High-Selectivity Microstrip Bandpass Filter Using Triple-mode Resonators

In this paper, a miniaturized highly-selective six-pole filter with wide stopband is reported. For this purpose, a triple-mode resonator loaded with stub and patch is proposed, which is characterized both by theoretical analysis and full-wave simulation. The designed six-pole filter is composed of two identical triple-mode resonators. The two resonators are coupled to each other to increase the selectivity. There are two transmission zeros at each side of the passband, which increase the stopband attenuation. The other two transmission zeros suppress the harmonic frequencies, which extend the 20dB stopband rejection levels up to 6GHz. The center frequency of the filter is 2.2GHz, the fractional passband is 13%, and the 30dB regular coefficient is better than 0.6.