Substrate Integrated Folded Waveguide Filter with Out-of-Band Rejection Controlled by Resonant-Mode Suppression

This letter presents a new filter, based on substrate integrated folded waveguide (SIFW) technology, which exhibits compact size and good out-of-band rejection. The filter is based on a SIFW cavity, which guarantees size reduction, and the out-of-band rejection is controlled by the suppression and tuning of the high-order cavity modes. A detailed investigation of the cavity mode spectrum is presented, to illustrate the operation principle and the design of the filter. The interesting feature of this filter is the possibility to design the pass band and the return band by simply tuning the mode spectrum of the cavity, which is practically unaffected by the connection to the excitation ports. The fabrication and testing of a prototype operating at 4.5 GHz validate the proposed filter topology.