Miniaturized high-selectivity microstrip bandpass filter using capacitively coupled stub-loaded stepped-impedance resonators

This paper presents a high-selectivity compact microstrip bandpass filter (BPF) based on electromagnetically coupled, stub-loaded, folded, asymmetric stepped-impedance resonators (SIRs). The main advantage of the proposed BPF lies in its highly miniaturized effective size (0.2985 × 0.2298 λg2) and a wide upper-stop-band response. The high-impedance sections of the SIRs were loaded with edge-coupled, fork-shaped stub loads in order to exploit the enhanced electromagnetic coupling of the SIRs, miniaturize the BPF effective size, and improve the skirt selectivity. In addition, the capacitive coupling of the SIRs with input and output as well as the use of fork-shaped stubs and simple uniform-impedance stubs generated three transmission zeros in the upper-stop-band, resulting in a wide stop-band response. The measured pass-band minimum return loss (26.6 dB) and maximum insertion loss (1.21 dB) for a protocol BPF with a central frequency of 5.17 GHz, three upper-stop-band transmission zeros with a minimum suppression level of 30.31 dB, and good agreement between the simulated and measured results verified the validity of the proposed high-selectivity BPF. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:2004–2010, 2016