What is a Dual Directional Coupler?

A regular directional coupler is a 4 port network, similar to what is 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. The power passes from the input port (port 1) to the output port (port 2) in transmission line 1. A portion of the power is coupled with transmission line 2, the amount of power coupled depends on the coupling value of the specific coupler. This coupled power is the output of port 3 (called the coupled port). Port 4 is usually terminated to 50 ohms (or to match the system impedance) to minimize any reflections within the system.

In a bi-directional coupler, the input and output ports are interchangeable, in the case where the port 1 is the input port, it works like a directional coupler as explained above. In the case where port 2 is the input and port 1 is the output, the coupled port and the isolated port switch i.e port 4 becomes the coupled port and port 3 becomes the isolated port. In this type of coupler, the directivity of the coupler depends on how well the isolated port is terminated. So if it is not terminated well, then there will be more reflection and will result in a lower directivity.

A dual directional coupler, can be thought of as two directional couplers placed back to back, with both the isolated ports of each coupler terminated to 50 ohms (or the system impedance). The outer ports i.e ports 3 and 4 can be connected to any mismatched load without affecting the directivity or the other coupled port. This type of coupler can also be used to simultaneously monitor both the forward and reverse power of a system.

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