Efficient eigenvalue analysis of rectangular coax for slot-fed patch antennas

An integral-equation technique is presented to solve for the mode spectrum of a rectangular coax line with magnetic side walls. The method is based on directly expanding the tangential electric field at the interface plane in terms of basis functions. These basis functions are chosen so that they not only comply with the boundary conditions of the electric fields, but also satisfy the edge conditions at the metallic corners. Therefore, the solution depends only on one parameter, namely the number of basis functions, which eliminates the phenomenon of relative convergence. The results show that rapid convergence is achieved and that, using only few basis functions, sufficient accuracy is obtained, not only for the cutoff wave numbers, but also to properly represent the fields. The method can be applied to many other structures with, in most cases, increased efficiency and reliability in comparison to standard modal field-matching techniques.