Wideband Bandpass TM Cavity Filters With Wide Spurious Free Band

Recently, the spurious self-suppression method has been introduced and applied to the bandpass TM cavity filters to improve their stopband performance. However, this method has the limitation of being applicable only to TM narrow-band filters. This paper shows how to overcome this limitation. In particular, it shows how to increase the bandwidth and improve the stopband performance by removing the spurious resonance of the coupling irises due to TE10 mode. The conventional rectangular iris is replaced by a new bent c-shaped iris. In this way, the resonance of the iris TE10 mode can be shifted to lower frequencies (below the cut-off), improving the stopband performance of the filter. Moreover, this new iris shape provides large couplings that allow wide passbands, and strong excitation of the spurious resonant modes that allows for an efficient implementation of the spurious self-suppression method. Furthermore, double slot iris has been used to connect the two central cavities to increase the central coupling of the filter. Design examples of 4th order and 6th order TM cavity filters with wide spurious free range up to 18 GHz having fractional bandwidth (FBW) of 5% and 4%, respectively, are presented. Finally, the 6th order TM cavity filter has been manufactured and measured. Good agreement has been achieved between simulated and measured frequency responses.