Practical realisation of multiband planar filters based on a reactance transform method

A recently proposed reactance transform method using rational mapping functions to synthesise multiband filtering functions is applied to dual-band filter design. The paper focuses on practical aspects related to the physical implementation on multilayer substrate. As is shown, the selectivity is influenced not only by the order of the basis filter, but also by the number of bands, the spacing of the bands and the relative bandwidths. During the standard numerical tuning based on a group delay method, the use of group-delay based tuning requires special care. A de-tuning method is used for efficient design.