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CP v Linear polarisation



I apologise if my original post on this thread didn't directly take on the
issue of CP v Linear. It was written to offset the idea that CP was not
needed in any circumstances and that seemed to me to be the message that was
coming through in the exchange and that newcomers may get the wrong idea.

I think we are all aware of the "3dB difference". This is a theoretical
(mathematical) difference which can no doubt be demonstrated using an
antenna range and careful measurement. In real life however all satellite
signals have to pass through the ionosphere and the ionosphere is a very
fluid medium indeed.

In 1983, DL1BU et al did a series of controlled experiments using the
real-world signals from the beacon of Oscar-10. The full results were
published in "VHF Comms" in the Autumn issue of that year. The tests
involved a number of antennas which were instantly switchable. They included
linear horizontal, linear vertical, 45 deg. linear, 135 deg. linear, RHCP
and LHCP. The tests were conducted at both high and low satellite elevations
and at times around apogee and perigee. Quite a complete and impressive
test. The test results came up with three clear conclusions.

1/ although there were some rare and unrepeatable circumstances when linear
pol. yielded slightly better signals, overall switchable R/L CP was the best
by far.

2/ if you were constrained by circumstance to linear polarisation, vertical
polarisation won the day because it performed much better at low elevation
angles.

3/ rarely if ever could the theoretical 3dB difference be demonstrated in
practice on real-world satellite signals.

An extract of the results including copies of some of the chart recorder
print-outs (enough to make the above points) appeared in the AMSAT-VK
newsletter #3, May 1985.

Keep smilin' and keep those cards and letter rolling in folks.

Bill Magnusson ... vk3jt
Milawa
Australia.


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