Abstract: For conventional uniform transmission-line directional couplers and interdigital couplers, it is difficult to realize fractional bandwidth greater than 50%.By using the S-parameter characteristics of even and odd modes of nonuniformly coupled transmission lines, this paper presents a new approach for designing 3dB couplers with high directivity and broadband.Cascaded ABCD matrixes are employed to calculate the S-parameters of the couplers.Simulation results show that coupled non-uniform transmission lines are suitable for designing broadband directional coupler.

Key words: interdigital couplers, S-parameters, broadband, directional couplers, non-uniform transmission lines