Full-wave perturbation theory based upon electric field integral equations for coupled microstrip transmission lines

A full-wave perturbation theory for the system of N coupled microstrip transmission lines is developed based on an integral equation description of the microstrip circuit structure. The perturbation theory is suitable for application to nearly degenerate multistrip coupling. The physical justification of the perturbation approximation is based upon the current distributions on the strips. The associated computational procedure is relatively simple. For the case of two coupled lines, it is found that the propagation eigenvalues split and shift symmetrically away from the average of their isolated limits as the microstrips become closely spaced. Numerical results obtained by the perturbation theory are compared with those obtained by a more accurate method-of-moments solution of coupled electric field integral equations. To verify the perturbation theory, an experimental implementation of two coupled microstrips is set up in a PC-board configuration, and the electric field distributions along the strips are measured. The theoretical and experimental results are in good agreement