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