Guilin Sun
Ansys Employee

I guess you have used PML BCs? if so, since there are many PML modes to be discarded, the actual number of modes is not what expected. You can either try to use Metal BCs but to identify the true modes, or use more numbe of trial modes, say 999.

When metal BCs are used, you may also need to use more number of modes, since there are many non-propagation modes.