I apologize -- I read your earlier posting too hastily. Your antenna
requires the full version of EZNEC because of the inductor. While a
short inductor can be modeled as a lumped load, that doesn't work well
in cases like this where the current changes significantly from one end
of the coil to the other due to radiation. The coil has to be modeled as
a helix.

A full model of your antenna with the coil properly modeled shows a
feedpoint impedance of 5.03 - j212 ohms at 112 MHz over perfect ground.
Loss in a real ground system will of course increase the resistance. It
resonates at 171 MHz, where the feedpoint resistance is about 17 ohms.
My result is far from your finding of resonance around 112 MHz, so maybe
I didn't interpret the design correctly -- the antenna I modeled is a
total of 11 inches high, the center 3 inches of that being the 1 inch
diameter 5 turn coil.

With EZNEC and similar programs, you can only connect directly only to
perfect and MININEC-type ground; connection to Real, High-Accuracy
ground results in an unpredictable resistance that has no physical
meaning. More information can be found in the Modeling Ground chapter of
the EZNEC manual under Building The Model.

Roy Lewallen, W7EL

dansawyeror wrote:
Roy,

Thanks, I was able to enter the data into eznec and run the simulation.
The output was 'confusing'. Eznec predicted 100 Ohm over various
grounds. Other models predict 5 Ohms for the antenna plus ground
resistance with a total of 10 Ohms or so. Which is correct? How can the
differences be reconciled?

Thanks again - Dan

Roy Lewallen wrote:

dansawyeror wrote:

All,

I am creating a 'scaled' model of a loaded vertical. The model is
complete and in place. However I immediately realized I had no idea
of the 'theoretical' response. How can I calculate the expected
impedance response of a loaded vertical? It a mid load construction,

the base in #10 solid copper 4 inchs,
the coil is .6 pitch, 1 inch diameter, 5 turns #12, measuring .72 uH,
and the top is 4 inch #12.

The antenna appears to resonate at about 112 MHz.

The instrumentation, a directional coupler measuring reflection is
located at the base, directly under the 'ground plane'. This allows
measuring input and reflected signal strength.

The question is: How can I plot the expected impedance from say 100
MHz to 130 MHz?

Thanks - Dan



The free EZNEC demo from http://eznec.com will give you the
information you need.

Roy Lewallen, W7EL