A design technique for stepped circular waveguide dual-mode filters for broadband contiguous multiplexers

A stepped circular waveguide cavity provides an effective way for shifting or even suppressing unwanted spurious modes in a circular waveguide dual-mode (CWDM) filter. This useful property may be used in a wideband waveguide contiguous channel output multiplexer (OMUX) in satellite payloads. In this paper, a technique for automatically designing a stepped circular waveguide dual-mode (SCWDM) filter is presented. A K-inverter model of a short-circuited multi-port network is proposed to find the physical dimensions of a SCWDM filter in an iterative fashion. By taking into account the self-couplings as well as the effect of the stepped waveguide, without requiring optimization, this design process only needs EM simulation at the center frequency of the filter. The rapidly-convergent solution corresponds to the desired coupling matrix circuit model very well. A design example of a wideband 17-channel Ku-band contiguous multiplexer with two SCWDM channel filters is presented, in which the EM designed multiplexer responses match closely those of the synthesized circuit model.