A highly selective bandpass filter based on microstrip multi-section coupled-line resonator

Novel microstrip multi-section coupled-line resonator is proposed. This kind of resonator can give us more flexibilities to design the microstrip line filter since it covers two different regions to settle the coupling coefficients of the filters and these two different regions are connected by two un-coupled transmission line. A highly selective bandpass filter centered at 1.8 GHz is demonstrated by cascading two identical multi-section coupled-line resonators. Two attenuation poles are achieved in the upper and lower stopband of the filter to implement the highly selective property since cascading of above resonators can simultaneously satisfy electrically and magnetically coupling paths and these two path signal can cancel each other in the output port at certain frequencies.