That diagram looks like it is for an isolated dipole in free space. I would
be careful about using it in this case.
That being said, I am not disagreeing with you. I'm only trying to
understand what the model is doing, and what it means by "surface wave."
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Richard
Fry
Sent: Sunday, September 15, 2013 13:05
To: topband@contesting.com
Subject: Re: Topband: More anecdotal "stories" to cause one to stop and....
"AJ4SQ" wrote:
>... I'm not sure what plot you are referring to. Was this for a
>wavelength of something like 160m?
Yes.
> I don't see how field values at a 100m (0.625 lambda) range and 9m
> (0.05625 lambda) altitude can tell us anything about the far-field
> behavior.
Note that such radiation may be relatively close to the radiator but is in
its far-field, regardless of the terminology/convention used by NEC. The
near-field/far-field boundary is located at one radiansphere from the
radiator, which is lambda/2pi (see clip below). So at the 1.85 MHz
frequency of my plots, that boundary is located at about 25.8 meters, and my
calculations for 100 meters are about 3.88 times beyond that boundary, into
the far field.
For background, I contacted Gerry Burke in January, 2012 when I was
researching the basis for the comments I have been posting here. Probably
most will recognize Gerry Burke as the co-author of NEC software, working at
Lawrence Livermore National Laboratory.
I sent him NEC surface wave plots similar to the one I posted in this
thread, and asked him, "...would you expect the fields at elevation angles
of 1 to 10 degrees in these plots to continue on to the ionosphere, and
under the right conditions be reflected back to the earth as skywaves?
His reply was (quoted with his permission): "The low angle 1/R fields should
reach the ionosphere, although perhaps not accurately predicted by NEC,
since it does not include the effects of earth curvature and the
ionosphere."
G. Burke's reply should be rather conclusive on the subject.
Clip taken from Kraus' ANTENNAS FOR ALL APPLICATIONS, 3rd Edition...
http://s20.postimg.org/xavzwzxwt/Kraus_Graphic_Fields.jpg
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Topband Reflector
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Topband Reflector
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