An efficient method for analyzing nonuniformly coupled microstrip lines
This article presents an efficient method for analyzing nonuniformly coupled microstrip lines. By choosing a modal-transformation matrix, the coupled nonlinear differential equations describing the symmetric nonuniformly coupled microstrip lines are decoupled using even- and odd-mode parameters; the original problem is thus transformed into two single nonuniform transmission lines. A power-law function of arbitrary order and having two adjustable parameters is chosen to better approximate the equation coefficients. Closed-form ABCD matrix solutions are obtained and used to calculate the S-parameters of nonuniformly coupled microstrip lines. Numerical results for two examples are compared with those from a full-wave commercial package and experimental ones in the literature in order to demonstrate the accuracy and efficiency of this method. This highly efficient method is employed to optimize a cosine-shape 10-dB codirectional coupler, which has good return loss and high directivity performance over a wide frequency range. (c) 2005 Wiley Periodicals, Inc.
nonlinear differential equations coupled transmission lines distributed parameter circuits microstrip couplers