To determine whether you really have a feedthrough capacitor, use a
simple capacitance meter to measure the shunt C. (Surely an electronic
parts measuring company has the capability to measure capacitance.) A
feedthrough capacitor will probably measure somewhere between 100 pF to
0.1 uF, and the Z0 specification is meaningless, as others have pointed
out. But if it's a coaxial transmission line feedthrough, the shunt C
will be a pF or so at most.

To measure the Z0 of a coaxial transmission line feedthrough, you'll
probably need a high speed TDR, like the Tek TDS 820, 11801, or older
gear with high speed TDR plugins. HP makes similar gear. The problem
with using a network analyzer is that the line is probably too short to
make a good measurement, unless you have a 40 GHz or higher frequency
analyzer, a really good termination, and know how to interpret the
results. The advantage of a TDR is that it graphically gives you the Z0
as a function of physical position, so you can see the Z0 of the
feedthrough itself, independent of the connecting cable and the
termination. However, to get the necessary resolution so you can see the
Z0 of a physically short line like the feedthrough, you need a high
speed TDR. That's exactly the sort of job that high speed TDRs are
designed to do. There's also a cost issue. The last time I checked --
some time ago, admittedly -- a new good high speed TDR was about $25k,
and a good 50 GHz network analyzer was roughly $500k. This price ratio
would be about the same for rental units, also.

Roy Lewallen, W7EL