Partially Air-Filled Substrate Integrated Waveguide Filters With Full Control of Transmission Zeros

This paper presents the investigation and testing of novel substrate integrated waveguide (SIW) bandpass filters, based on air-filled cavity resonators. Removing the lateral parts of the dielectric substrate in SIW rectangular cavities enables the use of the TM120 and TM210 modes to implement a dual-mode resonator. In addition, the fundamental TM110 cavity mode is adopted to implement a transmission zero. This paper demonstrates the complete control of the central operation frequency, the pass bandwidth, the spurious-free bandwidth region, and the location of the transmission zeros. After a comprehensive theoretical investigation, design rules are derived and applied to the implementation of a variety of dual-mode cavities, which have been manufactured and tested. Moreover, these basic structures have been used as building blocks to design higher order filters: simulations and measurements of four-pole filters are reported for validation.