Design of tight coupled backward-wave directional coupler using coupled lines with dielectric guide and finite metallisation thickness

A design for a tight coupled backward-wave directional coupler using coupled lines with dielectric guide between two finite metal strips is described. A mode-matching method, simple and fast quasi-static analysis, has been used to analyse the propagation characteristics of the structure. To provide the validity of the results obtained, comparison with other available data is made. The numerical results confirm that, with the use of dielectric guide in addition to finite metal conductors, it is possible to overcome the shortcomings of the conventional backward-wave directional coupler such as weak coupling, low directivity and extremely narrow gap distance for tight coupling, which are very difficult to realise. It is clearly shown that the dielectric guide between two strips and finite metallisation thickness can be new parameters for tight coupling in the design of backward-wave directional couplers, and this allows the RF designer another control variable in the design of high-performance three-dimensional monolithic microwave integrated circuits.