Strip-line 3-db directional couplers

This paper describes the performance of two basic types of strip-line 3-db directional couplers. One coupler is one-quarter wavelength long at the center of its frequency band, and has a coupling variation of db over a 2:1 frequency band. The other directional coupler is three-quarter wavelengths long at the center of its frequency band and has the same coupling variation of db over a 4.5:1 band. Theoretically, both types of couplers have infinite directivity and unity VSWR at all frequencies. Experimental models of these directional couplers have been found to have good directivity, low VSWR and coupling variations almost exactly predicted by theory.