The measured impedance of cz={44.6133821915143, 24.2820051981698}, the red point, corresponds to Zant(with stub)=35.252-j16.472 ohm, the green point, if you consider the cable (5D-2V) length of 20m.
Without the stub match, Zant=27.182-j40.050. Therefore, it is presumed that the stub works something like this way.
