New coupled-line bandpass filter with stepped impedance hairpin (SIH) resonator

In this paper, a new coupled‐line bandpass filter with a stepped impedance hairpin (SIH) resonator is proposed. The design formulas are derived based on coupled‐line filter theory and the equivalent‐circuit network. This filter not only achieves better selectivity, but also suppresses the 2nd‐harmonic response, as compared with the three‐stage parallel‐coupled microstrip‐line bandpass filter, which we use as the conventional case. The measured results show that the proposed coupled‐line bandpass filter with an SIH resonator increases the lower‐ and upper‐band selectivity by 13.8% and 41.9%, respectively. In addition, the level of the 2nd‐harmonic response is improved 35 dB, and the fractional 3‐dB bandwidth is reduced 10.5%. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 44: 80–83, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20553