[CQ-Contest] Is HF HF propagation reciprocal?

[Steve Harrison]

At 08:21 PM 6/4/2007 -0700, you wrote:
>>I'm working on posting additional texts where I take various HF
>>propagation topics, and use modern software to model the concepts. 
>
>------------ REPLY FOLLOWS ------------
>
>IMO, modeling software is only useful when all parameters are known.
>That is not the case for so-called one way skip. Your software focuses
>on the signal to noise ratio, which is fine, but only part of the
>issue.
>
>for years, it has been theorized that one way skip is caused by a tilt
>of the ionosphere, so that signals going in one direction strike it at
>a very shallow angle, whereas signals coming the other way strike it
>at a more acute angle. Due to the angle of incidence vs MUF, one
>signal will be reflected and the other will not.
>
>I don't know if this has ever been convincingly proven or not, but it
>sounds reasonable. Does anyone have any further information?

Other theories hold that the noise floor on the eastern (dark) side rises
so much more after sundown that the S/N ratio drops sufficiently as to make
western signals uncopiable, if not inaudible in the east. While this is
certainly true, it doesn't explain why many high-power western stations
cannot be heard on the eastern side when eastern conditions are quiet.

New England large stations are particularly interested in this subject
since we can often hear Europeans several hours before our sunset on 160
and 80. I rember when I lived in California during the '60s and '70s, we
could always hear hundreds of eastern stations on 40m throughout the sunny
summer hours on 40m in Field Day, but could rarely work them until sunset
in W6-land.

OTOH, it's not at all unusual for eastern stations to work west several
hours after their sunrise. I rember playing hooky from school sometime in
the early '70s and going to K6UYC's (now K6RR) Orange County superstation,
where Bob had a huge fullsized 5L 40m yagi. I was running JAs at 10 am LA
time, some 3-1/2 hours after our sunrise, until Bob shooed me outta the
station as he had to go to work  ;o((((  IIRC, I also worked a few
Scandinavians direct-path that late in our morning. It was quite a riveting
experience for a young kid who'd only DXed with inverted vees until then
8-))))))

One has to rember that the D-layer altitude effectively rises and falls
with darkness and sunlight; clearly, the reflective and absorptive edges
have to be tilting to some degree as ionization occurs. During the summer
months as sunlight occurs earlier further north, there should also be a N-S
tilting which would skew a given path.

Steve, K0XP