RF directional couplers are passive devices that couple power travelling through the device to another port enabling the signal to be used in another circuit.RF directional couplers can be implemented using a variety of techniques including stripline, coaxial feeder and lumped or discrete elements.
Directional couplers can be realized in microstrip, stripline, coax and waveguide. They are used for sampling a signal, sometimes both the incident and reflected waves which application is called a reflectometer, and it is an important part of a network analyzer. Directional couplers generally use the distributed properties of microwave circuits. A directional coupler consists essentially of two coupled transmission line designed to couple a certain fraction of energy from one line to the other and in a direction dependent on the direction of power flow. Directional couplers find important application in power monitoring, power splitting and in microwave measurement systems. Well-designed couplers typically have directivities in excess of 30 dB so that the ration of powers coupled in wanted to unwanted directions is usually better than 1000 in such devices.