The SWR analyzer I used can also measure the antenna's impedance.

The radio is a Becker AR3201. The power wiring for the averager is not
close to the radio wiring because the radio is the bottom of the
intrument stack and the B40 is at the very top ( 12-14" away). I tried
to trace the antenna's coax back along the inside of the fuse and
determined that the cable must end somewhere between the control stick
access and the access panel in the deck behind the pilot's head. It's a
DG-300 and there is no seat pan that can be removed (great design).
Anyway, I suspect that means the antenna is located under the permanent
seat pan. Where would the antenna normally be located and is it's
distance to the panel a potential cause of my problems?