I'd appreciate and welcome reports from anyone who finds a significant
difference in insulated wire results between EZNEC and NEC-4, in the
region where both should be accurate (typical wire insulation values).

Roy Lewallen, W7EL

Owen Duffy wrote:

Roy, I went back and re-read Cebik's notes about the way in which the IS
card is processed vs using and LD card to add inductive loading. He
passes on an explanation:

===quote
In calculating the E-field for each segment in a model NEC calculates a
"cosine" component, a "sine" component, and a "constant" component. NEC
then sums the three fields to arrive at a total field value for each
segment.

LD commands modify the "constant" component of the E-field calculation,
whereas IS commands (in NEC-4) modify the "cosine" component. As a
consequence, any workaround formulation will result in satisfying only a
limited range of geometries.
===equote

It was with this in mind that I chose to head down the NEC4 path.

Owen

Roy Lewallen wrote in
:

I'd appreciate and welcome reports from anyone who finds a significant
difference in insulated wire results between EZNEC and NEC-4, in the
region where both should be accurate (typical wire insulation values).


Roy,

My comment is not intended to impune EZNEC in any way. I don't know how
EZNEC v4 models insulation, and cannot comment on it.

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.

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.

Owen

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

Roy Lewallen wrote in
:

....

Thanks for the thoughts Roy, appreciated.

Owen