Thirty years ago, at work, we had two -
hp- devices for coaxial impedance measurements (an 8753 network analyzer and a 4191A), and a full set of -
hp- impedance standards.
For measuring balanced networks (resonant circuits with several k\$\Omega\$ resistance at resonance), we made simple adapters using MCL plastic-cased transformers with appropriate frequency and ratio specifications for the specific network under test, in the 5 to 80 MHz range.
We had a simple coaxial connection to the 50 ohm winding on the transformer, and a two-socket, 0.2" spacing cheap female connector on the balanced high-Z side.
The important thing was to do the calibration with a mating two-pin male header: we used an open, short, and a 1206 SMT resistor equal to the nominal impedance on the high-Z winding.
The 8753 firmware did not support a calibration impedance value that high, so we lied to it and divided the characteristic impedance setting by a factor of 10.
We could use a reasonable length of good coax between the port on the -hp- equipment and the 50 ohm winding of the transformer.
We had a small collection of 1206 resistors on male headers, of different values for different transformers, and verified the calibration by measuring them with another transformer ratio.
An example of MCL's current production, similar to the models we used, is
https://www.minicircuits.com/pdfs/ADT8-1T+.pdf with an 1:8 impedance ratio from 50\$\Omega\$ and the relevant output pin spacing of 0.2 inch.