A new mathematical design method of microstrip tapped-line filters

In this paper, a new mathematical design method of tapped-line pseudo-combline and interdigital filters is proposed. The parameters of tapped feeding are obtained by the input admittance of filter's prototype, and the tapped feeding structure is designed by the open stub and the additional transmission line with negative electrical length. The electrical lengths of the stub and line are functions of line impedance. The designed tapped feeding method is applied to pseudo-combline and interdigital filters and the results are well matched to the filter's theory.