dansawyeror wrote:
Roy,

Several weeks back, and confirmed by frequency sweep model runs, you
indicated that minimum impedance is close or equal to the resonance
point. I tuned the antenna to the frequency of interest and then used
the Autek to verify the resonance point. That minimum value was 36 Ohms.
I am assuming this is or is very close to the resonance point for the
antenna system.

Yes, that should be correct.

What does your running of the model show for resonance frequency? At
resonance my running of the model shows close to 20 Ohms for the
relatively large values of R used in the model.

NEC-2 shows resonance (and minimum SWR) at 3.55 MHz, where R = 16.12
ohms; NEC-4 says resonance is at 3.51 MHz., where R is 16.08 ohms. (I'm
using EZNEC implementations of both.) Although small, I don't usually
see that much difference between NEC-2 and NEC-4. I suspect it's because
of the very low height above ground -- the two programs implement the
Sommerfeld ground somewhat differently. An average gain test shows good
average gain, indicating that NEC isn't having numerical difficulties.

I'm getting pretty convinced that the problem is the use of lumped loads
for the inductors. With this short an antenna, I'd expect the inductor
currents to be quite different at the ends(*), making the lumped load
models inadequate. This can lead to pretty severe errors.

(*) due to inductor radiation and unsymmetrical coupling of the inductor
to the rest of the antenna and to ground.

Roy Lewallen, W7EL