TopBand: Ducting

Tue, 8 Jul 1997 20:48:06 +0000

> From:          Robert Brown <bobnm7m@baker.cnw.com>
> Date:          Mon, 7 Jul 97 19:26:54 +0000

Hi Bob,

> between VK2 and VE7.  The path starts in darkness over VK2 and has F-hops
> til it gets north of the geomagnetic equator.  Then, when it gets into
> field lines with a greater vertical dip angle, the ducting begins and 
> when the rays get closer to the dawn terminator, the path begins to 
> skew more (but not much).  That duct is only about 4,000 km in length; 
> however, it does not bounce off the sea or any ground, for that matter.  

I think you and I both agree ducting is speculative, and can 
not be measured. Since it can not be measured, we can't know many 
exact dimensional quantities. At this point it's just a theory. The 
only reason to accept it as a tentative fact is if it fits better than 
other theories or explanations, unless we can measure it..

So far most of the explanations have been pretty "thin", and 
do more harm to the theory (making it look like a wild reach) 
than good. 

The short length duct fits general observations much better than 
other explanations, and appears to be well thought out. But we need 
proof and not guesses.

I do know that my path to Europe, over the poorly conductive and 
scattering Appalachian range (much closer than 4000 kM) greatly  
affects my signal. I do miserably compared to other stations outside 
that path, and the mountains start far beyond my groundwave range.

Your theory is not much different than W1BB's theory. In Stu's 
theory the twilight densely ionizes the upper layers before the D 
layer is formed, and this concentrates the signal (since the signal 
REFRACTS, rather than cleanly hops or reflects) and focuses the 
signal just ahead of the area where the D layer is rapidly forming. 

Call that what you like, but the term "duct" paints the wrong picture 
in most respondents minds, I am not alone in thinking it indicated 
long distance propagation

> Now as for my remark,
>  
> > Finally, MUFs have nothing to do with propagation on 160 meters as the 
> > critical frequencies are ALWAYS greater than 1.8 MHz, even in solar 
> minimum. 
> > The VK0IR operation was wonderful testimony for that!
>  
> and your reply:
>  
> >Then the books I have are incorrect, because they state maximum 
> >signal strength is obtained near the MUF for a given path. And my 
> >experience is wrong also, because for the last 30-years-plus 160 has 
> >been hottest when twenty meters is stone dead and I can't work short 
> >skip on 75 meters.
>  
> I'm not saying your books are wrong, just that they have "the right
> answer to the wrong question", the HF bands and not Top Band.

During the 70's, I kept a log of MUF and signal levels over  paths 
to VK and Europe. 160 signals always were best when the MUF 
was at the very lowest frequency for that path. Years of that 
"coincidence" tells me MUF can be directly related to 160 
propagation, at least in my data base.

> But that is way up there on HF, not down 1.8 MHz.  The 1.8 MHz RF never 
> gets to the peak of the F-region, even at solar minimum.  If you go 
> to the databases, the minimum critical frequencies foF2 up in the 
> polar regions or at night are never less than about 3 MHz.  That 
> means one can operate on 40 meters or below and not have to worry 
> about MUFs as they are about 3*foF2 (The "Rule of Three" in HF 
> propagation).  So below 40, in quiet conditions, it is largely a 
> matter of ionospheric absorption and ground losses.    

That's agreeable. Ionospheric absorption, barring disturbances, 
is reduced as the lower layers disappear. That goes hand in hand with 
a lower MUF. I'm not sure 160 can be sawed off from 80 meters and 
tossed in its own woodpile. I prefer to look at it as more like 
HF than LW or MW. Especially during solar activity minimums.   

> All the lore that I hear suggests that propagation is BETTER on Top Band 
> during high K-values but so far, I have never seen any sort of data 
> collected on the subject that I could sink my teeth into.  But it sure
> sounds interesting.

I've logged short enhancements during a transition from a very low to 
a suddenly high Boulder K indices. But these are ABNORMAL conditions 
and not the reliable 160 propagation.

> Geomagnetically disturbed conditions can tear up the F-region and the
> HF bands, lowering high-latitude electron densities by as much a 25%.
> That's when 20 is dead.  Maybe 160 is hot then, I dunno.

It sure is, as a general rule.

> The problem I have is that I am trying to refine or distill all that
> I hear to the point where I can one apply some physics to the questions.
> The murkier the statements, the less chance I have of doing anything
> with them.  In that regard, before the VK0IR affair, I urged folks to
> give COMPLETE statements of the circumstances surrounding any sort of
> skewed paths - date, time, frequency, direction, geophysical data - 
> and with one exception, all that I saw on the Reflector was fragmentary, 
> at best.  If you look into the VK0IR book, you'll see more of the same.

When VK0IR peaked in the northeast, he was peaking here northeast 
but just out of the noise floor along my horrid NE path along the 
mountains. Prior to, during, and AFTER the brief NE peak he had a 
more consistent signal  east south-east. Nether signal was workable 
from hear. On the NE path, he appeared to have about the same dB 
disadvantage as any signal from Europe along that path, when compared 
to reports given by Northeasters.

I suspect if the east south east path was better, more inland 
southern stations would have worked him. 

My directional headings are pretty accurate, since I phase multiple 
elements in a large receiving array, rather than relying on a 
multi-lobed long wire type pattern. There absolutely were two paths 
open at the same time on two days.

Just like with my skeds with VS6DO, where his signal appeared 
just south of west and north-west at the same time many mornings 
with different fading rates.

73, Tom W8JI 

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