Compact microstrip diplexer based on a novel coupling topology

A compact microstrip diplexer with second-order bandpass response has been reported in this paper. It is composed of two quarter-wavelength resonators and two folded half-wavelength resonators. By taking advantages of the new coupling topology proposed, two transmission zeros are generated around each passband to improve frequency selectivity and isolation performance. In addition, no extra matching circuits are needed for the designed diplexer, which greatly simplifies the design procedures and reduces the circuit size. For the demonstration, a diplexer sample occupying a compact size of 0.29 λ_g by 0.23 λ_g, has been designed and fabricated using single-layer printed circuit board (PCB) technology. Figure of merits such as low loss, high frequency selectivity, fairly good isolation and stopband rejection (>20 dB) have been achieved, which exhibits a satisfactory agreement with the EM simulations.