Hybrid TM-Mode / Coaxial Triple-Band Bandpass Filter

In this paper, a novel realization of a triple-band filter offering a large spurious free range is presented. Three different resonator types (dielectric TM-mode, dielectric quasi-TM-mode and coaxial resonators) are implemented in order to increase the out of band rejection of the filter. Each of the chosen resonator types are used to implement one of the passbands in the filter response. The proposed topology leads to three real frequency axis transmission zeros between adjacent bands without the need for any complex cross coupling or coupling with a negative sign. Additionally, the topology reveals possibilities for an adaption of the bandwidth and distance between the individual passbands. The presented filter design leads to a large spurious free range up to 3.5 times the center frequency of the center band.