Discrete shape optimization method for the non-uniform transmission line directional couplers

In this paper a new method for a non-uniform directional couplers design is presented. It is modeled on the cascade connection of a large number of short, equal length sections of a uniform coupled transmission line. The coupling coefficient vector is optimized with the non-smooth sub-gradient algorithm. The performance of the optimization algorithm is surprisingly good. The algorithm generates many, close to optimal, solutions, with a variety of shapes. Initial profiles are almost smooth. However, at the end of optimization, shape tends to converge to a technological constraint. The optimal structure consists of a cascade connection of uniform coupled transmission line sections (of maximal coupling), separated by sections of a non-coupled transmission line.