Application Note: RF Directional Couplers And 3dB Hybrids Overview

Couplers and hybrids are devices in which two transmission lines pass close enough to each other for energy propagating on one line to couple to the other line. A 3dB 90° or 180° hybrid splits an input signal into two equal amplitude outputs. A directional coupler normally splits an input signal into two unequal amplitude outputs. This terminology "directional coupler", "90° hybrid", and "180° hybrid" is based on convention. However, the 90° and 180° hybrids could be thought of as 3 dB directional couplers. Despite these similarities, the parameters used to describe signal flow in directional couplers and the application, in actual use, is sufficiently different to warrant separate considerations.