Broadband 4 × 4 Butler matrices utilizing tapered-coupled-line directional couplers

The analysis and design of a 4 × 4 Butler matrix that utilizes tapered-coupled-line directional couplers is presented. As a basic element of the matrix, the previously designed and measured 3.3-dB directional coupler has been used. Two different realizations of a 4 × 4 Butler matrix are considered, i.e. a network consisting of tapered-line couplers and a Schiffman phase shifter, and a network in which the appropriate phase characteristics have been ensured by substituting one of the 0/180° directional couplers by a 3-dB/90° directional coupler. In this solution, the needed coupler has been designed as a 5-section symmetrical coupled-line coupler in which the tightly coupled section is designed with the use of tapered-coupled-lines. The proposed technique allowed for achieving very good amplitude characteristics of the designed directional coupler due to the fact that the large discontinuity between coupled-lines having different coupling coefficients is avoided. The operational bandwidth of the presented 4 × 4 Butler matrices exceeds two frequency octaves.