Strategies for the improvement of the out of band behavior of TM dual-mode filters

An investigation on the stop band behaviour of TM dual-mode filters has been conducted in order to identify ways to suppress or alleviate the presence of spurious out-of-band resonances. Two types of spurious resonances have been identified: iris and cavity resonances. Both resonances can be shifted to higher frequencies: the former by using multi aperture irises or multiple irises, the latter by modifying the cavity geometry. In this paper the conventional square cavities are replaced by circular cavities, taking advantage from some modal properties of the circular geometry. The presented strategies are used in order to improve the upper stop-band of a manufactured four-pole TM filter with four transmission zeros presented in [12], obtaining an increase up to 370%.