An aperture-backed sextuple-mode ultra-wideband bandpass filter

In this paper, a sextuple-mode ring resonator ultra-wideband bandpass filter is presented. The objective is to enhance the selectivity of a quintuple-mode filter by introducing an additional transmission pole within the passband. The extra pole comes from two open stubs inserted symmetrically inside the ring resonator. Analysis of the filter is done to show the relationship between the six resonant frequencies, and characteristic impedance and electrical length of the stubs. The filter is designed, fabricated and measured. The measured responses are in good agreement with simulations. The filter exhibits 108.02% 3dB fractional bandwidth at 6.85GHz centre frequency.