Novel inductance-incorporated microstrip coupled-line bandpass filters with two attenuation poles

A microstrip coupled-line bandpass filter with good stopband rejection is proposed. By introducing an additional shunt inductance to the conventional coupled-line filter, along with the capacitively coupled gap between coupled-line sections, two attenuation poles at upper and lower stopbands may be achieved without increasing the circuit area. The proposed filter exhibits good stopband rejection with no repeated passband at twice the center frequency as compared to the conventional coupled-line filters. Specifically, a microstrip coupled-line bandpass filter centered at 2.4 GHz with 5.4% 3-dB bandwidth and two attenuation poles is implemented.