Dan,

Be interested to see what the exact dimensions of the coil are. Anyway, it
seems we have some agreement on the 600 nH value, although my physical NEC
helix models do not agree based on my estimate of your coil dimensions. I
understand that EZNec uses a "Minninec" ground, which allows antenna contact
with a perfect ground, but uses actual ground parameters to analyze the
reflections. I am not sure about this, but would assume from the point of
view of the input impedance, that the ground would be considered perfect;
and therefore lossless. I also noticed I had some borderline NEC warnings
when attempting to construct a coil with #10 AWG, so sometimes had to resort
to a much thinner conductor.

My models showed about 17 ohms at resonance when connected to a perfect
ground. The only time I observed impedances as low as 6 ohms was far from
resonance when the antenna was highly capacitive. Ideally I should
construct a ground screen, but for the time being will consider a perfect
ground.

A free space dipole might be easier to model, but I am curious to understand
why there are discrepancies in the monopole modeling.

Frank

PS, be interested in any comments on my NEC code:

CM Loaded 2 m monopole
CE
GW 1 15 0.4 0 5.6 0.4 0 1.6 0.025
GH 2 50 .32 1.6 .4 .4 .4 .4 0.025
GM 100 0 0 0 0 0 0 4 000.051
GW 3 15 0.4 0 4 0.4 0 0 0.025
GS 0 0 0.025400
GE 1
GN 1
EX 0 3 15 00 1 0
LD 5 101 1 15 5.8001E7
LD 5 102 1 50 5.8001E7
LD 5 3 1 15 5.8001E7
FR 0 41 0 0 135 2
RP 0 181 1 1000 -90 90 1.00000 1.00000
EN
"dansawyeror" wrote in message
...
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