Chapter 7 - Directional Couplers

Abstract

This chapter opens with definitions and basic properties of directional couplers. A directional coupler as treated in the chapter is a passive, reciprocal four-port coupler in which power incident on one port, the input, is split between two other ports, the coupled and through-ports, and little or no power emerges from the fourth isolated port. The chapter discusses different classes of couplers: waveguide couplers, coupled-line couplers, and branch-line, branch-guide, and rat-race couplers. Waveguide aperture couplers, depend on the electromagnetic properties of one or more apertures that cut into the common wall between two waveguides. Included in this class are the Bethe-hole, the multihole, the Riblet short-slot, the Schwinger reversed-phase, and the Moreno cross-guide couplers. Branch-line and branch-guide couplers are formed by coupling two main transmission lines together with two or more quarter-wavelength-long transmission lines, spaced by quarter-wavelengths along the coupled lines. The chapter concludes with the development of the design of basic microwave directional couplers.