Topband: circular polarization on 160m

Tue Feb 4 14:02:05 EST 2014

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 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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