This is a long thread so this might have been mentioned already. I bet I am way out of line. Sorry.
Why can't the measurement be made with a 50 ohm network analyzer? We're just looking at impedance so it can easily be converted to SWR or other formats mathematically referencing 75 ohms. Maybe this is what the formulas show above are doing?
It is not easy to fabricate a 75 ohm OSL cal kit using discrete components. If this is an HF app maybe.
