John S wrote in :

....
What I want to do is build an antenna based on its radiation
characteristics (as shown with EZNEC) and then measure its impedance (at
the end of a few inches of parallel conductors) so that I can put in a
matching network to give my source what it wants.

My mention of the common mode current path is very relevant.

You have test equipment that is 'not-balanced' and a load that is balanced
or more likely 'not-prefectly-balanced' in a different way. That is likely
(certain) to cause common mode current, which means the common mode current
path directly participates in radiation, making the antenna different to
your design.

Owen

On 11/8/2011 4:04 PM, Owen Duffy wrote:
John wrote in :

...
What I want to do is build an antenna based on its radiation
characteristics (as shown with EZNEC) and then measure its impedance (at
the end of a few inches of parallel conductors) so that I can put in a
matching network to give my source what it wants.

My mention of the common mode current path is very relevant.

You have test equipment that is 'not-balanced' and a load that is balanced
or more likely 'not-prefectly-balanced' in a different way. That is likely
(certain) to cause common mode current, which means the common mode current
path directly participates in radiation, making the antenna different to
your design.

Owen


Yes, I am aware of that problem. Perhaps I will build a current pick-up
loop and run it up and down the coax. I can use the sensitive input of
the vector voltmeter or maybe my Boonton RF voltmeter. I can then try to
minimize the common mode current.

John