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Eroica
2006-Dec-06, 12:57 PM
Radio-detection of meteors has been around for sixty years or so. It has obvious advantages over the traditional method of observing meteors with the naked eye: radios can "see" meteors during broad daylight as well as during the night; they can detect meteors through clouds and pouring rain; they can detect meteors just as well when there is a full Moon in the sky; and they can see meteors too faint to be detected by the human eye.

I never realized, however, that it was so easy to detect meteors by radio, but with little more than a radio (e.g. a car radio) and a simple antenna (e.g. a car antenna, a V-shaped dipole antenna, or, better still, a homemade Yagi antenna) any amateur can begin to observe meteors 24/7.

Step 1 - Procure a radio. An ordinary car radio will do, so long as it has digital tuning.
Step 2 - Connect the radio to an antenna. A simple car antenna will do; so will one of those standard indoor, V-shaped dipole antennas that often come with portable TVs. Better than either is a homemade Yagi antenna (http://en.wikipedia.org/wiki/Yagi_antenna) (see below).
Step 3 - Consult an atlas and locate a city approximately 1000 km due east or due west of your current location.
Step 4 - Consult the World Radio TV Handbook (http://www.wrth.com) (doesn't seem to be online, but most libraries shuld have a copy - observers in North America can consult the FM Atlas (http://members.aol.com/fmatlas/home.html)) and find a suitable radio station that broadcasts from the city located in Step 3.
Step 5 - Tune your radio to the chosen radio station. You should be receiving nothing but white noise. You are now ready to detect meteors.
When a meteoroid big enough to create a meteor (i.e. about 100 microns in diameter) passes through the atmosphere it leaves an ionization trail in its wake. This trail can scatter radio waves, which is how a radio can detect meteors. If the meteor is created somewhere between you and your radio station, the radio waves been broadcast by the station may be forward-scattered by the meteor's ionization trail, allowing you to detect them on your radio. Typical meteors (i.e. faint meteors of short duration) result in a short-lived audible "ping" interrupting the white noise on your radio, while brighter meteors of longer duration (including fireballs (http://en.wikipedia.org/wiki/Bolide)) might result in several seconds of clear broadcast, during which words and music may be made out.

You can hear what a "ping" sounds like at the Marshall Space Flight Center (http://www.spaceweather.com/glossary/nasameteorradar.html) in Huntsville, Alabama, which also has a live audio stream from their own radio receiver (tuned to 67.25 MHz - λ = 4.46 metres).

Further information can be found at the Jordanian Astronomical Society (http://www.jas.org.jo/radio.html).


Yagi Antenna
A simple Yagi antenna can be made at home quite easily. Instructions for a 3-element one can be found at Sky Scan (http://www.skyscan.ca/3ElementYagi.htm). A 4-element one is described by Ilkka Yrjölä (http://www.kolumbus.fi/oh5iy/msobs/msobs.html). The dimensions of the antenna are based on the wavelength (λ) to which your radio is tuned. The lengths of the various elements are as follows:

Reflector Length: 0.495λ
Driven Element Length: 0.473λ
First Director Element Length: 0.440λ
Second Director Element Length: 0.435λ
Third Director Element Length: 0.430λ
The spacings between the various elements are:

Reflector to Driven Element: 0.125λ
Driven Element to First Director: 0.125λ
Spacing of Directors: 0.250λ

Of course, for radio waves c = λf, so λ is the speed of light (3x108 ms-1) divided by the frequency (in Hz) to which your radio is tuned.