Roy,

Thanks for the exhaustive answer.

I appreciate your opinion on the short segment issue. I must admit
playing with it and not noticing things go pear shaped as frequency was
reduced (so that segment length become relatively short).

To those of us that haven't dived into the internals, the warnings are a
concern and prima facie, I would be reluctant to accept and use results
from a model with unresolved warnings.

The helical problem I posed is not unusual, many if not most low HF
helicals wind up with close spaced turns at the top, and the absence of
model accuracy in that part of the antenna renders any model of very
limited value. (I am not suggesting that the Corum approach can deal with
a variable pitch helical with close spaced turns at the top either.) It
is just a real implementation that seems a problem to model.

So, NEC can model your helical so long as your helical is one that NEC
can model!

I am familiar with the GH card, Arie's 4NEC2 has it. I think my version
of EZNEC can generate a helix, but I know that it cannot save a NEC
compatible deck.

I have tried a NEC-2 model of the coil that Tom described with 8
segments/turn (warning: segment length = 0.00013wl). The inductive
reactance is a little different to the Hamwave Corum based calculator,
the R is very different, the self resonance is different (understandably,
likely to be affected by the coil terminations) and we don't know a
measured value for Tom's coil anyway or the true stand alone self
resonance.

Owen