>I do not know what is the correct behavior, but can I assume that QUCSSTUDIO is correct?
I don’t know. You could post this in one of these forums:
https://www.edaboard.com/#analog-design.10
I asked an rf question some while ago and got some very knowledgeable answers.
FYI The repetetive “dips”in the S11-shape – I think – originate from fractional reflections at discontinuities (e.g. change from port to microstrip). If the discontinuities are n * lambda/2 apart, they cancel each other out and you get a perfect match (very negative S11 value)