In an older post,(I now fail to find), Kurt explained it is possible to use,
after calibration, a long high quality coax to test and improve the correction
factors of the calibration standard.
You may have read my interpretation (#3515) of page 3 in:
... which now states:
"Providing your short and open calibration standards are well defined (like the Rosenberger kit from SDR-KITS)
incl. the precise value of the load resistance (which is measured by SDR down to 0.01ohm accuracy)
there is only one source for fine tuning and that is the fringe C of the load which varies from one example to another."
.. and Kurt suggests tweaking that to minimize oscillations in
"a S11 dB trace with e.g. 0.2dB per division (the first red trace) which is oscillating across the frequency range"
I sort of confirmed that for myself, using 2m LMR-400..
Oristo