Optimal subparametric finite elements for the computation of cutoff wavenumbers in waveguides

In the present work, the computation of cutoff wavenumbers in waveguides with both straight and curved edge boundaries have been carried out using subparametric transformations. As compared to the conventional finite element methods, the subparametric transformation takes the advantage of mapping curved boundaries with greater accuracy. Under this transformation, any triangle with two straight sides and one curved side can be mapped to a standard right-angled triangle. This method has been applied to a regular L-shaped rectangular waveguide and also on an irregular curved geometry. The obtained cutoff frequencies of regular geometry are in close agreement with the existing values found in literature and those of irregular boundary have converged very well.