Topband: circular polarization on 160m

Tue Feb 4 12:03:18 EST 2014

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 at gmail.com>
To: <topband at 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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