"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