Hi,
I need to determine the resonance frequency of a coupling slot in CST Studio Suite . I’ve placed the slot between two waveguides and configured three waveguide ports.

I set the boundary conditions to ET=0 in all directions and swept the slot length. I then identified the optimal S11 corresponding to the resonance frequency.

However, when I change the boundary conditions to open (add space) in all directions, the resonance frequency shifts( My waveguide ports and PEC walls establish the boundary conditions for the structure, ensuring accurate simulation. Additionally, the output boundary should not have anyfor significant effects on the results.)

I switched the solver from Transient to Frequency, but the same behavior was observed—the S11​ resonance frequency differed between the two solvers. 

I need to understand why the resonance frequency changes for this (bounded structure)  when using different boundary conditions