Frank,

I will re-measure the coil dimensions. The recall the coil measured 600nH. That
is the value I used when I modeled this antenna using EZNec. It showed resonance
at about 145 MHz and 12 Ohms. (That was using an average real ground.)

If I assume the antenna measurements are correct then is it the ground that
accounts for the difference between 6 Ohms and the modeled 12 Ohms?

Now I am on to model and measure a center loaded dipole.

Dan

Frank wrote:
"dansawyeror" wrote in message
...

Frank,

The antenna I am trying to model is a center 'loaded vertical'. It is a 4
inch base, 5 turns at 40 percent spacing on a .8 diameter inch form and a
4 inch tip. The material is Num 10 solid copper. I adjust the frequency by
stretching or compressing the coil. Currently it is resonant at about
141.7 Mhz. The 8405a shows a phase shift of 1 degree per 30 kc change in
frequency.

I have used both the vertload model and the EZNEC model. Both predict an
antenna R of about 5 Ohms. The 25 Ohm load shows a 12 db power difference
between forward and reverse. The antenna shows a 10 db power difference
between forward and reverse.

Thanks - Dan


Dan, I have modelled a 5 turn inductor, 0.8" diameter, varying in length
from 0.8" to 1.6". The inductance values are 380 - 490 nH. An,
approximately 9" long monopole, with a 5 turn helix appears to be resonant
at about 190 MHz, with a highly reactive 6 ohm input impedance at 141 MHz.
Using a lumped element simulation the required load inductance, for 141 MHz,
is about 600 nH.

The only way to resolve these discrepancies is to do a standard single port
network analyzer calibration and measure the actual input impedance of the
antenna.

Frank