Quasi-Reflectionless Short-Through-Line-Based Bandstop Filters: IEEE Microwave and Wireless Technology Letters

Highly miniaturized input quasi-reflectionless bandstop filters (BSFs) are reported. They are based on a complementary RF diplexer design approach where the bandstop branch is formed by cascaded short-through-line (STL)-based bandstop stages and the bandpass branch by a single first-order bandpass filter (BPF). When compared to conventional input quasi-reflectionless BSFs, the proposed approach leads to a significantly smaller size for multistage implementations. The operating principles of the STL-based BSF concept are provided through circuit-simulated examples demonstrating scalability in terms of bandwidth (BW) and selectivity. For validation purposes, the single- and two-stage microstrip prototypes were built and tested at 2GHz. The single-stage prototype exhibits: center frequency (f0: 2GHz, stopband rejection: 25dB, 3-dB stopband BW: 135MHz (6.75%), and 10-dB input return-loss (RL) BW: 2GHz. The two-stage prototype demonstrates: (f0): 2GHz, stopband rejection >35dB, 3-dB stopband BW: 228MHz (11.4%), and 10-dB RL BW: 2GHz. © 2023 IEEE.