TV and FM DX - Meteor Scatter Propagation

Meteor Scatter Propagation

Meteor scatter occurs when a signal bounces off a meteor's ionized trail.

When a meteor strikes earth's atmosphere, a cylindrical region of free electrons is formed at the height of the E layer. This slender, ionized column is relatively long, and when first formed is sufficiently dense to reflect and scatter television and radio signals, generally observable from 25 MHz upwards through UHF TV, back to earth. Consequently an incident television or radio signal is capable of being reflected up to distances approaching that of conventional Sporadic E propagation, typically about 1500 km. A signal reflected by such meteor ionisation can vary in duration from fractions of a second up to several minutes for intensely ionized trails. The events are classified as overdense and underdense, depending on the electron line-density (related to used frequency) of the trail plasma. The signal from overdense trail has a longer signal decay associated with fading and is a physically a reflection from the ionized cylinder surface, while an underdense trail gives a signals of short duration, which rises fast and decays exponentially and is scatter from individual electrons inside the trail.

Frequencies in the range of 50 to 80 MHz have been found to be optimum for meteor scatter propagation. The 88 – 108 MHz FM broadcast band is also highly suited for meteor scatter experiments. During the major meteor showers, with extremely intense trails, band III 175 – 220 MHz signal reception can occur.

Ionized trails generally reflect lower frequencies for longer periods (and produce stronger signals) compared to higher frequencies. For example, an 8-second burst on 45.25 MHz may only cause a 4-second burst at 90.5 MHz.

The effect of a typical visually seen single meteor (of size 0.5 mm) shows up as a sudden "burst" of signal of short duration at a point not normally reached by the transmitter. The combined effect of several meteors impinging on earth's atmosphere, while perhaps too weak to provide long-term ionisation, is thought to contribute to the existence of the night-time E layer.

The optimum time for receiving RF reflections off sporadic meteors is the early morning period, when the velocity of earth relative to the velocity of the particles is greatest which also increases the number of meteors occurring on the morning-side of the earth, but some sporadic meteor reflections can received at any time of the day, least in the early evening.

The annual major meteor showers are detailed below:

• January 3 – 4: Quadrantids
• April 22 – 23: Lyrids
• May 5 – 6: Eta Aquariids
• June 9 – 10: Arietids & zeta-Perseids
• August 12 – 13: Perseids
• October 21 – 22: Orionids
• November 3 – 5: Taurids
• November 16 – 18: Leonids (Note: activity varies, outburst only at about 33 year interval)
• December 13 – 14: Geminids
• December 22 – 23: Ursids

For observing meteor shower-related radio signals, the shower's radiant must be above the (propagation mid path) horizon. Otherwise no meteor of the shower can hit the atmosphere along the propagation path and no reflections from shower's meteor trails can be observed.