Reduced-Length Two-Section Directional Couplers Designed as Coupled-Line Sections Connected With the Use of Uncoupled Lines

Broadband coupled-line directional couplers having the properties of two-section coupled-line directional couplers are proposed. The presented couplers are composed of two-coupled line sections having the same coupling coefficient which are connected with the use of two sections of uncoupled transmission lines. It is shown that in such a structure it is possible to achieve an equal-ripple coupling characteristic similar to the coupling response of classic-two section coupled-line couplers. It is also shown that, in comparison to the classic counterparts, in the proposed couplers the required coupling between the coupled-lines is slightly weaker and the overall electrical length is reduced. The theoretical analysis and measurement results of a 3 dB directional coupler operating at a center frequency of 1.8 GHz are presented.