – the time it takes for the meteor transit through the E Region from about 120km to about 90km above the Earth’s surface. Several phenomena dictate the trail duration for which a trail supports scatter, refraction, or reflection. These are described below.
Let’s first get a sense of the outcome of the propagation effect.
The figure above shows a plot of received signal with time.
Three meteor bursts exceed the receiver noise floor and are usable for communications. I discuss the threshold of reception in the presentation on data modes,. I indicate that using WSJT-X to decode messages, this threshold is in fact around -8dB relative to the 2.5kHz noise for MSK144 modulation. The absolute value is not important for the discussion here. Suffice that the bursts of signal are of short duration.
The efficacy of any one burst depends on three things: trail density, trail orientation and the receiving system.
Trail density
Most meteors are tiny. Dust, in fact. The diagram below tries to illustrate the relative sizes. Note the logarithmic number scale. A small number are big enough to create a trail and be useful in communications. And a very small number are big enough to fall to earth. Our interest here is meteors with mass around a few grammes.
Elsewhere I’ve discussed two types of meteors – sporadic meteors and shower meteors. Sporadic meteors are more frequent, peaking at around 250 per hour in summer. Shower meteors are less frequent at around 100 per hour. The energy of a meteor depends on its mass and velocity. Of the two, shower meteors tend to have the greater mass, and hence greater energy.
Meteors create trails of ionised material – plasma. The trails have a density measured in electrons per cubic centimetre or per cubic metre. Typically shower meteors cause trails with the greater density.
Meteor trails are classified as overdense and underdense. Overdense meteor trails tend to reflect or refract radio waves and are most useful in communications. Underdense trails tend to scatter radio waves and result in weaker effects.
Trail orientation
Trails can be in line with the transit between the two stations, orthogonal to it, or indeed at any other angle with it.
Trails are most effective in reflecting, refracting, or scattering radio waves when in line with the transit. That’s logical since the trail and the radio wave are coincident for longer. Conversely, if a trail is orthogonal to the transit, it’s only useful for a short time.
This angle between the transit (between the two stations) and the direction of travel of the meteor hugely influences the meteor burst duration.
Receiving system
The tail density and orientation lead to high reflection, refraction of scattering efficiency. Whether the outcome is a usable communication depends on the system value of the communications system.
The system value is a function of a) the modulation and coding system, b) the effective radiated power, and c) the receiver threshold. I highlight the receiving system here because the other two, the modulation and coding system, and the effective radiated power are similar in all ham radio stations. It’s at the receiver that differentiation becomes apparent.
Given that most hams use similar equipment – a rig and a low-noise pre-amp – the effective receiver threshold depends on the environmental noise floor. At VHF, that can be high. The effect is to reduce the burst duration. The image above shows that if the receiver noise floor could be reduced, the burst duration would increase (the dotted lines extending from the bursts above the threshold).
Overall, meteor burst communications succeeds because a good number of meteors create a useable trail. To exploit the trail radio amateurs must use short burst signals such as fast Morse code of transmission schemes like MSK144.