TUNING INTO METEOR SHOWERS
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Using Radio techniques meteor watchers can study 24 hours a day, through daylight, cloud and moonlight.
Whenever a particle enters the Earths atmosphere, it collides with air molecules producing a great amount of heat which in turn evaporates atoms from the meteorite surface, these move off with a velocity that approaches that of the meteoroid itself. Collisions between these high velocity atoms and the surrounding air result in the production of heat, light and ionisation.
It is this ionisation which we use as reflectors for radio waves. Exactly the same way as we use the atmospheric ionisation to reflect HF radio waves back down to other stations many miles away. Well beyond "Line of sight". No great ionisation occurs usually, until the meteoroid is about 180 km above the atmosphere, below that to about 80 km, where the meteoroid vaporises rapidly, most meteoroids totally vaporise before they reach 80 km altitude.
In the radio detection of meteors, RF energy is produced on a ground based transmitter and a small fraction of this energy is reflected and scattered off the ionised stream of particles left by the meteoroid. These reflected radio waves can then be received by another station beyond the radio horizon.
Most radio meteor work is confined to 15 Mhz to 100 Mhz range. The maximum frequency normally used is around 100 Mhz. The reflected power is proportional to the cube of the wavelength, so higher frequency operators need higher power transmitters and more sensitive receivers. At frequencies below 30 Mhz, interference from distant stations via ionospheric propigation can be a serious problem, particularly during Solar maximum, and daylight.
Using forward scatter, many amateurs have been able to detect meteors. It is possible to detect radio or television signals which lies beyond the radio horizon - usually 1,000 Km to 1500 Km distant. All the observer has to provide is the antenna to pick up the distant signals, a radio or TV and maybe some sort of recording apparatus. When an ionised trail occurs, with the path being inside the path of the TX station and RX station, reflections will be detected as brief snatches of speech or music or TV pictures in the case of a TV. Observers around EI/G are fortunate enough to have many commercial stations both TV and radio in Scandinavia and Eastern Europe. Particularly those in the 66 Mhz to 72 Mhz region in Eastern Europe. Several stations in Poland, Hungary, Romania, and Bulgaria are ideally located for EI/G. Details for frequencies can be found in the World Radio and TV handbook. It's best to choose a station that broadcasts 24hrs a day so as to avoid the possibility that it will be off air when the meteor shower being studied is at a maximum.
The 4m band (70 MHz) between 70.0 and 70.5 should be avoided as local interference from Amateurs and other services will be present. A wide choice of high quality, moderate-cost radio equipment is now available. For receive, a 5 element beam is quite adequate. The receiver should be tuned to the distant station frequency, and the beam mounted at an altitude of at least 5 meters should be angled very slightly so that it points about 100 Km to one side of the TX - RX path. Allowing for the curvature of the Earth the main lobe should be directed at an elevation of approx 10 degrees, 4 degrees and 1 degree for path lengths of 1000 Km, 1500 Km and 2000 Km respectively. The maximum range possible with the antenna exhibiting a main lobe at 0 degrees is about 2300 Km.
With the antenna aligned properly, all that will be heard will be the hiss of the background receiver noise, until a brief burst of speech or music from the station is momentarily forward scattered to the antenna by the meteor trail. The number of events occurring in any given period of time can be observed by simply listening to the receiver, but this will soon become tedious, so for longer, more detailed study some form of automated recording is necessary. Many receivers have an external output socket where it is possible to get an output which is proportional to the input signal strength, this can be fed directly into a chart recorder. Each spike represents a single meteor...
Use of a twin display chart recorder will allow the display of regular timing signals as time reference. Extreme care must be taken to illuminate interference from mains electricity or RF interference.
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