Jim - NN7K wrote in
. net:

....
work! Oh well, sorry for the trouble- thought you
talking about VHF! Jim NN7K
....

There seems a theme in some of the postings that Sporadic E is a VHF
phenomena.

Go look at the ionograms from an ionosonde, it can measure the critical
frequency (don't know what that means? consult a text book) of the layers
above itself at the time, and it discovers E and Es layers. The highest
return frequencies indicate the critical frequency, and it isn't
(usually) at VHF, it doesn't need to be to facilitate VHF paths.

If the f0Es is say 10MHz, consider the length of the paths, the virtual
height of the layer and the MUF for the path (approximated by the secant
of angle of incidence * f0Es). If you approximate the earth as flat, a
500km distance to a virtual reflection point at 100km height with f0Es=
10MHz yeilds an angle of incidence of 78deg, sec(78)=5.1, so MUF=51MHz.
The earth is not flat, so a slightly longer path is needed for MUF
50MHz... but the paths commonly worked are 1000 - 3000km, and more. You
don't have to do the calcs, some of the space weather sites will fake up
an oblique ionogram for you, certainly ours can.

Sporadic E is not the only mechanism in long distance VHF contacts, and
contacts are not necessarily dependent on a single mechanism.

But back to the original question, if there is a sporadic E layer
overhead at night, why would it not support NVIS 4MHz contacts?

The maps of f0F2 look pretty, but there is reduction of a lot of
measurements to a single statistic, interpolation and a delay between
measurements and publication that questions their use to prove that the
contacts you had could not happen.

I still ask the question "how can they measure f0F2 when there is such
strong "reflection" from a sustained E layer that no signal returns from
the F layer?".

Owen