Topband: skew paths

[Dan Robbins]

What an interesting discussion!  I do have some observations.

1.  While skew paths often seem to occur during disturbed geomagnetic
conditions, I suspect that the association is not that disturbances
cause skew, but rather disturbances shut down normal paths allowing us
to hear an alternate path.  I have heard skew under very quiet
geomagnetic conditions, also.  In those cases the normal path was shut
down by seasonal conditions such as a polar path being bathed in
sunlight.  We may have skew paths going unnoticed because the normal
path is not impaired.

2.  I've heard lots of LDEs on 80 and 160 of the type that Eric
discussed, but I don't believe this is the major cause of the skew path
on 160.  First, to hear the LDE requires a very weak ionosphere.  This
is commonly only found at night,  in the dead of winter,  near the
sunspot minimum and at a high latitude where most of the 24 hour "day"
is bathed in darkness.  The signal can leak out of this depleted
ionosphere, travel in the magnetic tubes out in space and be bent around
to the opposite pole.  It reflects off of top of the ionosphere at the
other pole and works its way back - that is the general theory as I
understand it.  The big story here is that if one pole is bathed in
darkness, the other pole must be in sunlight.  The signal will only leak
through the depleted nighttime ionosphere, not the more energetic
daylight.  One end will be a hole and the other end will be a mirror.
Everything is high angle, too.  There may be a specific season and
geometry that does support leakage at both ends, but skew apparently
occurs at all times of the year and over most places on the globe and
during all phases of the sunspot cycle.

3.  There may be more than one cause of skewing.  As people have put
forth different ideas, I can think of one instance that seems to fit a
certain hypothesis, then immediately think of another instance that
refutes it.  The second instance seems a good fit to a different
hypothesis, however, so maybe there is more than one way to skew a
signal.

4.  My pet theory is backscatter/sidescatter from ocean waves.  While
this certainly would not account for all cases of skew path, it does
have several advantages.  The backscatter mode is well known and has
been used for many years for oceanographic research from a distance.
The strength of the scattered signal varies tremendously according to
the shape, height and direction of waves in the sea.  At times the
backscattered signal can be unbelievably strong even after many hops
even with amateur power levels and simple antennas.  The direction and
height of the waves can cause changes in both the horizontal and the
vertical angle of a propagated signal.  The scattering surface is huge
and sea water is an excellent reflector.  Ocean backscatter is
independent of the ionosphere and the solar cycle and may only be weakly
correlated with the seasons on certain parts of the earth.  Finally
there have been instances where both stations involved in a skew path
have been able to gauge the direction of the signal.  In at least some
cases, the two beam headings met in the middle of the ocean with no
other apparent scatterer to be found.

5.  One would think a couple dedicated graduate students could set up
some nice directional antenna arrays and investigate skew paths with
sophisticated equipment.  Probably work a lot of DX on 160 on the side,
too.

Dan KL7Y


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