Topband: strange conditions

Eric Scace K3NA eric at
Fri Feb 24 05:39:45 EST 2006

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 

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.

    -- Eric

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