All,
Bah, humbug. We got another 5 inches of snow overnight. We're running out
of room to put the snow. At least I can stay inside and play on the radio.
Tom makes some good points. Two comments.
First, when I said "advantage", all I meant was there is less fading on HF
when receiving on a circularly polarized antenna. That's the common
conclusion of those "studies" that I referenced. Remember these studies are
HF (80-10m), not MF (160m).
Second, polarization is not purely random. There is more order to
polarization that we generally think due to the ionosphere being immersed
in a magnetic field. What's important is where the wave enters and
exits the ionosphere - and how well the polarization of the ordinary and
extraordinary waves that propagate thru the ionosphere couples to the
polarization of your antenna. In my mind that theory translates nicely to
the real-world. One of G2HCG's conclusions from his 10m study unwittingly
confirmed magneto-ionic theory. I don't think he was even aware of the
effect of a magnetic field on a plasma, so that makes his conclusion all
the better. Yes, the ionosphere is dynamic and varies over the short-term -
so there is some randomness imparted on the what the ionosphere dictates.
For the record, G2HCG's conclusion referenced above stated that "It was
immediately apparent that the number of hops to the ionosphere and back was
totally irrelevant. The polarization of signals must therefore be
controlled by the last hop."
I agree with Herb's comment about implementing a cp antenna on 160m -
a very tough job. The big question in my mind would be how do you separate
out the difference in vertical patterns when ground is taken into account?
Having said all the above, I still say circular polarization on 160m would
not be beneficial due to just the ordinary wave being useful.
Carl K9LA
On Tue, Feb 4, 2014 at 12:03 PM, Tom W8JI <w8ji@w8ji.com> 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
>> _________________
>> Topband Reflector Archives - http://www.contesting.com/_topband
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>>
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>>
>
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