Direct-coupled waveguide filters for the lower gigahertz frequency range

An analysis method based on the coupled-integral-equations technique (CIET) and the mode-matching technique (MMT) is presented for the design of a variety of direct-coupled waveguide filters suitable for applications in the lower gigahertz frequency range. The method is verified by comparison with data obtained through other numerical techniques and measurements. With reference to standard waveguide filters, the issues of filter miniaturization and stopband extension toward higher frequency bands are investigated. For given frequency specifications in the 2-GHz frequency range, examples of rectangular coaxial waveguide filters, ridge waveguide filters, ridge waveguide filters including coupling irises and T-septum waveguide filters are presented. It is demonstrated that the rectangular/square coaxial waveguide filter achieves the highest degree of miniaturization, but that ridge and, especially, T-septum waveguide filter technology is advantageous with respect to stopband performance toward higher frequencies. In particular, a T-septum filter centered at 2.155 GHz is shown to block the entire frequency range up to 7.5 GHz.