Design of broadband asymmetric tapered-line 3-dB directional couplers

In this paper we present design of broadband tapered-line 3-dB coupled-line directional couplers. Novel functions of the coupling coefficient distribution have been proposed in order to minimize the cut-off frequency of the couplers. Two different designs of 3-dB tapered-line directional couplers are presented (i) a tandem connection of two 8.34-dB couplers and (ii) a 'single-element' coupler which is rarely reported in literature. Both theoretical analyses and experimental results are shown in which the bandwidth of 6:1 has been achieved.