The assumption is wrong. Scattering parameters are usually referred to a nominal, real Z0, in most cases 50 ohm.
TY for replying but I think I have been misunderstood.. I agree that Scattering parameters are usually referred to a nominal real Z0, but I want to clarify my question asking you a thing related to TRL calibration.
Assume somebody gives you coaxial standards for calibrating a VNA with TRL technique, namely two coaxial lines ( the longer is the "line" and the shorter is called the thru) and a coaxial short ( it is the R). (All the 3 standard are made from the same kind of coaxial, let say rigid, cable and are properly assembled with suitable connector in order to be measured on the VNA).
Perform now the TRL calibration so that you are able then to take measurement of other two ports DUTs.
The numbers you get measuring DUTs, namely scattering parameters of the DUT, are normalized to a Zc ... which one?
I would say Zc= characterisitic impedance of the coaxial cable.. ( this is what i understood of TRL calibration, if it is wrong advise).
Now, it could happen that the coaxial cable has a "not real" characteristic impedance (Zc=sqrt( (jwL+R)/(jwC+G))
https://en.wikipedia.org/wiki/Characteristic_impedance
The question is : how should I manage the numbers I got from the measurement after the TRL calibration using that standards?
Are they suitable S parameters? To what Zc are thy referred?
Not to 50 ohms I would say, to the Zc of the cable then.... rigth but it is not real !! so tho which impedance are they referred?
It may be the Real part fot the Zc of the cable... but this is naively not consistent (as different cables can have the same real part of Zc but measuremetns will be different).
Many thanks
Eventually