On 4/9/2009 2:07 PM, Steve Katz wrote:
> Actually, *all* inospherically refracted signals come down with circular
> polarization; that's been verified lots of times.
No, virtually never circularly polarized. The general case for a 100%
linearly polarized wave that propagates through the ionosphere (a
magnetized plasma) is for the wave to be converted into an elliptically
polarized wave having some intermediate degree of ellipticity between
linear and circular.
Just how elliptical depends on several factors, but primarily the
initial orientation of the polarization and the direction of propagation
relative to the Earth's magnetic field. As the direction of propagation
relative to the Earth's magnetic field varies over a great path,
computing a wave's polarization state upon exiting the ionosphere is
non-trivial as it requires performing a full ray trace.
Circular polarization represents one extreme case with specific, and
highly contrived, initial conditions and propagation path with respect
to the Earth's magnetic field.
There are other equally contrived initial conditions and propagation
path for which the wave would be purely linearly polarized when exiting
the ionosphere.
Independent of the degree of polarization conversion that takes place,
the plane of polarization (the major axis for elliptical polarization)
of a wave propagating through the ionosphere will be rotated (Faraday
rotation). And small variations in the ionosphere conditions along the
path of prorogation are capable of producing the large rotations in the
plane of polarization that we observe.
> This is why when you have both horizontally and vertically polarized
> antennas, you can watch an ionospheric signal fade on one, switch quickly to
> the other and catch a peak until that fades, and then switch back to the
> first one and watch the signal gain strength again.
Well, if it were actually a circularly polarized wave that was coming
down, then that would of course be received at the same strength on a
horizontally or a vertically polarized (or any other orientation of
linearly polarized) antenna.
The effect that you're describing is the result of time varying Faraday
rotation.
> The ability to switch quickly can be an advantage,
Definitely. And even more an advantage (for reception) if that switching
can be done instantly and automatically as in diversity reception.
73,
Mike K1MK
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