A new method of synthesis of multiple-decade band directional couplers

A new method of synthesis of a prototype of multiple-decade band coupler is presented this paper. Optimally designed “stepped” coupler usually consists of a cascade connection of large number of two kinds of uniform coupled transmission line sections. Coupled transmission lines, of maximal acceptable coupling are separated by non-coupled or minimally coupled sections. Analytical solution for the problem is usually not available, therefore synthesis can be solved only by numerical optimization. Modern optimization solvers can successfully coup with huge number of parameters, however a suitable starting point for optimization is required. Therefore a three stage synthesis algorithm is proposed. Firstly, a starting point is generated by a direct shape optimization method. A non-uniform coupled transmission line is evolved from scratch. Secondly, the non-uniform line is converted into a stepped one. Finally, the coupler is optimized with dynamically growing frequency band. An example of synthesis of 0.1 to 10.6 GHz 10 dB coupler with ideal uniform coupled transmission lines is presented.