Back-to-back simulation of waveguide to microtsrip

[Salving]

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

 

[johnjoe]

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

 

[Salving]

It doesn't help, but thanks anyway

 

[johnjoe]

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.

 

[Salving]

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