Leakage analysis of higher order modes on coupled microstrip transmission lines

This paper examines the dispersion and leakage characteristics of higher order modes on coupled microstrip lines. The full-wave, spectral domain electric field integral equation (EFIE) formulation used in this work has a dyadic Green's function in the form of a spectral Sommerfeld integral. Based on the migration paths of the surface-wave pole(s) and the branch point(s) implicated in the integral, the appropriate integration paths for evaluating the integrals in all three propagation regimes are determined. For numerical solution, the integral equation is discretized via the method of moments, where entire domain Chebyshev functions incorporating appropriate edge factors are used for current expansions to provide convergence with relatively few terms. Numerical results in the form of propagation constants, leakage constants, and current distributions for the first three higher-order coupled modes are presented. Results from this research will serve as a useful reference for the design of novel coupled microstrip devices and for further study in this area.