I guess a revisit to using an impedance matching network to solve this issue is required.
Once a upon a time we spent significant effort building RX bridges, noise bridges, etc... and spent much time on finding ideal variable R's and C's as well L's and involved transformers. Application of some of that effort may be re used and supplemented now that a low cost vna is available.
Consider building a frequency independent L matching network or even a PI although it adds the challenge of adding a 3rd element to the challenge. That challenge is to incorporate L and C's whose self resonate frequency is far removed from the measurement frequency. Example, a 20 meter bobtail antenna has a driving point Z of nearly 5k with -j290 series ohm. Not easy to measure. However, readily matched to 50 ohms, the Qm is only 9.8 and the element values are 5.5 uH and shunt 23 pF at 14.1 MHz.
Both of these components are constructed with SRF values well above 20 meters, with care. And will provide as measured by the vna a return loss in excess of 30 dB. Reading off the value of the L and C at the measured frequency will permit you to find the antenna port Z. So in advance of the measurement, if you have some idea of what Z is expected, it should be straight forward to construct a match system that facilitates that measurement. I have done this simple exercise with some unknown R-X values with high gamma and it works out fine.
Alan
