Directional Coupler Application Note from TRM Microwave

Directional Couplers are used to provide a simple, convenient, accurate means for sampling and monitoring incident and reflected power, as sampling devices in control loops and in a wide variety of signal processing applications.

Directional Couplers are passive, matched, reciprocal and essentially lossless devices except for incidental, unavoidable dissipation. By construction, these are devices in which two transmission lines are positioned such that energy propagating on one line can couple over to the other. Couplers are commonly used for sampling a single direction of power which is flowing through a transmission line.