I'm trying to ensure phase advance and minimum reflection from the coupler. 3 structures are designed (coupler cavity, 1st accelerating cavity, and 2nd accelerating cavity short-circuited). Background lossy metal copper, boundary conditions are perfect electric and simulated in CST-FDM. The reflection phase (calculated phase of S11) calculated isn't correct as there should be a difference of 240 degrees between the three structures mentioned above.
My question is what should I do to get the correct phase?