When operating at 3B9C, I frequently noticed the band
becoming very quiet in preparation for a good opening to
North America. The change was quite distinct. Sometimes it
happened quickly, and at other times it might take 15-30
minutes to develop.
My favorite hypothesis is that the ionosphere is developing
a broad (large azimuth range) E-F layer ducting region. In
the directions of the duct, the geometry of the duct may
make it more difficult for QRN sources which sit along the
way (and under the duct) to couple their noise into the duct
or otherwise propagate back toward my receiver. In other
words, the duct provides some shielding.
Looking at 3B9C logs, we can see many situations where
stations in large regions where unable to contact us while
stations further away had extremely strong signals. On one
memorable night with outstanding signals from the USA, not a
single station from New England was logged (New England
being the closest part of the USA to 3B9C). The structure
of the E-F duct on that night simply did not allow signals
from New England to propagate well. If we imagine a large
region of thunderstorms in that area, all that QRN would
have been similarly restricted in propagating back toward 3B9C.
With less QRN propagating back toward my receiver, the band
became quieter.
On some nights, during the development of the duct, we
observed European signals drop significantly in strength as
well.
It should be noted that, as sunrise approaches, QRN
propagating from the newly sunlit sectors encounters high
absorption. But this is routine quieting ... not the sudden
or unusual quieting mentioned in the notes from Larry N7DD
and Steve KK7UV.
73,
-- Eric
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|