Numerically efficient determination of the return loss of cavities using a finite difference eigenmode solver and modal analysis

A novel method is presented which allows for the numerically efficient computation of the return loss of almost arbitrarily shaped cavities. Modal analysis is used for the calculation of the cavity-waveguide coupling, whereas a finite difference method is applied for the determination of the cavity and waveguide eigenmodes which serve as expansion functions for the modal analysis. It is demonstrated that the required coupling integrals between the 2D waveguide and the 3D cavity eigenmodes can be calculated using the electromagnetic field simulator MAFIA. Moreover it is shown that the in general poor convergence of the modal expansion can significantly be enhanced by using two sets of cavity eigenmodes, namely, modes with a short circuit and an open circuit as boundary condition in the aperture of the feeding waveguide. It is furthermore discussed under which conditions quasistatic electric and magnetic cavity eigenfunctions are excited and how their explicit use can be avoided. Numerical results are given for an X-band rectangular waveguide cavity which is fed by a coaxial line. The presented results prove the excellent accuracy and numerical efficiency of the new method.