Design flexibility of an open-loop resonator filter using similarity transformation of coupling matrix

In this letter, the design flexibility of an elliptic-response open-loop resonator filter is achieved through similarity transformation of the coupling matrix. To verify the proposed method, a compact four-pole elliptic-response bandpass filter is designed using the conventional structure with open-loop resonators. Designed filter has the resonators with very narrow spacing for the required coupling coefficient and it is impossible to implement a filter with wider fractional bandwidth. Two possible configurations to overcome this problem are presented through similarity transformation of the coupling matrix. From this design flexibility, a filter with wider fractional bandwidth is designed with realizable design parameters.