I have simulated simple rectangular waveguide & cylindrical waveguide separately and optimized their length for fundamental mode efficient propagation. Now When I m adding these to form a mode converter from TE10 to TE11, the obtained S-parameters are very poor. I simulated for different lengths of transition section between both also but yet reflection coefficient is very high at rectangular port., though not acceptable at other port as well. Can anyone help me out with the issue. For reference I m adding the cst file of the converter.