Microstrip lowpass filters with improved frequency responses using coupled-line hairpin resonators

In this paper, microstrip lowpass filters (LPFs) with improved frequency responses are developed by cascading a coupled-line hairpin resonator with quarter-wavelength microstrip lines. The equivalent transmission line circuit and lumped element circuit of the filters are analyzed, and formulas are derived for determining the frequencies of reflection zeroes and transmission zeroes, as well as the 3dB cutoff frequency of the filters. Based on the derived formulas, a number of LPFs are designed and fabricated, and their measured frequency responses agree well with the predicted ones. The desired filtering characteristics, like flat passband, sharp roll-off skirt, and wide stopband, are realized.