A pivotal component in a network analyzer, a directional coupler is used to test the incident and reflected waves of RF frequency signals. Comprised of four ports, each port is carefully paired to minimize any losses. These passive RF units take a small amount of energy from the primary transmission and redirect it to the other ports. The level of isolation between components can affect the performance of the coupler. Here are the three basic types of directional couplers.
Standard Directional Couplers
This is the most basic form of RF directional coupler. It has one coupled port with a port that is internally terminated. This is useful for measuring power in a single direction. If the reverse of power needs to be measured, the component must be physically reoriented in the flow.
As a four-port component, a bi-directional coupler can measure both directions of power simultaneously. With no terminated ports, this part does not need to be transposed to monitor reverse or forward power. One important thing to note, though, is that the loads on the coupled ports must be matched at 50 Ohms. As long as the input has the intrinsic impedance of its transmission line, the energy should have minimal reflections. However, if the port is mismatched or open, the power can be reflected back into the main transmission path causing overheating and failure of the component.
Dual Directional Couplers
Comprised of two standard directional couplers, the two coupled ports – for monitoring the forward and reverse power—are each individually terminated in this component. This makes a distinction between the forward or reverse ports. This is particularly beneficial in reflectometer measurements and, unlike bi-directional couplers, is not dependent on matching the power loads on the coupled ports.
All three of these types of directional couplers are used to monitor power injection or the flow of power through a system. It is a tiny but integral part of most microwave and semiconductor industries.