Back-to-back simulation of waveguide to microtsrip

Hello everybody. I have designed a waveguide to microstrip transition for 60 GHz (using CST) that shows very good matching, when the length of PCB component (microstrip line) does not exceed 5-7 mm. If i increase this length the parameter S11 starts to oscillate as you can see on picture. Is this related to surface waves or something else? any help would be great

Try to use the option "electrical shield" for your waveguide ports

It doesn't help, but thanks anyway

Oh sorry, I didn't sorry your pic. It is not oscillation, check the port signals. If you increase the length of TML, you have more n*lambda/2 lengths. Maybe post also a pic of your results before you increase the length and maybe a short pic of your structure.

Unfortunately I can't post a pic.. The structure contains of two striplines connected between each other with simple microstrip line, everything on the same substrate with no baluns.Herу are results before increasing the length