Stopband performance improvement of rectangular waveguide filters using stepped-impedance resonators

Rectangular waveguide stepped-impedance resonators (SIRS) are analyzed and employed in the design of an X-band filter with center frequency fo = 10 GHz and a bandwidth of 100 MHz. An attenuation of 80 dB is held up to 23.1 GHz and, compared to standard uniform-impedance-resonator filters, a reduction in length of 55% is achieved at the expense of an increased insertion loss from 0.6 to 1.5 dB. The second resonance of the fundamental TE10 mode can be controlled by adjusting the length and impedance ratio of each resonator. A design procedure that takes into account step discontinuities is described and applied to the design of a number of SIR filters. Finally, the presented theory is supported with experimental results.