My program is not Nec based so I am not familiar with that routine.,
I cant model my antenna because of the pre twisted wires. I can
however
model a combination of two helix antennas without interweaving into
one circuit.
where one helix antenna is slightly larger diameter than the other
My program shows instances of gain! So the question becomes is your
model
based on zip cord which is suggested by one and secondly what was the
reason for the combimation of two antennas
to shrivle away into dust or the equivalent of a dummy load. It is
also suggested that antennas such as this
are breaking the laws of nature so does your program show the arrival
of doom ?.
I am making this antenna very frequently and all these things
predicted are just not happening. But then I would never use
zip cord or violate the rules of nature if I was advised which one I
was violating but apparently that is a secret.
So Frank what is the antenna configuration etc that your program
represents so that I can understand the particulars that you have so
genouresly probided so all may share

I have tried two variants of the model: Interleaved, and one slightly
inside the other. I used uninsulated # 14 AWG copper, and designed for 7
MHz.
The copper conductivity used is 5.8001E7 S/m. At resonance the
the input impedance is 3.1 ohms. The radiation resistance computes
to 0.14E-3 ohms. For 100 W input the total radiated power is
2 mW. At 1000 m, normal to the axis of helices, the field strength
is 3.7E-4 V/m (RMS). I have found, for such closed spaced helical models,
that segmentation is critical. My model uses 2200 segments,
and takes 28 seconds to run. The "Average gain test" returns "Good".
The "Average gain test" is a method employed by Nittany Scientific's
NEC based programs to estimate model validity.

The interleaved model version appears to have a slightly better
performance than the "Coaxial" structure. It is essential,
when testing such a higly compact radiator, that a balun
be used to feed the antenna. The "Tak-tenna", for
example, has a very low radiation efficiency. It is
designed for use witout a balun, and the bulk of the radiation
occurs from currents flowing on the outside of the
coaxial shield.

73,

Frank