Owen Duffy wrote in
:

What have you done to make the 259B appear as an isolated impedance
meter?

I did mean to add that in this case, the impedance at the end of the
isolated feed line is not necessarily (and not likely to be) the same as
when it is connected to your transmitter. Again the same issue arises about
the path to ground for common mode current, and the influence that has on
the antenna feed point impedance transformed by the transmission line.

You need to think about the purpose of the measurement.

Owen