A novel coplanar-waveguide directional coupler with finite-extent backed conductor

A novel edge-coupled coplanar-waveguide (CPW) directional coupler with a floating backed conductor is proposed. For design purposes, a theoretical transmission-line model for the coupler is established, which is based on the characteristic parameters of the related coupled CPW structures. To this end, the effective dielectric constants and characteristic impedances of the conductor-backed edge-coupled CPW structures are analyzed using the full-wave analysis based on the spectral-domain approach. Specifically, the coupling coefficient of this new coupler can be controlled by adjusting the width of the backed conductor. In this paper, two conductor-backed edge-coupled CPW directional couplers with moderate (-6 dB) and tight (-3 dB) coupling coefficients are designed and implemented based on the theoretical transmission-line model. The theoretical results are compared with the measured ones, and the agreement among them confirms the usefulness of the coupler design model, as well as the full-wave approach for the related coupled CPW structures.