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

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

Thanks. I have two coupler systems, one with very good directivity and one
with only moderate directivity. The antenna is resonant at about 141.6
MHz. There is a difference of about a couple of hundred kc between the
systems.

Now on the the "R" measurements. Both systems indicate an antenna
resistance of over 30 Ohms. This antenna has been modeled with three
different models. They all indicate a Rr in the range of 5 Ohms. The
antenna is grounded by a 3 foot by 5 foot screen mesh. While this is not
infinite, it should provide a good ground. Do you have a suggestion of
where to look for the discrepancy?

Dan


Dan, the frequency difference is most likely caused by the poor directivity
of one of your couplers, so would not worry about it. Just take the reading
with the better coupler. The forward power coupled into the reverse port
will cause a phase error.

As for your modelling: what program are you using, and how do you describe
the ground screen? I ran a quick model above a perfect ground -- using
NEC2. According to my program the input impedance is 36 ohms at resonance.
The radiation efficiency, with a copper conductor, is 99.6%, so the
radiation resistance is obviously very close to 36 ohms. I can run a model
with a ground screen, but would like to replicate your structure as close as
possible. The actual height of my model monopole is 0.505 m (19.88"), and
is resonant at 141.8 MHz.

Frank