Stability and accuracy of coaxial waveguide port algorithm for the time-domain finite element method

The finite element method (FEM) is an attractive choice for full wave analysis of electromagnetic problems with large variations in length scales for the geometry and the field solution. An example is the modeling of devices fed by coaxial cables and we suggested a time-domain FEM for such problems at last year´s meeting (Rylander, T. and Jin, J.M., IEEE Antennas and Propagation Society International Symposium, 2003). We prove that our proposed scheme is stable up to the Courant limit of the explicitly time stepped transmission line equation and demonstrate its accuracy on a simple test problem. We also suggest a new and alternative approach to compute the field distribution of the dominant mode, which considerably simplifies the computer implementation of the port algorithm.