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re: (meteorobs) radio meteor detection system
The best FM receiver to use for radio meteor observation is digitally tuned
and has a scan or search capability. These will search for a station
strong enough to provide good clear reception. To accomplish this
capability they internally develop an automatic gain control voltage (AGC),
also some times called receiver signal strength indicator (RSSI) voltage,
that is proportional to the strength of the received FM signal. On most
receivers this comes off of the 10.7 MHz intermediate frequency (IF)
amplifier. This is almost always an integrated circuit that uses two or
more little brown three terminal ceramic filters. Normally the IF amplifier
is located fairly close to the antenna input terminal. One way to locate
the correct pin of the IF amplifier is to switch from a frequency with no
stations to one with a strong station while looking on each of the pins for
a DC voltage changes when the signal strength changes.
A better approach may be to place a second IF amplifier in parallel with
the original one and use a integrated circuit with a good RSSI output. This
can be implemented for $15 or $20 in parts. This is easy for some one to
do if they know their way around electronics, but can be a real problem for
one who does not understand how FM receivers work and does not have some
good electronic tools and test equipment available.
If one has the money (in the range of $600 to $800 for a basic system),
monitoring the video carrier frequency for low band VHF TV (channels 2-6)
will probably produce better results than monitoring FM frequencies. A TV
frequency system has several advantages: It has greater sensitivity due
to the much narrower bandwidth. It provides a pronounced Doppler shift
that aids in separating meteor reflections from other propagation modes and
lightning. And it does not suffer from the proliferation of small stations
that makes it very hard to find a clear FM frequency to monitor, unless one
lives in a very rural area.
Jack Meyer
<jrmeyer@erols.com>
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