RF directional coupler basics tutorial

RF directional couplers are electronic items that couple a defined amount of the electromagnetic power in a transmission line to a port enabling the signal to be used in another circuit.

One key feature of directional couplers is that they only couple power flowing in one direction. In this way, power entering the output port is coupled to the isolated port but not to the coupled port.

RF directional couplers can be implemented using a variety of techniques including stripline, coaxial feeder and lumped or discrete elements. They may also be contained within a variety of packages from blocks with RF connectors, or solder pins, or they may be contained on a substrate carrier, or they may be constructed as part of a larger unit containing other functions.

RF directional coupler basics

An RF directional coupler has four ports and these are normally defined as in the list below:

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

Terms in brackets refer to alternative names for the ports that may be seen on occasions- they give a little more explanation of the function of the coupler port than just the name.

Typically the main line is the one between ports 1 and 2. Normally this may be more suited to carry high power levels and it may have larger RF connectors, if it is a unit with RF connectors.

The other ports on the directional coupler are normally more suited for lower powers as they are only intended to carry a small proportion of the main line power. Ports 3 and 4 may even have smaller connectors to distinguish them from the main line ports of the RF coupler. Often the isolated port is terminated with an internal or external matched load which would typically be 50 ohms.

Four port directional coupler circuit symbol — Four port directional coupler symbol

While specific ports are given labels on a device, this is normally more of a physical constraint as some ports will be manufactured to carry higher powers than others. In fact any port can be the input, and this will result in the directly connected port being the transmitted port, the adjacent port being the coupled port, and the diagonal port being the isolated port.

RF directional coupler specifications

As with any component or system, there are several specifications associated with RF directional couplers. The major RF directional coupler specifications are summarised in the table below.

TERMINOLOGY / TERMINOLOGY	DESCRIPTION OF DIRECTIONAL COUPLER SPECIFICATION
Main line loss	Resistive loss due to heating (separate from coupling loss). This value is added to the theoretical reduction in power that is transferred to the coupled and isolated ports (coupling loss).
Coupling Loss	Amount of power lost to the coupled port (3) and to the isolated port (4). Assuming a reasonable directivity, the power transferred unintentionally to the isolated port will be negligible compared to that transferred intentionally to coupled port.
Isolation	Power level difference between Port 1 and Port 4 (related to directivity).
Directivity	Power level difference between Port 3 and Port 4 (related to isolation). This is a measure of how independent the coupled and isolated ports are. Because it is impossible to build a perfect coupler, there will always be some amount of unintended coupling between all the signal paths.

Couplers are not as widely used as some RF components like attenuators, mixers and the like, but in areas closer to the antenna where power may need to be sampled, they are widely used. For example a directional coupler may be used within a meter detecting the standing wave ratio, but they also have many other applications.