HFSS predicts active behaviour below hollow waveguide cutoff frequency

Hello,

when simulating a hollow waveguide configuration within Ansoft HFSS,
the scattering parameters show ACTIVE behavior (s21_db > 0) at frequencies below the hollow waveguides cut off frequency.
(interpolating sweep)
Increasing frequency samples and simulating more passes does not solve the problem.
The solution frequency was set below and above cut off frequency.
Does anybody know about this problem ?

Thanks.

---------- Post added at 20:31 ---------- Previous post was at 19:40 ----------

RE:

sorry, I didnt recognize there is port deembeding activated.
Actually, the active scattering parameters belong to a hollow waveguide port reference plane where the evanescent modes are in high gear.
It is still a mystery, but may be would should take it is a rule.
Port reference planes of hollow waveguides should be placed at distances with monomode behavior.

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