On 4/10/2009 11:08 AM, jim Jarvis wrote:
> accepted the literature which said
> that all signals arrived horizontally polarized.
I can't recall ever having seen this bit of folklore written down
anywhere. Does anyone know a source? Just my curiosity.
FWIW, the phenomenon of "skywave" fading was first studied by Bell Labs,
RCA, NBS, etc., in the early days of shortwave radio, the 1920s. They
pretty much got it right back then :-)
> This morning, I learned that it may be
> polarization that is the principal
> variable.
And at times when multi-path (spread) propagation is present, that can
be a significant source of fading.
Very broadly speaking, near the MUF when only a single, stable
propagation path is present, polarization fading will be the dominant
cause of fading. Below the MUF or when the ionosphere is disturbed, then
multi-path effects are significant. Far enough below the MUF, where
absorption limits the number of alternate paths, polarization fading
once again becomes important.
To counter polarization fading one would like to have an ability to
switch between antennas with orthogonal polarizations. To counter
multi-path fading one would like to have an ability to switch between
physically separated antennas, i.e. spatial diversity. To counter
ionospheric propagation mode switching, e.g single-hop F2 vs. double-hop
F2 vs. single-hop Es, etc., one would like to an ability to switch
between antennas emphasizing different elevation angles.
Once again the bottom line is, as it has always been, having more
antennas from which to choose is better than having fewer.
Oh, and those antennas are on towers. :-)
73,
Mike K1MK
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