Cecil Moore wrote:
How do you guys model the parasitic effects of
a tower? With its base in concrete, it is neither
floating nor grounded. Is there some sort of
modeling rule for such?

For instance, if a vertical with sloping radials
is mounted on a tower with the radials insulated
from the tower, the tower still has an effect.
If we model the tower as mininec grounded, it has
some extreme effects which may not be realistic.

I'll be a bit glib here..

I wouldn't use mininec to model it, just because it doesn't do a good
job for those sorts of things.

Hie thee to NEC4, which is specifically designed to deal with antennas
near, on, touching, partially and fully immersed in the ground.

And even then, you can get bitten.

in practical terms, the impedance to "ground" of a concrete base is very
low.. large contact area with the soil, reasonably good conductivity of
the concrete itself (compared to soil, and especially compared to air).

if that level of subtle detail is important, you need some sort of
Finite element or finite differences code.

When I want to assess the sensitivity with something like NEC, I start
by just modeling the "widget" with a single wire, or maybe, a few wires.
Then, see how much difference it makes to change that simplified model
a bit (maybe move it a few inches or change the length a bit). if it
doesn't make much difference, there's not much interaction, and the
inaccuracy of the simple model will be small.