What is a Bi-Directional Coupler?

A bi-directional coupler is a type of RF coupler where the input and output ports are interchangeable. When port 1 is the input port, it works like a regular directional coupler. 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 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.