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First a bit of information about how radio waves
propagate. When they leave a transmitting antenna,
radio waves propagate in a straight line and unless they are reflected
or bent, they pass out of the atmosphere and into space and are
therefore not detectable over the horizon. However, at low
frequencies such as AM and short wave broadcast frequencies, radio
waves can be refracted by the ionosphere and reflected back
down to the earth past the local horizon. This effect varies
with frequency and time of day. For our purposes, we want to
tune to the frequency of a transmitter that is sufficiently distant and
high in frequency that it is not normally detectable at the receiving
locaiton. It also needs to be close enough that a meteor
trail in the upper atmosphere is visible to both the receiving and
transmitting site. Under those conditions, the receiver will
be able to detect the transmitter when a meteor produces an ionized
trail in the upper atmosphere.
Television transmitters on analog TV channel 2
make ideal transmitters for the study of meteor echos. The
video carrier at 55.250MHz is fairly strong and the frequency is
sufficiently high to make them weak to undetectable at distances that
are still close enough to be able to detect the ionized trail of a
meteor passing between the transmitter and the receiver.
Fortunately, at my location in the northern
suburbs of Detroit, there are still several Canadian stations on analog
channel 2. This may change soon, but for now it is easy to
detect radio meteors. I have not tried using the signal from
a digital television transmitter.
My system is fairly simple. I use a Kenwood
TS-480
amateur radio transciever as the receiver. It is an amateur
radio
transciever with an excellent receiver but any receiver that can tune
to 55.249MHz (USB) will do. The antenna is a non-directional
halo
on the roof. That setup alone is sufficient to detect and
analyze
meteor echos by ear. To produce a spectrogram and analyze the
echos visually, I use a free program called Spectrum Lab.
The audio output from the receiver is connected to the sound card on my
PC and Spectrum Lab does the rest.
In
operation, I tune to 55.249MHz USB, which produces a 1KHz tone from the
55.250MHz channel 2 video carrier if the transmitter is not completely
inaudible at my location. Meteor echos are generally Doppler
shifted upwards. Audibly they sound like pings, or if the
trail
is sufficiently intense, they sound like a quick chirp as the meteor
decelerates, followed by a steady tone as the traail dissipates.
Visually, they show as vertical lines (the chirp) followed by
a
horizontal line (the echo as the trail dissipates).
Paul Goelz
2010
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