On 13/03/15 07:30, Jeff wrote:

If you understand what I wrote above, you'll see my point was about
local conductivity and how it affects the radiation pattern after being
launched from an antenna, rather than the variability along a signal
path, although I did mention for completeness that models can now take
such variability into account.

I think it is your choice of words that is causing the confusion. It is
not normally the practice to consider the ground conductivity over the
entire transmission path when considering the radiation pattern of an
antenna. It is usual to have a 'local' radiation pattern and then
consider what happens on the path as a separate (path loss) issue.

Mmm...my original question was concerned with how much power wound up
where, I'm sure this sort of thing was asked after sky-wave propagation
was discovered ~90 years ago. I believe that ground-wave/space-wave
propagation was understood before then.

Obviously with a sky wave path the intervening ground has no effect, but
with a ground wave signal it can have a huge effect, particularly is
there is water in the path. The ground causes the lower portion of the
wave to be retarded so you can think of the wave-front as starting to
slope, the degree of additional slope along the path depends on the
ground properties (conductivity and permittivity) at any point. The wave
will propagate like this until the 'slope**' becomes too great an angle
for the wave to propagate.

Yes, I'm familiar with the concept, I've mentioned it several times
before now, usually in terms of an ultimate maximum surface-wave range.

Interestingly, the ITU ground-wave curves that were referenced in this
thread show no such phenomenon, even at 30 MHz, or suggest that this is
a much gentler in action than might otherwise appear - perhaps this
implies there is a limit to the veracity of the modelling?

**apologies to Jeremy Clarkson
..
I recall a cartoon about Spiro T. Agnew in the 1960s.....Unsurprisingly,
I can't seem to find a reference to it. I think it was published in Time
magazine, BICBW. Today, it's what's known as 'not politically correct',
the current version of doublethink applying here.

[1] Once upon a time, the group I worked in had brought in the UK's
leading theoretical electromagneticist to act as an adviser. Afterwards,
I buttonholed him and asked for his view on an e/m issue[2] I was
responsible for. It was a straightforward question, but he had
difficulty understanding it, nonetheless he said he'd look into it. He
got back to me two weeks later. In apologising for the delay, he said
that no-one had ever asked this question before, and he'd spent the
intervening time researching the issue. I'm well used to asking awkward
questions of experts, it's their replies that give them away as to
whether they know their stuff or not.

[2] related to what happens when an e/m wave meets a surface, so not
entirely disconnected from this thread. It's a subject one would think
would have been well covered, but apparently this was not the case, and
asking a simple question revealed that.


--
Spike

"Hard cases, it has frequently been observed, are apt to introduce bad
law". Judge Rolfe