Sharp-rejection microwave planar filters using the branch-line directional coupler

The design of new sharp-rejection microwave planar bandpass filtering topologies using the branch-line directional coupler is presented in this paper. The basic idea consists on using the branch-line coupler as a transversal filtering section, by loading the coupled ports of the coupler with transmission line segments ending in an open circuit and taking the isolated port as the output node. Thus, under the signal-interference philosophy involved in conventional transversal filtering schemes, bandpass transfer functions with perceptible stopbands and sharp cutoff slopes can be obtained. Furthermore, the main characteristics of the synthesized filtering response, such as the bandwidth or the out-of-band performance, can be easily controlled by means of the design parameters of the transversal section. The theoretical results are experimentally validated with the design, construction and characterization of a microstrip filter prototype at 3 GHz based on branch-line coupler filtering sections.