The eigen mode problem for microwave transmission lines with absorbing boundary conditions

The scattering matrix is computed from the electromagnetic field using a three-dimensional boundary value problem for Maxwellian equations including Absorbing Boundary Conditions. As a first step one has to calculate the field distribution on the transmission lines solving an eigenvalue problem for complex matrices. Estimating the maximum propagation constant toe show that one can avoid the computation of all eigenvalues to find the few required modes.