Directional couplers with reduced coupling requirements as connection of coupled-line sections and left-handed transmission lines

A new class of directional couplers realised as a connection of two symmetrically placed coupled-line sections and two sections of uncoupled left-handed (LH) transmission lines is proposed. It has been proved that such circuits allow for achieving a higher nominal coupling of the resulting coupler than the coupling level of the utilised coupled-line section. Moreover, the proposed networks feature the properties of the ideal coupled-line sections, therefore, they allow for the realisation of broadband, high-performance directional couplers with relaxed coupling coefficient requirements. The proposed directional couplers have been theoretically analysed, and approximate design equations have been derived, which allow for the calculation of the required electrical length of the LH sections against the assumed coupling of the applied coupled-line section. Moreover, it has been shown that in such circuits the required minimum coupling of the utilised coupled-line section depends on the assumed nominal coupling of the resulting coupler, and this dependence has been derived analytically. The presented theoretical analysis has been verified experimentally by the measurements of two planar 3-dB directional couplers. The obtained results proved the correctness of the presented analysis, and confirmed that the presented method allows for the realisation of the broadband 3-dB directional couplers with the use of a single layer microstrip structure.