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Re: (meteorobs) radio meteor polarisation diversity
Hello Phil
I think I see where the misunderstanding is happening here. I don't
think Doug (the non-list person involved in this) is concerned as
much about polarization changes in any particular individual burst as
the possibility that you may not detect a weak burst of unfavourable
polarization at all and therefore have your data biased by
polarization effects... which have no relationship to the actual
meteor events.
The FM broadcast stations I listen to are of polarization type
"mixed"... whatever that means. It has been suggested to me that it
is simply 45 degrees. I don't know what their power output is.
Personally, I'd just like to have a nice automated system, though
Doug and several of his very experienced friends, are not the first
"radio persons" to immediately raise the question of polarization
when I've discussed radio meteor detection with them.
Anyway, rather than me become caught in the middle of this (though
I'm interested in the outcome), I think you should discuss the finer
points of this with Doug himself. A nice report back to the list of
the "concensus" would be good too. ;-)
Maybe crossed yagi's are the way to go. What would be a good physical
arrangement for that? I'm currently using a FM 3 element yagi.
thanks
Bruce
>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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