Quasi-Reflectionless Short Through-Line Based Bandstop Filters

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 significantly smaller footprints for multi-stage implementations. The operating principles of the STL-based BSF concept are provided through ideally-synthesized examples demonstrating scalability in terms of bandwidth (BW) and selectivity. For proof-of-concept validation purposes two microstrip prototypes were built and tested at 2 GHz. They demonstrated the following unique characteristics. Single-stage: center frequency: 2 GHz, stopband rejection: 25 dB, 3-dB stopband BW: 135 MHz (6.75%), 10-dB input return loss (RL) BW: 2 GHz. Two-stage: center frequency: 2 GHz, stopband rejection >35 dB, 3-dB stopband BW: 228 MHz (11.4%), 10-dB input return loss (RL) BW: 2 GHz.