Is there not a built in loss of 3db on both TX and RX with a CP antenna
compared to an Axial mode antenna? Not that it makes that much
difference on RX but 3db is 3db. Another issue with CP I understand is
the difference between LHCP and RHCP for space communications is
supposed to be infinity for space communications. I do not know if the
same rules apply for HF with skip involved. Although I have seen this
on terrestrial UHF paths when the screw sense is reversed and a complete
loss of signal results. I would also presume that the construction of a
good CP antenna for 160 would be very difficult to perfect. I have seen
some antennas for AMSAT work attempting to produce a CP type antenna by
have two interlaced yagis, one vertical and the other horizontal, one
space 1/4 wave in front of the other, and with a quarter wave delay
line at the feed point separating each. If this could be replicated
between a TB horizontal vertical and a horizontal dipole some distance
away...I just don't know if this would even end up providing a CP wave
front. If they were far enough apart maybe there would be some
diversity gain./
Herb Schoenbohm, KV4FZ
On 2/4/2014 1:03 PM, Tom W8JI wrote:
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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