Roy Lewallen, W7EL, posted:

No, that's not the problem. The problem is
that you're looking for a single voltage
between two points separated in space. There
is no single value for that voltage. If you
made some kind of "artificial ground" close
to the antenna, then there are an infinite
number of possible voltages between it and
the Earth.

Well, I chose one particular configuration
and one particular integration path because
I was curious about the original question -
something about how much voltage would the
end insulator have to handle for 100 watts
of radiated power.

I chose a vertical, half-wave monopole fed
against perfect ground and looked at the
driving source data with EZNEC. The feed-
point impedance was 2188 +j66 ohms and a
driving current of .213 amps produced a
radiated power of 100 watts and a feedpoint
voltage of 466 volts. 1500 watts scales
that up to 1805 volts. Symmetry about the
ground would increase that to 3600 volts
for the free-space case. That is what I
would adopt as my design-to target for
end insulators.

I know it's crude, but I was just looking
for a ballpark figure.

Jim, K7JEB