CST - Using CST to simulate a waveguide struture but failed

Hi community,

I am a beginner in cst studio, and im trying to simulate a waveguide like struture that is made by metal wire mesh, as shown in the figure below.

 

I use time domain to solve it, and I put waveguide ports on z boundary like this, and i set all boundaries to opend add space.

My mesh setting is like this and I got 6M mesh cells. I use adapting mesh refinement in time solver.

 

But when the simulation was done, the S parameters look so wrong and ugly, and I had no idea how to solve this problem.

 

Can anyone tell me how to simulate this structurer or what should I do to get the right result, please? Thank you so much!

CST Studio Suite