Two-section asymmetric coupled-line impedance transforming directional couplers

Two-section impedance transforming directional couplers have been proposed. It has been shown that such asymmetric coupled-line directional couplers can provide equal power split between the coupled and transmission ports as well as impedance transformation in the same broad frequency range, as classic two-section coupled-line couplers. The theoretical analysis has been presented and appropriate formulas allowing for calculation the required per-unit-length parameters of broadband two-section impedance transforming couplers have been derived. Moreover, it has been shown that the impedance transformation ratio of the presented coupler can be increased by the appropriate modification of the two-section coupled-line directional coupler in a way that the directly fed and coupled-line transformation ratios are different. The proposed modification allows for the design of broadband impedance transforming directional couplers with increased transformation ratio over the limit fixed by the coupling coefficient of the tightly coupled section and having at the same time acceptable impedance match and isolation between the respective ports. The presented theory has been confirmed by the measurements of a two-section 3 dB impedance transforming directional coupler having bandwidth f_u/f_l = 3:1 and impedance transformation ratio R = 2.