Circular polarization cannot have an advantage on average, or over time. The
problem with circular polarization on skywave is the wave has no set
rotation, level, or phase.
The circular antenna would be fine combining two phase-quadrature fields
with a certain lead or lag (depending on rotation or sense), but the
arriving signals at HF would be random. They would be just as likely to
subtract as to add.
Worse, the noise from both systems sums. If you use circular polarization,
you are guaranteed a reduction in signal-to-noise the vast majority of time
for a small improvement a fraction of the time.
This is why microwave links and HF links that have random paths or multiple
paths "vote" with signal-to-noise detectors to pick a single polarization
that is optimal at any moment of time. With line-of-sight the signal could
have a set, known, repeatable, rotation. With things multi-pathing and
bouncing all around, there is no phase or rotation consistency, so they have
to "vote" to the best polarization and ignore the other at any instant.
There could also be a system that detects phase and corrects phase to add,
but it would have to be a smart system with signal phase correction.
In the systems we have, the only practical combining is through stereo
diversity. Your brain has to learn to process independent identical
phase-locked channels from two different antennas. It does not even have to
be polarization differences, spatial differences alone will be enough on HF
and MF.
For example, two identical Beverage antenna systems here separated maybe 3
wavelengths or more will have entirely different fade times. Signals can be
completely out on one, and still workable on the other. Your brain can then
learn to sum the independent signals in each ear (if they are phase locked)
and make maybe 3-6 dB improvement when both ears have signal, and not be
distracted by the left ear noise if only the right ear has signal. Phase
coherence is not critical, but lock is.
This goes partly away if the channels are not locked. Even 0.1 Hz unlock is
deleterious.
This ALL goes away if the channels are a few Hz or more out of lock.
The advantage goes away if channels are combined, except for seconds or
minutes of "luck" followed by equal times of "bad luck".
I can sit here and flip switches to parallel channels, either into a
receiver or on the output, and these results are repeatable. I can combine
dipoles (which by the way are only horizontal broadside to the dipole,
tilting to vertical off the ends) and verticals, Beverages and loops,
Beverages and Beverages, verticals and Beverages, and it all repeats over
and over the same way. I can shift phase between channels bringing wide
spaced or cross-polarized systems in matched level and phase, and a few
seconds to a few minutes later it is back at 180 out or one channel is
adding nothing but noise.
I'm afraid just like in commercial systems with scattering or multipath
propagation, a circular polarized system is a net detriment.
73 Tom
----- Original Message -----
From: "Carl Luetzelschwab" <carlluetzelschwab@gmail.com>
To: <topband@contesting.com>
Sent: Monday, February 03, 2014 12:16 PM
Subject: Topband: circular polarization on 160m
I hope everyone has had a chance to work FT5ZM on topband.
With respect to circular polarization on our HF bands (3.5 - 28 MHz) and
on
6m, theory says both the ordinary and extraordinary waves propagate thru
the ionosphere with pretty much equal ionospheric absorption. Thus
circularly polarized antennas can provide an advantage. Some of
the real-world examples I'm aware of have been documented by G2HCG on 10m
(in the old Communications Quarterly), by the original K6CT on 20m (in the
RSGB Bulletin) and by WA3WDR on 75m (a web paper). I'm sure there are
others out there, too.
On 160m, theory says the extraordinary wave incurs much more ionospheric
absorption (more heavily attenuated) due to 1.8 MHz being so close to the
electron-gyro frequency. Thus in theory only the ordinary wave is useful
on
160m, which says circular polarization wouldn't do any good.
Now things happen on 160m in the real-world that we simply don't
understand. For example, an ordinary wave can excite an extraordinary wave
under certain ionospheric conditions (if you'd like to read more, curl up
in a warm place on a cold night with Chapter 3 in Ionospheric Radio by
Kenneth Davies). Could this be happening? I don't think we can rule it
out.
In my opinion based on all the reports on this reflector over the years,
it
seems to me that having selectable elevation angles is more important than
polarization. But I also admit that there hasn't been much work in the
polarization field (no pun intended) on 160m (except for N4IS with his
horizontal Waller flag - which makes sense with theory for roughly
East-West propagation close to the geomagnetic equator).
Carl K9LA
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