John Ferrell wrote:
I don't find the Helix feature in my copy of EZNEC+ V4.

You'll find it in the EZNEC manual index under "Helix Creation".
(There's lots of other good information in the manual, too.)

I believe I can get the same results by inserting an appropriate load.
It will just be a little more cumbersome.

It'll be less cumbersome, but you probably won't get the same results.
The amount difference will depend on the geometry of the helix. However,
a lumped load is adequate to illustrate the phenomena you're investigating.

I was not aware that the feeding of the radiator was affected by the
driving source. My "lab models" require a base inductance to get the
radiator into a range I can feed. I have not been including that in
the models.

I added a base inductor of 100uh(r=3) and got the results you
predicted. The 28 foot radiator is now showing -3.75 dbi gain!
Lesson learned!

The loss is due solely to the inductor's resistance -- you'll get the
same result by replacing the inductor with a 3 ohm resistor. The amount
of gain reduction due to the inductor will get worse and worse as the
antenna gets shorter and shorter, for two reasons. The first is that the
antenna's radiation resistance drops as it gets shorter, so the inductor
loss becomes a greater fraction of the total feedpoint resistance. The
second is that you need a larger inductor for a shorter antenna and,
assuming a constant Q, that means more inductor resistance.

. . .

Roy Lewallen, W7EL