New Integral Equation Approach to the Characterisation of Coupled Microstrip Transmission Lines

An accurate numerical procedure is presented for calculating the normal mode parameters of symmetrical coupled microstrip lines. The Green´s function approach leads to a singular integral equation of the first kind which is reduced to an equivalent Fredholm equation of the second kind. It is shown that this equation may be approximated by a set of linear equations. The solution of these equations gives the charge distribution on the strips from which the normal mode parameters are computed. Results for the characteristic impedance and the effective relative permittivity are compared with those obtained by other authors.