Richard Clark wrote:

EZNEC has the capacity to model wires with insulation. I presume that
is a legacy of NEC, but I will await tutoring on that point from other
posters.

Although it's a feature of NEC-4, it's not part of NEC-2. The insulated
wire capability of EZNEC was developed independently from other sources.

Carry that a bit further, it has at least "some" capacity to
model wires with material nested between them. After all, the
difference is in degree, not in concept, and the degree is hardly
remarkable.

No, they're different things. The insulated wire feature slightly
modifies the field from a wire, and is valid only for thin insulating
layers. Insulation between conductors has a considerably larger effect
on the field and consequent coupling between them. Adding insulation to
a parallel wire line gives you a model of something like an
air-insulated ladder line made with insulated wire.

When I observe common window line, it is not all that
different from two insulated wires.

It's enough to drop the differential mode velocity factor down to
somewhere around 0.91 - 0.95 (from various sources - I haven't measured
any), which indeed isn't very different from the common mode velocity
factor of insulated wire. Whether or not the difference is significant
depends on the application.

Further, there is nothing
remarkably different to the degree that the Lattin analysis is so
entirely thrown off as to be wholly useless. For that matter, I
haven't observed any postings here on any Lattin analysis other than
my own. If this all hinges on TV type twin lead, then too much credit
is being given to too little plastic.

You could probably make a model with EZNEC which would be fairly close,
then manually adjust it to optimize performance. A real antenna would
have similar performance if optimized for the type of line it's
constructed from, although the final dimensions would be a bit different.

Roy Lewallen, W7EL