I understand there are measurement issues (and certainly assumption
issues
for Rrad). Isn't is fairly certain that increasing the number of radials
(of proper length) until the feedpoint R (at resonance, at the antenna)
no longer drops, is a reasonable approximation of "high efficiency"? The
only issue I see, is determining the target Rrad to compare it to when
trying to "estimate" efficiency.

Are you saying (for example), that the feedpoint R of a 1/4 w vertical
against perfect ground cannot be reliably estimated at 37 ohms? If it can,
then isn't 37/R a measure of efficiency?

Again, I'm thinking of the efficiency of the ground system... I have no
way to look at field strength.

Is it really possible to reduce ground losses to the absolute minimum and
not have a corresponding increase in field strength?

This is starting to turn into "black magic" for me. I can understand
questioning a particular "number" for efficiency based on the simplistic
Rrad/R formula. If the implications go further...indicating there is no
meaning to Rrad/R, then I'm lost.

Perhaps the issue is that it's known how to maximize efficiency, it's just
completely unknown what that efficiency really is, and there is no simple
way to measure it. If that's what your saying, then I understand.

That position does seem to muddy up the "how many radials and of what
length" efficiency info presented in ON4UN's book and referenced in other
texts. They all seem to acccept some sort of accuracy for the Rrad/R
formula with 1/4 w verticals. If I understand you correctly, the formula
is rejected outright as hopelessly simplistic, and of no particular value.

Do I have it now? If so, I'll refrain from using it in the future.

I had always assumed that a NEC model of a perfectly conducting
monopole above a perfect ground would provide the radiation
resistance. For example, considering your antenna of 18.3 m
at 3.62 MHz, the input impedance is 27.5 - j 64.7. The radiation
resistance would therefore be 27.5 ohms. This appears to be
fairly close to your estimate of 25.4 ohms.

Frank