Owen Duffy wrote:
. . .
I have read a description on Cebik's page about modelling the effects of
insulation by loading all segments with a constant value of L which
apparently makes a insulated wire half wave dipole model in NEC2
calibrate well with NEC4' IS model. It would seem that the amount of the
inductance was chosen empirically, a curve was fitted to the experimental
model data. It occurs to me that the results depend to an extent on the
current distribution on the conductor, and might be different for a small
loop where the current is almost uniform.

An underlying assumption in any NEC model is that the current in any
given segment varies only a small amount from one end to the other.
Therefore, any criteria applicable to a loop should also apply to other
geometries. If not, then the other geometry has an insufficient number
of segments. I would, however, use NEC-4 if in your shoes, because the
source code is available for scrutiny. Be careful, though, since there
have been quite a few modifications to NEC-4 since its introduction, and
various compilations can also give slightly different results. None of
the changes have been in the insulated wire calculation that I'm aware
of, however.

The reason for the study is to be prepared to fend off criticism from
opponents (the BPL industry, the FCC etc) who might criticise performance
modelled on bare wire when everyone wants to make the loops from
insulated wire. I can defend a NEC4 IS model, but if there are questions
about the accuracy of LD loading of a small loop based on empirical data
from half wave dipoles, I am in a weaker position and possibly worse than
NEC2 models of bare wire.

Modelling the insulation is probably splitting hairs. If I address that,
the next question will be over using stranded wire vs single core.

It is indeed splitting hairs. The effect will be so small it will be
hard to see in a model (except possibly in the resonance of a very high
Q loop) and probably impossible to discern by measurement of a real
loop. But then your modeling and measurements will show you that.

Stranded wire might noticeably reduce the Q of a very high Q loop, but I
couldn't be sure. Its effect can be seen in coaxial cable, but the field
distribution around the center conductor is very different than for an
antenna wire.

Roy Lewallen, W7EL