On Mon, 30 Mar 1998 08:31:01 +0000 Tom Rauch
<10eesfams2mi@mass1-pop.pmm.mci.net> writes:
To: <topband@contesting.com>
>> Date: Fri, 27 Mar 1998 17:59:47 -0500
>
>I would like to ask a question.
>
>What do people mean by a "duct"?
>
>When I read some of the comments, it appears a duct is credited
>for signals "focusing" in one area and no (or very very little)
>signal appearing elsewhere, even a few hundred miles away.
>
>So am I correct in assuming a duct should create a no signal zone,
>where stations only at the opening of the "duct" hear the signals???
>
>Is that correct, or do I misunderstand how a "duct" is supposed
>to appear to us?
Speaking from strictly a VHF/UHF perspective this has been my experience
Tom:
The duct is most often preceded by or during weather conditions
associated with storm fronts, extreme high atmospheric pressure and the
like. Not all produce ducts or they may just be formed in areas with no
ham population.
A typical event will start in Florida and will proceed up the coast with
Florida still an anchor point. As it lengthens out it becomes harder for
stations along the middle to access. So for instance when the duct
extends from South Florida to say Virginia, just about everyone can make
use of it to some degree. But when the duct reaches New Hampshire or even
Nova Scotia the stations along the New Jersey thru GA coasts are often
"frozen out". I have also heard stations on Cape Cod and Rhode Island
have their own private openings with nary a southern station heard here.
Some ducts appear frequency sensitive and the prevailing theory is that
they act as waveguides. One example was the 1296MHz record from MA to FL
a few years back...it appeared as a point to point event.
There are other events that may confuse the issue such as storm generated
E Skip and simple enhancements....this will manifest itself as stations
in Alabama for example accessing the duct from its East Coast/ Florida
Keys point.
I can still remember one good December Northeaster that had 2M 10W mobile
stations in FLA with 20 dB + over S9 signals up here. Outside was a
raging blizzard.
Hurricanes have generated similar openings.
How this can relate to 160M or other HF, I do not know since I have never
listened on those bands during an opening.....the feeding frenzy is too
intense and after all who is looking for loud US on HF ?
Here is something that I will toss out for discussion....is it possible
on 160M for a very intense, very low altitude tropo event to cause some
form of extended groundwave? This would assume that a low TOA and
coastal stations are involved. This "groundwave" would have very low
attenuation since it is over salt water.
Some of you who were in the Navy, etc can attest to the extreme ranges
that an AM BCB station can be heard at sea...in this case I am talking
about modest power and mid-day but take this a step further to darkness
when the absorbtion is minimal...forget skywave for a moment.
OR would it require an almost 90 degree TOA if the tropo was at a higher
altitude with a sharply defined top and bottom and could not be
penetrated by a low glancing angle?
None of the above would seem to preclude or deny that most long haul
propagation is via the more normal ionosphere method. Instead they would
be random, often localized and short lived events.
Indonesia to Georgia this past week a duct....I doubt it....but I never
heard a peep. The KL7 to NA East Coast.....possibly but for only the KL7
to OA coast area and even then just for a portion that was weather
enhanced.
What could explain an event back in 1986 or so ( those logs are in the
attic) that had 160M covered with low power SSB Southern European and
Asiatic Russian and Turkish stations. The path appeared to propagate
along the Mediterranean, into the Black sea. My "basic" Russian
attracted over 20 stations already in local QSO's and at the same time
other NA stations appeared oblivious to it all. The path opened and
closed in about one hour if I remember correctly. Many signals were S9+
and several commented they were running only 10W. At
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the same time the
more standard EU signals such as EA, I, F, G, etc, seemed to be at poor
to moderate strengths.
There just appears, at least to me, some events that do not fit models of
predictable propagation which are strictly ionosphere based. Those models
are valuable for the purpose they were developed....100% predictability
particularly for military use.
Hopefully this message will generate some good discussion without various
proponents "dissing" views of others. The discussion a year or so ago
degenerated to a no-win situation. Surprisingly, I was strictly a
bystander during that one!
73 Carl KM1H
Just a thought as I reread the above but perhaps we should all consider
the term "tropospherically enhanced propagation" instead of "ducting". It
might make the possibilities a bit more palatable.
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