Topband: polarization

[Carl Luetzelschwab]

Thanks to everyone for your comments on polarization. I certainly agree
with Tom W8JI in that this discussion is a brain exercise (he also said
enjoyable). It forces me to delve deeper to asses whether I have a
screwed-up understanding of all of this. And he has a good point in that
this is a system analysis.

It also seems to me that polarization in the ionosphere is a bit fuzzy to
many people. First, an HF electromagnetic wave upon entering the ionosphere
splits into TWO waves that propagate thru the ionosphere - one of these
characteristic waves is called the ordinary wave and the other one is
called the extraordinary wave - they rotate in opposite directions - and
the polarization ellipse at HF for both is pretty much circular (a special
case of an ellipse). On MF, the polarization ellipse tends towards a thin
ellipse - with the ordinary wave essentially being vertically polarized
since the major axis of the ordinary wave is parallel to the magnetic field
lines at these latitudes - which in general dictates vertical polarization
on 160m at mid to high latitudes in the northern hemisphere. Did anybody
note the signal difference between OH8X's horizontal Yagi (before it came
down, of course) and a vertical (or 4-Square) also in OH-land?

Second, the magnetic field dictates that only two characteristic waves
can propagate in the ionosphere - and as I said above they are pretty much
circularly polarized at HF. Chapter 3 of "Ionospheric Radio" goes thru the
math that shows this (equation 3.7). Again we're talking about waves that
propagate in the ionosphere. Of course the polarization of the wave from
your antenna to the entry point of the ionosphere is that of the antenna -
and it should be very constant until it enters the ionosphere. So the job
of your transmit antenna is to deliver the most power at the azimuth angle
dictated by the ionosphere, at the elevation angle dictated by the
ionosphere, and at the polarization that couples the most power into
the characteristic wave (ordinary or extraordinary) that is propagating
best thru the ionosphere.

Tom asked "What ratio of V to H signals levels do you expect?"
I can't answer his question as it's really an ellipse - not a V component
and H component. What I can say is on 160m, from my work with the PropLab
Pro ray tracing software, I would expect the ratio of the major axis to the
minor axis of the ordinary wave's polarization ellipse when it comes out of
the ionosphere to be in the neighborhood of 2.5 (the major axis length
is 2.5 times the minor axis length). For reference, circular polarization
would be a ratio of 1.0 (both axes are the same length).

That's enough for now. This is interesting stuff for sure (at least to me!).

Carl K9LA