I will try to explain.
If you're measuring antenna VSWR, you're needs to use antenna impedance for VSWR base. Otherwise such a measurement would not make sense.
For example, ideal half wave dipole has impedance 73 ohms, if you will try to measure it with VSWR meter calibrated for 50 ohm it would not make sense, because there is no 50 ohm. You're needs to measure VSWR for base impedance 73 ohms or use impedance matching circuit to match it with 50 ohm properly.
