Design of UWB BPF using SIR with open- and short-circuit stubs for bandwidth enhancement

In this paper, an ultra-wideband (UWB) bandpass filter (BPF) composing of a multi-mode resonances SIR is proposed. The SIR provides three resonances to form the passband response. However, the acquired bandwidth (BW) is insufficient and it is remedied by an open and a short-circuit stub, attaching to the SIR in the middle. The added open and short stubs with appropriate length furnish the SIR with two extra resonances, below and above its original ones. The sandwich-coupled-lines structure is implemented in the feed structure to provide the tight coupling needed by the UWB BPF. This feed structure alleviates the manufacturing tolerance caused by a small gap used in a conventional coupled-lines design. To further enhance the coupling strength of the sandwich-coupled lines, a slot is embedded in the ground plan beneath the feed. Experiments are conducted to verify the UWB BPF and agreements between the simulation and the measurements are observed.