Dual-Mode Branch-Line/Rat-Race Coupler Using Composite Right-/Left-Handed Lines

A novel dual-operational mode directional coupler, functioning as a branch-line coupler at one frequency but identically to a rat-race coupler in the other one, is proposed and investigated. To fulfill the unusual design, the characteristic impedance and phase response of a conventional composite right-/left-handed line are controlled simultaneously at both operating frequencies. Analytical design equations are derived and verified by experimental results. The dual-mode coupler shows typical responses of a 90° and 180° coupler, respectively, in the first and second (1.5/2.5 GHz) operating bands. The fractional bandwidths are 8% and 16%.