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Re: (meteorobs) radio meteor polarisation diversity
At 12:08 2000-09-22 +1000, you wrote:
>In one email he brought up the issue of loss of sensitivity due the
>probable change in polarization of the radio signal after reflection
>not matching your antenna. He then speculated on the use of helical
>antennas.
I agree with the comments by Phil Gebhardt. But, as he indicated, I'm not
aware of much work either vertical or crossed polarization. I've used
meteor scatter for communications since 1955. Just about all work that I'm
aware of has been horizontal. This is primarily because everybody was
already horizontally polarized, as tests many years ago indicated that
horizontal was slightly better for weak-signal VHF work.
I am aware of only a very few vert-to-vert tries, and they were generally
unsuccessful. However, the attempts were too few to really draw any
conclusions.
With circular, you of course lose 3 dB on a linear-polarized signal. I am
not aware of any cross-polarized tests, as few would be willing to atempt
them. Some years ago I did run circular polarization for a few schedules.
They were rather successful - but it's unknown how they would have compared
with my running horizontal for the same schedules. And all of these were
*not* direct forward scatter, but were side scatter (necessitated becaue of
the short distance between us). This means that the geometry of the
reflection may have caused a polarization shift, which might have favored
circular on my end.
Sporadic radio meteors can vary greatly over a period of an hour, or even
10 minutes. A slight change in the path direction and length can also make
huge changes in the number of underdense pings received. For example, I
recently ran with two stations to the NE at about 1000-1100 miles and
received only about 1 to 3 pings per minute. Running with two stations to
the west at about 800 miles produced up to 20 pings per minute! These were
at the same time of day, several times back-to-back schedules. (We were
testing for ionospheric scatter, not meteor scatter, at the time, so only
partial records of the pings were kept). And we've often seen 5 to 8 pings
per minute for a few minutes, then dropping to near zero, then back up, etc
etc. So if you have sensitive equipment, it can get interesting! (All this
sporadics - showers get even more interesting. But that's another subject).
So for most work, horizontal would probably be better. If you have the
resources, more experimenting along this line would certainly be
interesting, tho!
73, Shelby, W8WN
Shelby Ennis, W8WN - EM77bq - KY
w8wn@arrldot net
w8wn@amsat.org
Web: http://www.qsldot net/w8wn/
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