Reduced-Length Tandem Directional Couplers Composed of Coupled-Line Sections With Fixed Coupling Coefficient

A novel concept of tandem coupled-line directional couplers has been proposed. The proposed couplers consist of coupled-line sections having different electrical lengths and fixed identical coupling coefficients regardless of the required nominal coupling of the entire tandem coupler. It is proved that the available coupling in coupled-line sections constituting tandem directional couplers can be used for overall electrical length reduction of the entire network. As shown, this allows not only for reduction of the circuit's size but also enables flexible design of such couplers, since tuning of the coupling characteristics can be easily achieved by change in solely electrical lengths of the coupled sections with no change in their coupling coefficients. The thorough theoretical investigation has been presented of such tandem couplers composed of two and three coupled-line sections having fixed coupling coefficients followed by measurements of four manufactured networks operating at a center frequency of 2 GHz.