TopBand: Ducting

[Garry & Yelena]

w8jitom@postoffice.worldnet.att.net wrote:
> 
> I decided to post this after several e-mails about ducting.
> I'm not convinced ionospheric ducting is anything beyond pure
> speculation. Even the experts admit there is no way to measure the
> effect, that means it is pure conjecture.
> 
Sounds like the arguments for or against Creation and the Big Bang.

>From what I read here, "ducting" is an arbitrary label for certain
observed, but not measurable occasions of unusual propagation, which may
or may not have been due to paths not involving repeated encounters with
the earth's surface. Pretty nebulous, it seems. 

But there are several suggested means for such propagation--Pederson
rays, chordal hops and "ducts," the term as used by writers like NM7M.
I  am no ionospheric scientist like NM7M, but I recall that:

* Pederson rays are incident at medium-high angles, penetrate further
into the ionospheric layer, past the region of maximum ionization and
are more gradually refracted by that lower ionization region. In the
extreme, the curvature follows the earth.

* Chordal hops involve rays which do not refract enough to return to
Earth, but are then refracted symmetrically by successive grazing
encounters with the F-layer, and eventually return to Earth. When I was
a Stanford grad student in the early '60's, Bob Fenwick had just
finished his thesis on "Round the World (RTW)" propagation, now called
chordal hop. It is three and a half decades later, but the antennas he
used--a pair of LPDA's--still stand, mute, below the Big Dish at
Stanford. It would seem a Pederson ray could instigate such propagation.
Supposedly, the ionization gradient at a terminator causes the return to
Earth. Fenwick recorded many instances of multiple echoes--several trips
around the Earth.

* Bob Brown, NM7M has written of ducts bounded by the F-layer at the top
and the top of the E-layer on the bottom. I recall something about the
phenomenon being initiated by gravity waves. This is far beyond my
college training to understand analytically--the Magnetoionic Theory was
more than enough for me.

OK, I cannot personally prove any of this. But, anecdotally, several
incidents from the 1996-97 season remain emblazoned on my memory--and on
that of Tree, and a bunch of other West Coast topbanders. These were the
evenings when Western  European signals--normally weak out here, if
heard at all--were LOUD, very loud. At NI6T, there are ONLY high arrival
angles in that direction, due to my canyon; I had no opportunity to
compare arrival angles. However, I recall that Tree, more blessed with
land and antenna options, heard these signals best with a very low
dipole in a gully--certainly a high-angle antenna. But Tree is 700 miles
north of here--not a controlled observation.

The effect appeared to peak at sunrise on the eastern end. This is not
an unusual time for east-west signals to peak, of course, but these were
dramatic, high-angle peaks. Given the hour, most stations situated
further east in North America were not present, so the opportunity to
compare signal levels is probably not available. It would be interesting
to see the logs of stations active on those evenings.

Were these incidents due to one of the above scenarios? I don't know,
but it does seem possible.

-- 
Garry Shapiro, NI6T
Visit the Northern California DX Club home page:
http://www.ncdxc.org

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