A compact frequency-tuned bandpass filter (BPF) with sharp rejection characteristic is presented. It is composed of a trans-directional (TRD) coupled line and two short-circuited stubs. By changing the capacitor values of the coupled line and the electrical lengths of the short-circuited stubs, a frequency-tuned BPF with sharp rejection is obtained. For verification, a prototype tuned from 1.0 GHz to 1.6 GHz (46.2%) is designed and fabricated. The measured results show that the proposed structure exhibits the return loss of more than 17 dB, the insertion loss of 1.4 dB, and the 3-dB fractional bandwidth (BW) of 43.2-50%. Sharp rejections are also obtained, agreeing well with the simulation results.
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