A. S. Bondarenko, V. P. Meschanov, L. V. Shikova

A new structure of the electrical circuit of superwide-band directional coupler is proposed and investigated. It is a cascade connection of coupled nonuniform transmission line sections alternated with uniform transmission line sections. A coupling coefficient of each of the nonuniform transmission line section is a symmetrical function tacking a maximum value at the centre of the section. When solving the synthesis problem for the case of Chebyshev-s characteristics, section lengths of uniform and nonuniform transmission are varying. For the purpose of eliminating electrical discontinuities in the directional coupler structure, a special restriction was introduced. It equates coupling coefficient value at the ends of nonuniform transmission line sections to the coupling coefficient of uniform transmission line sections. Both symmetrical directional couplers structures and non-symmetrical ones are considered. A general analysis of the obtained results of solving the synthesis problem is presented. In this work, the attempt of approximation of a Class II stepped directional coupler structure by a structure on nonuniform lines is undertaken. A comparison of the synthesis results of the considered directional couplers with those of symmetrical and non-symmetrical Class II directional couplers has shown the followings. In fact, the uniform coupled transmission lines in the Class II structure were substituted by symmetrical nonuniform coupled transmission lines. The introduction of nonuniform transmission lines allows us to widen the frequency band of the directional coupler with insignificant rise of its length and coupling coefficients. In this case, the lengths of coupled nonuniform lines are longer than correspondent coupled transmission lines of Class II stepped directional couplers, and the section lengths of the loose coupling ( ) are shorter than corresponding to them uncoupled transmission line sections of Class II stepped directional couplers. Earlier obtained section lengths distribution along the coupling region in Class II structures remains valid. It consists in the following: for non-symmetrical structures, coupling line section lengths are increasing (decreasing) from the left to the right side, and uncoupled line section lengths are decreasing (increasing) in the same direction; for symmetrical structures coupled line section lengths are increasing in the direction to the vertical symmetry plane, and uncoupled section length are decreasing.