[amsat-bb] Polarization (attitude mostly)
WB4SON at gmail.com
Tue May 31 21:36:42 UTC 2016
That is an excellent observation about the use of horizontal or vertical
polarization for geostationary satellites -- obviously that works well in
that case (much higher frequencies and zero relative motion).
Perhaps it is a combination of relative motion and signals passing through
different parts of the ionosphere that causes the problem. When talking
about VHF signals coming from the ISS in particular there isn't any random
attitude change (unlike a tumbling/spinning satellite). Certainly there is
a slow and predictable change in attitude due to relative motion. If that
is all it was, then a single RHCP or LHCP antenna would do the trick 100%
of the time.
The ARISS crew highly recommends the use of CP antennas (certainly
understandable) AND polarity switches. I pushed back on that requirement
due to cost and availability issues, but went ahead and installed the
polarity switch on our CP antennas. With certainty I can tell you that at
random times it made a huge difference in downlink signal level.
The effect usually happened close to the horizon -- perhaps the VHF signals
are undergoing polarity changes due to tropospheric ducting. But the use of
a CP polarity switch proved to be very useful. Sometimes it might not
happen at all. And at least once it happened near the point of closest
it is an interesting issue for sure, and not much literature on it.
73, Bob, WB4SON
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