The first thing I'd try, then, would be to put a 1:1 balun (common mode
choke) at the feedpoint and, when using feedline, a second one about a
quarter wave down the line. The balun can be constructed by winding the
coax (if you're using RG-58 or smaller) 8 - 10 turns on a type 43
ferrite core. Or you can clamp or thread a few large cores over the
coax. You can use the Autek to measure the core impedance -- shoot for
500 - 1000 ohms total -- the angle of the impedance isn't important.
Unlike the 2:1 balun, this won't disturb your basic measurement in any
way, it'll just reduce any common mode current.

If you still get the same result, then there are only two other possible
causes I can think of. One is the modeling of the inductors. I've found
that a lumped model of an inductor isn't good if there's any appreciable
current change in the real inductor from one end to the other due to its
physical length. The solution is to model the inductor as a helix.
You'll have to add some extra R to the model, however, if the turns are
spaced closer than a couple of wire diameters, since the program doesn't
account for proximity effect.

The other possible cause is that there's some source of loss you're not
accounting for in your model. The inductors and coupling to nearby lossy
objects are the most obvious candidates.

Roy Lewallen, W7EL

dansawyeror wrote:
Roy,

No I have not decoupled the feed from the antenna. I will try that
tonight. I have wound a 2:1 balun for testing. (anticipating at least a
25 ohm input impedance)

I have measured the antenna with two different instruments. One is an
Autek analyzer at the antenna, the second is with an 8405a at the end of
100+ feed of cable. They both show the same results.

Thanks - Dan

Roy Lewallen wrote:
dansawyeror wrote:

. . .
The question is: What is the basis of the difference between
predicted and measured values?


1. Have you decoupled your feedline? If you're not using a feedline,
have you decoupled your measurement device?

2. Have you substituted a lumped impedance of about 16 + j133 ohms for
the antenna and observed what your measurement equipment indicates?

Roy Lewallen, W7EL