cst 1% circular latest
So,
first: use xmin, xmax, ymin, ymax open (add space), not only open, I think that is better.
second: 867 MHz is for set 1) and 868 for set 2) ? It is OK, CST is not absolutely precise
, 1 MHz is not accuracy. Now you must use "Adaptive mesh refinement", is it in "Transient solver parameters". Try it and wait, you can see, that the resonant frequency will be jumping (you can see in Navigation tree - 1D results - Adaptive Meshing - S [dB])
I hope, that I helped you
Added after 6 minutes:
btw CST help:
Open: Operates like free space. It is available for the electrostatic magnetostatic and thermal solver.
Open (add space lf): Similar to the open boundary condition, with some extra space added between structure and applied boundary condition.
Added after 4 minutes:
and 2. btw:
try in help search: Circular Patch Antenna, it is
third result, there is inspiration for you