Hi Max and QucsStudio community,
The document in the link below is helpful in resolving my query. In short, the Mason’s unilateral gain function in QucsStudio plots the inherent gain of device which is independent of matching networks while including stability factor and does not fix |S12| = 0. This makes it invariant and important FoM. The GTUMAX which I hand calculated is different as it assumes |S12| = 0 and conjugate match at input and output.
https://ieeexplore.ieee.org/document/137392
Thanks
Best Regards
Het (KJ5BEI)