Application of ‘C’ section based left-handed transmission lines for coupling enhancement of coupled-line directional couplers

An application of Schiffman `C´ section based left-handed transmission lines for coupling enhancement of coupled-line directional coupler´s is proposed. The higher nominal coupling of the directional couplers can be achieved by using two symmetrically placed coupled-line sections and two sections of uncoupled left-handed (LH) transmission lines. The proposed solution allows to simplify the design and decrease complexity of manufacturing process. Two exemplary planar 3-dB couplers, in which available coupling of the coupled-line section is as low as k_av = 0.42, operating at f₀ = 2.6 GHz and f₀ = 4.5 GHz have been designed and manufactured. The measurement results proved the usability of the proposed circuit.