What is the Difference between a 3-Port Coupler and 4-Port Coupler?

A directional coupler is usually a 4-port device with an Input Power, Coupled Port, Output Port and Isolated Port. In a 3-Port Coupler, the isolated port is terminated internally. The performance (directivity) of a 4-port directional coupler can be impacted by the quality of impedance match (termination) at the isolated port. As a result of this, 3-Port couplers where the isolated port is internally terminated can have more consistent and better performance.

In a 3-port coupler, the directivity can be calculated by measuring the loss from the main-line input to the coupled port with the main-line output terminated. The process is then reversed, i.e the loss from the main-line output to the coupled port is measured with the input port terminated. The difference in the two readings will give the directivity in dB.