Directional coupler
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In telecommunications, a directional coupler is a transmission coupling device for separately sampling (through a known coupling loss) either the forward (incident) or the backward (reflected) wave in a transmission line.
A directional coupler may be used to sample either a forward or backward wave in a transmission line. A unidirectional coupler has available terminals or connections for sampling only one direction of transmission; a bidirectional coupler has available terminals for sampling both directions.
A simple example of a directional coupler is the Bethe coupler between two similar rectangular waveguides, one carrying a TE-10 wave: The waveguides are set long side against long side and a hole is opened to the common wall. Typically either a round hole is placed in the middle and one of the guides is turned about the axis of the hole to control directivity, or the guides are rigidly attached and the distance of the hole from the short wall determines the directivity. The amount of coupled power is determined by the radius of the hole.