Topband: 160 Meter Propagation

[Thomas Giella]

Ford N0OQW Said:
I've also noticed that the A index varies depending on who takes
it--which implies where it was taken and not variability in the
measurement. Is there a local phenomena that can affect only the
appalachian mountains to the east and the Rocky's to the west?

Tom W8JI Said:
I've given up totally on watching the A and K indices to estimate how
the band is.

  There is the A index which is centered on Boulder, Co. and the Ap
index which is a sum of many A index measurments taken simultaneously
around the globe, including Boulder. It works in a similar way with the
k and kp index. An A index above 10 and K index above 3 generally
indicate reduced propagation on the bands especially 30 meters and
above. These indices are a measure of the health of the F layer, which
can be shredded by ionospheric storms. As one to three hop propagation
is via the E layer on 160 meters, the indices don't as readily
correlate. On long haul DX paths where E/F layer ducting and or
underside F layer guiding of the signal comes into play, the indices
more readily correlate. As 160 meters is a medium wave frequency located
band the E layer is king.   
   Irregularities in the shape and density of the aurora oval can create
seeming dead spots between say a Nebraska and Washington propagation
path or Nebraska and Maine, while a Nebraska to Colorado or Florida path
will be open. With numerous solar flares and ion storms going on during
a contest, propagation can seem a jumbled unpredictable mess. We want to
look at things in simple homogeneous terms when the ionosphere is a
heterogeneous medium on the quietest of solar event days.  
   Another little known phenomena is refractive and ducting properties
on 160 meter signals from temperature and dewpoint discontinuities, ie a
cold or warm front. 6 and 2 meter ops are familiar with this type of
propagation, though it's not exactly the same. An arctic frontal boundry
on say a line from Michigan SW to Texas, then curving back NW and N
along the front range of the Rockies to Canada, can enhance propagation
between Florida and Ohio, by focused refraction off of the frontal
boundry to the west and block by scattering and diffusion a path between
Florida on one side of the front and New Mexico on the other. The lower
the angle of signal takeoff the more pronounced the effect is. KK4TR and
I have observed this phenomena many times over the years.
  The better a station's antenna system like W8JI's, the harder it
becomes to discern and correlate the A and K indices to what you are
actually seeing propagtion wise on the band. Stronger more focused
transmit signals and more focused receive antenna's push the progagation
envelope past what a backyard operation can do along a propagation path
during a solar flare or ion storm. 
   160 meter propagation is not as mysterious as it's made out to be.
The explanations are out there, the available info must be read and in
some cases reread. Much knowledge can be gained by reading N4XX Ted
Cohen's CQ magazine propagation articles. I have a link to his articles
on my 160 meter website at http://kn4lf.tripod.com .Also a wealth of 160
meter related propagation info is hidden away in the receiving antenna
and propagation archives of the broadcast band dxing organization "The
National Radio Club".

Take Care,

Thomas KN4LF


--
FAQ on WWW:               http://www.contesting.com/FAQ/topband
Submissions:              topband@contesting.com
Administrative requests:  topband-REQUEST@contesting.com
Problems:                 owner-topband@contesting.com