James,
I don't know who Mr. Gordo is, but those comments directly contradict
the ARRL Antenna Book and the evidence from EZNEC.
I've posted the EZNEC elevation patterns here:
http://www.karinya.net/g3txq/temp/ground_zones.png
The blue trace is an 80m quarter-wave vertical over poor Sandy soil.
Notice the relatively high optimum take-off angle of around 30 degrees.
The green trace is the same antenna, but with Salt water immediately
below the antenna extending out a quarter-wavelength in all directions.
It's the equivalent of an excellent ground radial system. Notice that
the signal is generally stronger at all elevations, but the shape has
*not* changed. Optimum take-off angle is still around 30 degrees.
Now extend the Salt water out to 1000ft and you get the red trace. We
have now improved the conductivity far out from the antenna where the
elevation pattern is being formed, and we have dramatically changed the
shape of the elevation pattern. Optimum take-off angle is now about 7
degrees.
73,
Steve G3TXQ
On 06/01/2011 02:19, Richards wrote:
This is what Mr. Gordo says verbatim on Disc 6 of his Extra Class Exam
course:
"What strongly affects the shape of the far-field low angle
which we want? the low angle elevation pattern of a vertically
polarized antenna ? Well, that would be the conductivity
and dielectric constant of the soil in the area just below the
antenna. And the more conductive the soil is, the better
the low angle of radiation. How is the far-field elevation
pattern
of of a vertically polarized antenna affected by being mounted
over seawater versus, let's say, ah, dry rocky ground? Well the
low angle radiation increases and that is what we want is low
angle radiation.
Now don't read too much into this... when they say it increases,
it does not mean it goes up, it means that there is more signal
coming out of it at a low angle.
What is the main effect of placing a vertical antenna over an
imperfect ground... the big effect there is it will reduce the
low angle radiation and create high levels of radiation like
NVIS where you are bouncing signals and, as hams call
it, "warming the clouds."
So, if you are thinking of putting up a vertical antenna, on
your roof, then have plenty of ground radials, usually two
to three inch-wide copper foil strips, as the ground plane.
If you are thinking of doing a vertical antenna in your back
yard, relying on your soil as ground, unless you live right
on the water, I recommend don't. Put it up on a metal
shed or up on a roof, with ground foil radials, and you
will have a much better low angle radiation pattern."
So.... I guess I am re-thinking all of this and not so sure that Ol'
Gordo is sufficiently precise when he says all this about improving
vertical antenna low angle radiation. Or am I still missing something ?
Thanks.
=============== Happy Trails - James - K8JHR ============
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