Tuneable lumped-element directional coupler using metamaterial transmission lines

A combination of traditional right-handed transmission line sections and metamaterial left-handed ones is considered to design miniature tuneable directional coupler for wireless applications. Artificial right- and left-handed transmission line sections based on lumped-element Π-cells are employed. The use of tuneable capacitors provides tuning of the directional coupler characteristics in frequency. Estimation of tuneability of an artificial transmission line with tuneable capacitors is presented. In order to verify the theoretical prediction, a tuneable lumped-element rat-race ring has been designed and experimentally investigated. Semiconductor varactor diodes have been used as tuneable components.