Richard Clark wrote:
On Fri, 20 Apr 2007 07:58:16 +0100, "Richard"
wrote:


More like the radials make the ground below your feet more like say
a pool of salt water than the high resistivity ground that it normally is.
What do you think?


Hi Richard,

Actually salt water sucks as a local ground - it is as poor a "good"
conductor as you could imagine. Carbon is a far better conductor than
salt water, but no one yet has suggested building on top of a coal
seam.

You would be better off filling your yard with sand to the depth of 30
feet or so (yeah, sure). The testimonials attributed to salt water
comes with its far field qualities of a tremendous mismatch to air and
offering spectacularly low radiation launch angles.

mostly due to that epsilon of 80, more than the conductivity


So, copper replaces a very poor conductor (as a first pass
approximation). Invest your copper in close proximity to the base of
the antenna. That is, a lot of short radials, and a fair number of
medium size ones, and a few long ones.

Exactly...

In fact, one can do some numerical analysis to figure out an optimum
strategy, based on minimizing IR losses in the soil. The current
density is higher close to the base of a vertical, so, at first glance,
it would appear that improvements in conductivity there would have more
"value". I imagine there's some nice integration that covers it all.

All that analysis and spreadsheets out there all make the assumption
that you want all radials the same length, which isn't necessarily so.

What also throws a "wrench into the gear train" is that if you start
looking at verticals with nonuniform current distributions, especially
if they aren't representable by a "simple" form such as linear or
cos(h), an analytical approach gets tricky (hence the suggestion for
numerical methods). Consider, for instance, a half wave dipole with the
center, say, 3/8 wavelength above the ground. Or some sort of asymmetric
vertical with an elevated feedpoint, or loading coils. The optimum
radial layout gets a bit trickier to figure out.


Two things to consider. The ground closest to the antenna is
responsible for efficiency in loading. The ground further out
(between 5 and 10 wavelengths, or more) is responsible for launch
efficiency (offering lower launch angles).

73's
Richard Clark, KB7QHC