Improved Design Equations of the Tapped-Line Structure for Coupled-Line Filters

Coupled-line filters with tapped input and output are studied in this letter. The tapped-line structures are used for transformation of the filter impedances to lower values. This makes the filter more suitable for manufacture. However, the variation of the transformed impedances around the passband central frequency, due to the tapped feeds, distorts the filter responses. In this letter, design equations are proposed to improve the design by taking into account this variation. Finally, a filter design example is given to justify the study