Self Cancellation of Higher Order Mode Spurious Frequencies in SIW Filters

In this paper, a new approach for the suppression of higher order mode spurious frequencies in SIW filters is proposed. The proposed structure is a double layer SIW filter. In this structure, all couplings related to the higher order modes generating the spurious frequencies are maximized with the exception of the central one that is instead minimized This higher order mode coupling configuration allows for removing the spurious through a self-cancellation mechanism, in which spurious contributions cancel one another. In this approach, the only iris that need to be designed in order to minimize the coupling to the higher order mode is the central one, that is the iris between the two layers. This approach has been compared with a classical one in which spurious couplings are minimized as much as possible. In the classical approach, all irises should be as our central iris, resulting in a multilayer structure in which each layer contains a single resonator only. In this paper, it is demonstrated that the new approach has the advantage of being more effective in spurious suppression and it has an easier manufacturing. The two approaches have been compared through the design of 4th order filters. The filter based on the new method has been manufactured and measured, thus demonstrating the feasibility of the proposed approach.