Directional Couplers Definition: The Operational Description

Directional couplers function as a four-part network, although one of the ports may be internally terminated with a load. Directionality is usually reliant on internal symmetries and partnerships.

In a four-port configuration, one of the ports is secluded at another port; energy is then partnered from the referenced port. The energy balance, after losses in coupling, leaves the remaining port referred to as the “mainline”.

As the name implies, a directional coupler tests the energy based on the energy flow’s direction.

Directional Couplers: The Basics

As mentioned above, a directional coupler is a four-port device. The four of them are generally termed:

• Input (Incident, Port 1)
• Transmitted (Output, Port 2)
• Coupled (Forward Coupled Port, Port 3)
• Isolated (Reverse Coupled Port, Port 4)

Take note that the terms in parentheses refer to the alternative names given for each port.

Basically, the main line is the one in between ports 1 and 2. This may be more suited to carry high power levels and may have bigger RF connectors, if it’s a unit that comes with RF connectors.

Meanwhile the other ports are suited for lower powers as their main purpose permits them to carry only a small portion of the main power.

Ports 3 and 4 could have smaller connectors to differentiate them from the main port lines of the coupler. Regarding the above stated that one of the ports may be internally discharged, this is due to a matched load which would usually amount to 50 ohms.

Although the ports are labeled on the device, this is more of a physical constraint as there are ports that’ll be made to hold greater power than the rest. As a matter of fact, any port can be the input.

This will result in the directly connected port to be the transmitted port, the neighboring port being the coupled port, and the diagonal port transforming into the isolated port.