A Novel Highly Selective UWB Bandpass Filter Using Quad-Mode Stub-Loaded Resonator

In this letter, a novel ultra-wideband (UWB) bandpass filter (BPF) configuration with a quad-mode resonator (QMR) structure is proposed, which has a highly selective and compact performance. The QMR is composed of a funnel-shaped resonator loaded in the middle and a low-impedance folded microstrip line. Initially, the resonant frequencies are uniformly distributed in the UWB passband by varying the length of QMR physical stubs, and later three-line parallel coupled lines are employed to enhance coupling to obtain a fiat passband. The feed lines are then loaded with a pair of A/2 stepped impedance radial stubs (SIRSs) to provide excellent band-stop characteristics. Finally, a filter prototype is created, and its performance is evaluated using the generated data. The proposed UWB filter has sharp roll-off ratio of 99 and 63 dB/GHz, respectively, at the lower and upper edges of the passband.