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Re: (meteorobs) radio meteor polarisation diversity
Hi again Bruce,
Just saw your comment. I think part of the problem is that most
meteor-reflected signals are so short-lived. Therefore, I wouldn't expect
any variation in polarisation shift during an individual burst. Effects
such as Faraday rotation would affect a burst uniformly throughout its
entire duration. It's only bursts from different directions or angles that
would be affected differently. That is why we tried a pair of crossed
yagis--on any individual burst one or other of the yagis would provide a
larger signal. By using two yagis, we were covered in either case and the
antennas provided an input to the receiver which would be a combined signal
based on both polarisation possibilities--if the reflected signal were
neither purely horizontal or purely vertical.
Since underdense trails decay immediately, it is impossible to keep the
signal constant. The multi-path phase distortion characteristic of
overdense trails will swamp any other effects. However, since many signals
exhibit characteristics of both underdense and overdense trails,
polarization within a single refelcted burst should be not be an issue.
Therefore, I think the whole issue revolves around the question: Is there
ANY variation in signal rotation. If there is, then the next issues to be
addressed are: How much rotation is there, is it variable for a given
communications circuit, and is the rotation significant. This last question
may or may not be an issue dependeing on the signal source. As I mentioned
in my previous reply, since we use commercial FM broadcasters as our signal
source, and since the signals contain both horizontal and vertical
components, rotation may not be an issue for us. If you use a signal source
which is solely horizontally polarised, then that may have an impact on
your antenna choice.
Another issue is what your goal is in detecting meteors. If your aim is
simply to count meteors, then who cares what the signal level is? A burst
is a burst.
One of the ways we overcome the possible signal loss due to polarisation
effects (other than the crossed yagis) is to listen for high-power
stations. Our antennas are usually pointed toward cities which have 100 kW
FM stations. Because of the number of cities and stations in North America,
that's never a problem for us. I don't know, but that might be a problem
for you.
I guess the bottom line is: So far we haven't found any real difference
when we've used cross-polarised antennas versus a horizontally polarised
antenna.
If I've misunderstood or misinterpreted what your were saying, try again
and I'll try to do a better job of reading next time. <|:-)
All said and done, I agree with Shelby--if you have the time, money,
resources and inclination, then it's worth experimenting to find out what
happens.
Phil
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