Unconditionally Stable TDFEM Algorithm Based on “Brick” Element

The basic theory of an unconditionally stable time domain finite-element method (US-TDFEM) based on the “brick” element is elaborated. Starting from the matrix equation of traditional TDFEM (T-TDFEM), the high-order modes that cause instability are deleted, and a new matrix equation is obtained, and then the iterative calculation is performed in time domain. As a result, the US-TDFEM can use a larger time step that is only determined by the upper frequency of the input signal. The resonant frequencies of the cavity with a small deformation and the cavity filled with a non-uniform dielectric are calculated. The numerical results demonstrate the good performance of this method in the calculation of the resonant frequencies of resonators with fine features.