----- Original Message -----
From: "GEORGE PRITCHARD" <firstname.lastname@example.org>
> 3. True. A low gain omidirectional antenna would work almost as well...
generally, but the arriving difference in angle of radiation may cause
additional fading, or >enhancement and is less defined as a system. Having
the same gain and the circular wavefront seems to be a more defined system.
Sometimes I see NO >difference in circular polarization. Here is something
you may find interesting: Somtimes circular is 3 dB stronger... indicating
that the arriving signal has been >faraday shifted to circular and is
matching the "right-hand-sense" of my antenna!!! That was very unexpected
and a real eye opener for me, although that >condition is pretty rare. I
have aslo noticed that sometimes the signal is MUCH worse in circular,
indicating the arring signal has been made circular and is >OPPOSITE sense
and heavily attenuated. I watch what is going on and play the diversity
"game", with some benifit as I attempt to match the current conditions.
Another interesting thing to consider is how the ground reflections
modify the "stokes parameters" of an antenna. With the typical
circularly polarized feed, you will get a circulary polarized pattern
on boresight in free-space. When you put the antenna over ground,
however, I would expect the ground reflections for the vertical and
horizontal components of the pattern to introduce different phase
shifts. Also these phase shifts are going to vary as a function of
takeoff angle. As a result, the polarization "ellipse" of a circulary
polarized HF antenna should vary quite a bit as a function of TOA.
At some TOAs, it might be linear and tilted, while at others it might
even reverse if the phase shift is sufficient (RHCP becomes LHCP).
This could explain why Yuri's XJ3ZZ/1 antenna provided superior
fading characteristics even though it was an ostensibly linearly
polarized antenna. In free space, a corner-fed loop will give
linear "tilted" polarization (not circular), but when you put it over
ground, the reflections might provide the differential phase shift
between the horizontally and vertically polarized field components
such that the resultant polarization characteristics were elliptical
or circular (at least at some TOAs). I dunno if TA keeps track
of phase. If so, one could probably simulate all this.
73 de Mike, W4EF
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