Compact ultra-wideband bandpass filters with narrow notched bands based on a ring resonator

In this study, compact ultra-wideband (UWB) bandpass filters with single/dual-notched bands are proposed based on a ring resonator and transversal signal-interference concept. Broad passband and narrow notched bands with controlled even/odd-mode resonator frequencies can be adjusted conveniently by changing the characteristic impedance of the ring resonator. Transmission zeros are introduced to improve selectivity and harmonic suppression of the proposed UWB bandpass filters with notched bands. To verify the presented strategies, three prototypes (ε_r = 2.65, h = 0.5 mm) with 3 dB fractional bandwidth >120% are designed and fabricated. Simple structure and good in/out-of-band performances can be achieved in the proposed UWB bandpass filters.