Tom,
Perhaps it is much simpler than that.
Recalling KL7AJ's article,
http://www.nxtbook.com/nxtbooks/arrl/qst_201203/index.php#/42 he states
that the magnetic field of the earth splits every HF signal into a right
hand or left hand circular wave. Add to that the unevenness of the
ionosphere can tilt the wave as well. All this is something that seems that
it could cause slow fading.
An interesting question, to be sure.
Jim - KR9U
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Tom W8JI
I hope Carl K9LA has input, but I cannot think of a single way that a
circular polarization signal would have fading that would be corrected by
changing from a linear polarized antenna to a circular system. This is what
perplexes me about any advantage of using a circular RX antenna based on the
signal:
1.) If the wave was circularly polarized, that could not cause a fade on a
linearly polarized antenna. It rotates far too fast for that. It would just
be a steady 3 dB loss.
2.) A slowly rotating signal can go into fades as the electric field crosses
the minimum response of an antenna. Making an antenna that responds **in a
correct way**, so we don't have a skewed or sloped linear polarization
(because that would still fade), might cure that fade. The cure would always
be at a S/N penalty for half of the rotation or more. The tradeoff would be
a few moments large advantage (during the fade) for a longer time
disadvantage. If the horizontal antenna did not have comparable directivity
to the vertical, that system could "totally hose" S/N for all but a very
short time, that time being when there would have been no signal.
3.) On VHF, and even ten meters, we can build a directive vertical and
horizontal antenna with a good pattern at low wave angles. The wavelength is
short enough we can get away from noise, the earth, and have low angle
horizontal patterns. But.....a linearly polarized antenna would not fade to
zero from a rotating wave unless it was rotating slow. The period of
rotation for a circularly polarized wave is far too fast for that. I can
tune into FM BC circularly polarized signals with linear polarized antenna,
either a dipole or vertical, and not have a bit of fading. Any fading would
only come from my having the wrong rotation on a circular receiving antenna,
or a long term null of response from a very slow rotation.
This is what perplexes me...to have fading from polarization it has to
rotate slow. That is not a circularly polarized wave by the normal use of
the term. If the wave rotates slow, the R-H L-H sense of the antenna makes
no difference at all.
So why are experimenters hearing slow fade on a linear antenna, and
correcting that fade ONLY with a certain L-H or R-H antenna? It was
rotating fast enough to be circular, the antenna rotational sense would make
zero difference and it would not be fading on a regular single polarization
antenna. The wave rotation would, at best, only cause a 3 dB fade into a
linearly polarized antenna.
This gives me pause about what people are measuring and writing. If they are
correct, hundreds of FM BC transmitters need to change their antennas. We
have a WA3 claiming the rotational direction makes a difference, that
implies the wave is circular. But if the wave were circular, he would not
have fading on a linear antenna.
73 Tom
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