Review of the microstrip/negative-refractive-index transmission-line coupled-line coupler

The microstrip/negative-refractive-index transmission-line (MS/NRI-TL) coupler is constructed by edge coupling a regular microstrip line to a backward wave negative-refractive-index transmission line. This coupler is primarily operated in the complex-mode band in which the eigenmodes are characterised by simultaneous phase progression and attenuation (in a lossless structure) and is accompanied by contra-directional power flow on the two lines. The complex modes that are excited by the sources in this coupler can be controlled independently and by examining the multiple reflections of these modes between the two terminal planes, the scattering parameters of the corresponding four-port device can be derived. Under a first-order approximation, it is determined that the MS/NRI-TL coupler is capable of achieving arbitrary coupling levels (by increasing the length of the coupler) and very high directivity (by making the coupler length equal to an odd multiple of half a guided wavelength). Useful nomographs that enable the realisation of MS/NRI-TL couplers to meet arbitrary design specifications are included and several applications of the coupler are also presented.