Transmission Zeros in In-line Filters by Using Source-to-Load Paths With Suppressed Spurious Frequencies

This paper presents a new class of in-line rectangular waveguide filters with source-to-load (S-L) coupling for generating transmission zeros (TZs). The easiest way to obtain a direct S-L coupling is to connect source and load by a secondary path realized with a waveguide section. However, in an in-line filters, source and load are in general distant, this resulting in a long waveguide section that generate spurious frequencies that degrade the filter performance. In this paper, an alternative method where the secondary path is realized by the cascade of cavities is introduced. Resonances generated by the cavities of the secondary path are then removed by exploiting the spurious self-suppression method. Cavities in secondary path can have a very small volume and their contribution to the filter Q-factor is extremely limited. This results in a secondary path that has a limited impact on the dimension of the whole filter. To validate the proposed method, a 4th order filter has been designed, fabricated, and measured.