On the Doubt of the Simplified FEM Formulation for Waveguide Discontinuity Problem

It is a key step for designing an efficient numerical solution to simplify the mathematic formulation based on the physical characteristics of problems. A doubt in the simplified FEM formulation is presented for the waveguide discontinuity problem. The reason is figured out why the look-like reasonable formulation cannot give accurate results. The explanation and numerical results further demonstrate that the uniqueness on boundary condition is the key issue causing the problem.