Taylor Robertson
Ansys Employee
If you are concerned about how the object is being mesh I would suggest using an index monitor.
Holes, contractions and edges can perturb the mode propagation. Even if their are no anomalies in the meshing if the object has been constructed in a way where, for example, the cross section get's narrower you can have scattering into higher order modes. That is to say just because you inject a single mode if your geometry is not adiabatic then you will necessarily get a single mode out.
Additionally there may be issues with the simulation settings that cause the frequency monitor results to have issues. In particular I would check that the simulation is reaching auto-shutoff, as this is a pre-requisite for getting accurate results in the frequency monitors. Given that this is a reasonably large device you may need to increase the simulation time?