High Q-factor compact filters with wide-band spurious rejection

A stepped-impedance configuration for coaxial resonators is exploited to realise filters in the L-, S-, C-band exhibiting a wide spurious-free range. The proposed configuration is realised by optimizing the aspect ratio of a single rod resonator, by enlarging its top head, maximizing the ratio between the frequency of the first higher order mode and the frequency of the fundamental mode. This approach allows for designing a very compact-sized filter which shows high Q-factor performances and a wide spurious-free band, up to nearly 5 times the centre frequency. A 6^th-order filter was designed and manufactured and a comparison between the full-wave simulation and the measurement of the filter is reported.