A New Microstrip Bandstop Filter for Fully Canonical Cul-de-Sac Coupling Configuration
A new microstrip fully canonical Nth-order Cul-de-Sac coupling bandstop filter (BSF) is proposed in this paper. A Cul-de-Sac coupling matrix based BSF design allows to provide an equiripple stopband response with reflection zeros at passbands, keeping a constant filter length regardless of filter order. However, such a coupling configuration of BSF has not been physically realized yet in a single-layer microstrip structure. The proposed BSF structure is simply formed by the parallel arrangement of even- and odd-mode half-wavelength resonators with respect to a through line connecting input and output ports, which makes it easier to design a higher-order BSF than conventional planar configurations. The effectiveness of the proposed BSF is demonstrated through the design of a 2-GHz, 2%-bandwidth, third-order Cul-de-Sac coupling BSF having three RZs. A sharp cutoff response because of three RZs is numerically and experimentally achieved by the proposed BSF.