Guilin Sun
Ansys Employee

Not necessarily right. you can try to use 2*2 mesh and check the result. FDE solves eighgen values so it needs some details of the structure.

The most important thing is: when Metal BC (the default BC for mode expansion monitor) is used it has box modes, which are not necessarily the propagation modes, whereas using PML BCs can have a lot of PML modes that need to e removed. Since you set a certain number of tiral modes, when the solver reaches this number it will stop, so some higher order modes may not be found.