Dan, KL7Y has provided valuable information and interesting
comments recently on the subject of low freq propagation. One of his
main points is that most 160 propagation is "normal", ground to F
layer hops, with pure reflections. I agree. He also feels that there
is little reason to believe non-normal propagation plays an important
role. Here I disagree.
>
First, let me correct an error I made in an earlier post, where I
said that there was 6 db per hop polarization loss. I was thinking
this type of loss occurred twice per hop, when it is only once per
normal hop, thus 3 db per hop would be a better aproximation. The
mechanisms are very complex, and very frequency dependant, and do not
concern the issues involved in the recent ducting discussion, so I
will leave this topic for now.
>
Dan says:
> We've been talking about low band propagation here, which is accomplished on
> nighttime paths in the absence of the D layer.
This is a major issue.
For 160 meter propagation where the entire path is well into
darkness, the propagation may well be primarily normal F hops, with little D
layer absorption. One does not need to invoke ducting propagation
modes. This also applies to short range paths from East Coast to
Europe during darkness.
Where ducting modes become important is near sunrise/sunset, and along
the terminators. These are not "nightime paths in the absence of the
D layer" conditions. Long path 80 meter propagation from the West
Coast to Europe is a fairly common occurrence at West Coast sunrise,
as are openings around sunrise/sunset on 160 meters. Why are there
paths open at these times when the path closes as one moves into true
"nightime paths"? Conventional propagation programs that only model
pure reflections and F and E hops fail to show these openings. I
found that when I incorporated refraction at the E layer (bending
into a lower angle, but not a complete reflection) and "ducting"
(bottomside F layer reflection to an E layer topside reflection,
etc.), then my program showed long path 80 meter terminator openings
and also 160 meter predictions that corresponded better to what I was
hearing. The paths close as one moves into darkness because the
ducting stops. It stops because one is no longer getting
refraction into low angles at an E layer penetration and/or loss of
sufficient E layer ionization to allow bouncing off the topside of
the E layer at the angles the rays can achieve.
> I fail to see how signals below the Fcrit can be ducted. If all angles are
> returned to earth, as they are for frequencies below Fcrit, how are they
> able to enter the alleged "whispering gallery" ionospheric duct? Remember,
> we're talking topband here.
Yes, it is especially on topband where this happens.
The reason why is that the E layer is still able to reflect low
frequency waves back up to the F layer. For low enough angles,
1.8 MHz is below both the Fcrit and the Ecrit. Higher frequencies
would pass down through the E layer to ground, while the low
frequencies bounce off the top of the E layer and fail to reach
ground. The signals are able to enter the "duct", because of
refraction to a lower angle at the entry point, or because the E
layer allowed a lower penetration angle earlier in the path. NM7M has
described additional, more sophisticated, mechanisms for these ducting effects.
> BTW, if you can come up with a propagation
> program that will reasonably handle predictions in or near the auroral zone,
> I'd be glad to buy it. So far, I've just wasted my money.
>
>
Dan, you are absolutely correct. Until auroral effects are properly
modelled, propagation prediction will fail to deal adequately with
polar paths, especially for low frequencies.
>
>Let me try to explain this in the light of my experience. When using
>the backscatter radar or sounder, one of the great displays was a
>graph of freq vs distance. You could instantly see how any given
>frequency was being propagated. In the day, for example, I might see
>that 14 MHz had a dead
>
Dan, surely this radar system was one of the greatest potential tools
for studying propagation. Is it still sitting there in an operational
state?
I find your descriptions fascinating, but I have to wonder why you do
not recall seeing ducting propagation on your displays. I would like
to discuss possible reasons for not seeing it other than that it was
not there.
1) Are you saying that you always saw a ground reflection on every hop
(I suppose this means that you got a radar return), thus no cases of
a return on hop 5 with no returns on hops 1-4? Is this what ducting
would look like on the radar screen?
2) Was the radar actually operating on say 1.8 and 3.5 MHz at
sunrise along the terminator? I would not think there was any
practical reason for doing that. But it is mainly sunrise/sunset when
ducting should show up. When I used to hear the "woodpeckers", it was
very rarely on 80, and never at sunrise/sunset that I can recall, and
never on 160. Since propagation would have been better on higher
frequencies, I would think that the radars were operating on those
higher frequencies, where ducting is less likely.
3) Were you looking for long range (>8000 km) returns using low
frequencies where the takeoff angle would be 30 degrees or so to
penetrate the E layer? These would be >10 normal hop paths.
I have certainly had qso's with Africa, where the path goes over
Europe, where neither I nor the African station were hearing Europe,
when certainly there were Europeans operating. Are you sure you never
saw these kinds of openings on the radar? (Probably 7 MHz is the more
common frequency for this sort of opening.) I doubt that you or I could
ever work ZS4TX on 160 if the path over Europe provided propagation
to Bernie from the Europeans.
Dan, I look forward to hearing more of your interesting comments.
73, Rod VE7FPT
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