This letter presents a design of a compact ultra-wideband (UWB) bandpass filter (BPF), which can exhibit an UWB response from 3.1 to 9.9 GHz with low insertion loss (IL) of 0.8 dB and an extremely broad stopband region from 10.9 to 25.1 GHz with high rejection level of 20 dB. This UWB BPF is simply constructed by using only one single-stage parallel-coupled line and two rectangular stub resonators. In this design, the rectangular stub resonators provide an exciting mode to enhance the desired UWB response and also create tunable transmission zeros to extend a wide stopband by controlling the impedance ratio (R) properly. The filter was fabricated and measured to verify the design concept.
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