dykesc wrote:

I didn't mention that I have a homebrew 1:1 current balun installed at
the antenna feedpoint to eliminate common mode currents which would
otherwise result from the off center feed. Would my EZNEC model still
not be adequate? (I also didn't mention the 4:1 balun in my antenna
tuner which I am accounting for in my comparison.) Also there is
probably some coupling of the feedline to the antenna field due to
geometry, but I tried to keep the feedline perpendicular to the
feedpoint as much as possible. I believe I read in the HELP file that
EZNEC does not model this type of coupling. Am I correct on this
point? I am not experiencing any issues with radiation off the
feedline at my operating location, so I believe I've done a pretty
good job with the balun and feedline routing.

EZNEC does model this type of coupling from wire to wire, but no
coupling to a transmission line model. So the coupling will be accounted
for only if you model the transmission line as wires.

Keeping the feedline perpendicular to the antenna isn't sufficient to
eliminate coupling when the feedpoint is offset, since the line will
still get unequal coupling from the two antenna sides. About the only
way to positively reduce it to a small value is to use multiple current
baluns spaced about a quarter wavelength or closer. The common mode
current might otherwise be high or low depending on the feedline length
and the length of the path to ground. Without modeling the entire path,
the accuracy of the EZNEC model will depend on how successful you've
been in reducing the common mode current to a low value. It's often
difficult with an off-center fed antenna.

Would you still recommend I don't pursue modeling of my OFC with EZNEC
at least as far as transmission line and feedpoint parameters go. How
about radiation patterns given the additional info in this post?

I'd always be suspicious of the result unless I verified that the common
mode current on the line is low. You can, of course, measure it with a
simple transformer made from a clamp-on ferrite core clamped over both
feedline wires. Use about 10 turns for the secondary, and terminate the
secondary with 50 ohms. That also allows you to put the detector at a
distance, connected to the transformer with 50 ohm line, with the
termination at the detector end.

If I model the antenna feed with two additional wires will the limit
on segments in the demo version of EZNEC start having an appreciable
impact on the quality of results? I'm probably going to get the full
version anyway but I am curious about the affect that segment limits
has.

You'll probably start getting a noticeable change in results when the
segment lengths exceed about 0.1 wavelength.

One more question. Is there a way to set antenna orientation in EZNEC
so that it coincides with my actual antenna end to end compass
bearings? This would allow me to use the "bearings" option in EZNEC
directly. It appears I have to measure X - Y offsets to do this and
can't use a simple circular (degrees) input in EZNEC to do this.

You can either define the antenna wires to be in the correct orientation
to begin with, or you can define them in a convenient way then use the
Rotate Wire feature to rotate them to the desired orientation. If you
prefer to work with compass direction rather than degrees CCW from the X
axis, you can chose this in the Options Menu (Angle Convention selection).

Roy Lewallen, W7EL