Approach to the Design of Asymmetric Coupled-Line Directional Couplers With the Maximum Achievable Impedance-Transformation Ratio

A novel approach to the design of asymmetric coupled-line impedance-transforming directional couplers with the maximum achievable impedance-transformation ratio is proposed. It is shown that the couplers in which one conductor is completely shielded from the other can be represented by two uncoupled transmission lines, which significantly simplifies the design procedure. Furthermore, the proposed approach of the directional couplers´ analysis has been utilized for the derivation of a general condition describing the maximum impedance transformation ratio that can be achieved for directional couplers designed in inhomogeneous media. It has been also shown that impedance-transforming directional couplers, in which the capacitive coupling coefficient differs from the inductive coupling coefficient feature limited isolation, and that such couplers can be represented by two uncoupled lines having different electrical lengths. Subsequently, it has been shown that this isolation can be improved when the electrical lengths of the two uncoupled lines are equal, which corresponds to the equalization of capacitive and inductive coupling coefficients. The theoretical analyses have been confirmed by the measurements of two different 3-dB impedance-transforming directional couplers designed with the use of the proposed simplified approach.