calibration to 25 ohm?

Can a VNA be calibrated to 25Ω for balun measurements?
25Ω coax cable
25Ω resistor
Unfortunately 50Ω connectors

thanks

Why do you want to do that?
You can measure in 50 Ohm and then re-normalize the results to any other impedance level.

First u can find 25 ohm cables
second i think volker is right just measure under 50 and normalize...