Modal Analysis of Discontinuities in Planar Transmission Lines: using Orthogonality as the Criterion for Selecting Modes

The technique of modal analysis has been used together with the spectral domain approach (SDA) to analyze single and cascaded step discontinuities in planar transmission lines, particularly in boxed microstrip and finlines. These kinds of discontinuities appear frequently in MICs and MMICs. Convergence was studied as a function of the number of modes (propagating, evanescent and complex modes) in order to determine the scattering matrix parameters and represent each component of the fundamental mode on each side of the discontinuity. This study showed that these sets of modes may be non-orthogonal and, consequently, a selection of modes based on orthogonality is made. Finline and microstrip discontinuities have been studied with this criterion. It is possible to obtain good convergence and satisfactory behavior of the components of the fields if the number of modes used is sufficient and if they form a complete set.