On Thu, 15 Nov 2007 23:35:08 -0500, "Stefan Wolfe"
wrote:


"Richard Clark" wrote in message
.. .
On Thu, 15 Nov 2007 12:29:08 -0000, "Mike Kaliski"
wrote:

http://home.comcast.net/~kb7qhc/ante...pole/index.htm

I don't intend to go too far off topic but the near field results are so
strikingly different when you add the ground screen. This is an area that I
am not so familiar with

The old saw of dipoles not needing a ground (one of those things that
makes them so much "better" than verticals) is confounded because they
do need a ground - if you want more gain. I don't suppose this is
covered in the RSGB RadCom issue (and now we are back on topic).

It would seem that the strictly horizontal dipole crowd doesn't need
the gain....

The simple fact is that a ground screen reduces ground loss.

. At 80M does the program assume that the ground at
near field is basically a dielectric instead of a conductor, acting like a
large capacitor

Basically, more like a resistor. No new science is being marketed
here.

whereas the screen is a pure conductor?

Basically, more like a screen (as in shield). If you want to keep it
in the metaphor of "large capacitors" that is what near fields are
for. Remove the screen and you have a lossy capacitor. A large
inductor could as easily demand center stage and without the screen,
we are talking about a lossy inductor.

It would seem this
would have to be related to the explanation, correct? It sure shows at least
one possible advantage of using a lot of radials; it reduces near field (?)

Reduces loss. Near fields being reduced is something of an oxymoron.
They are still as near as they ever were, the page(s) only illustrate
the impedance of near space, expressed in terms of a ratio we commonly
call mismatch. In other words, the conventional 377 Ohms of
air/vacuum, in the proximity of a conductor, is quite different in
value. In some regions, it is represented as being as low as 120
Ohms, in other regions as high as 1200 Ohms. As both these values
represent a mismatch ratio of the order of 3:1 (or, contrariwise 1:3),
the colors are mapping only the ratios, not the 10-fold variation
(that would be too many colors). With finer grain resolution, these
values plunge and rise well beyond these arbitrarily restricted
values.

Someone is sure to exclaim this would only increase NVIS. Yes, the
skyward lobe is enhanced, but so is the lobe out to the horizon. A
reflector would contribute to the first gain, less loss accounts for
both gains.

I also analyze verticals and exotic designs (aka fractals) which have
mappings that look like tropical storms.

73's
Richard Clark, KB7QHC