Compact Waveguide Bandpass Filters for Broadband Space Applications in C and Ku-Bands

Design and manufacturing of ultra-compact C and Ku bandpass filters for broadband space applications is presented in this paper. The C-band filter is centred at 6.175 GHz with a fractional bandwidth FBW=8.3% while the Ku-band has a central frequency of 14.25 GHz with FBW=3.5%. Both designs have a bandwidth of 500 MHz. TM mode cavity resonators are used to develop both filters. The C-band filter is a 9
^th
order filter consisting of 5 TM single-mode resonators and 4 comb-line waveguide resonators. TM resonators allow for a very short structure with good performance in terms of in-band loss, while the 4 comb-line resonators mitigate the spurious harmonics generated by TM cavities avoiding the need of bulky low-pass filters. The Ku-band filtering function is instead obtained by cascading a TM dual-mode cavity filter with a low pass filter. TM cavities allow for a length reduction of roughly 75% with respect the conventional TE dual-mode resonators while maintaining relatively high Q-factor values (>4000). The low-pass filter is an 8
^th
order comb-line filter that allows the fulfilling of all out-of-band requirements up to 30GHz. Both the proposed filters show very good performance in terms of loss, compactness and wide spurious-free stop-band range. Results of environmental tests for space environment demonstrate their on-board applications.