What is a Directional Coupler?

A directional coupler is a 4-port device that is used to sample a small amount of input signal power for measurement purposes. As seen in the diagram below, Port 1 is the input port, port 2 is the output port, port 3 is the coupled port and port 4 is the isolated/terminated port.

When an input signal travels from port 1 to port 2, a part of this signal is coupled to port 3. The portion of the power coupled to port 3 depends on the coupling value of the coupler being used. For example if we use a 3 dB coupler, the power split between port 2 and port 3 would be 50%, however, if we use a 10 dB coupler, then this power split would be 9:1. The coupling is one of the key parameters on the basis of which a directional coupler is selected. Typical coupling values vary from 6 dB to over 30 dB. Another important specification of a directional coupler is directivity (Click here to learn more about Directivity).

Port 4 of the directional coupler is known as the isolated port. In an ideal directional coupler, no signal should appear at the isolated port, however practically, a small amount of power called back power is obtained at Port 4.

The directional coupler as the name suggests operates in a single direction i.e its ports cannot be interchanged. There are some other types of coupler configurations like Bi-Directional Couplers and Dual-Directional Couplers whose ports can be interchanged.

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