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).
Topband Reflector Archives - http://www.contesting.com/_topband