A universal method for the design and synthesis of wideband directional couplers using non-uniform coupled transmission lines

In this paper a complete procedure for the design, synthesis and construction of wideband directional couplers is presented. Cascading multiple quarter-wavelength line segments leads to an increase of coupling bandwidth up to 180% but also results in discontinuities of geometry and thus in strong reflections. Furthermore, the isolation is diminished. To avoid these effects a design technique for the generation of continous, non-uniform geometries is analyzed and implemented. Two methods for the coupler synthesis are proposed and compared. One is an approximational approach and uses two sets of equations. The other uses accurate 2D field simulation of the propagating modes and is universally applicable to any kind of coupled transmission lines. The completely synthesized couplers are simulated in CST Microwave Studio and achieve very good results for coupling, isolation and return loss. A nominal -10 dB wideband coupler at a center frequency of 15GHz and a nominal bandwidth of 164% is built in a multilayer stackup. Good agreement between measurement and simulation results is observed.