Enhancing the spurious free range in inductive rectangular waveguide filters

This work proposes a new strategy for the improvement of the out of band response of rectangular waveguide inductive bandpass filters. The strategy consists of selecting non-uniform coupling window thicknesses along the filter structure. The basic idea is that irises of different thicknesses can produce the same coupling values in the passband but have different loading effects in the harmonic resonances. To further increase the level of spurious rejection, the above technique is combined with the use of resonators of different widths. To demonstrate the effectiveness of the proposed idea, the design of a fifth order, non-uniform Tchebyshev bandpass filter is fully described showing how the technique presented in this paper indeed results in an important improvement of the out of band performance with respect to conventional inductive filters.