RF directional coupler basics tutorial

- overview, information and tutorial about the basics of the directional coupler and basic directional coupler design.

RF directional couplers are often used in RF design applications. Directional couplers are RF passive devices used to couple a specific proportion of the power travelling in one transmission line out through another connection or port. Directional couplers find many applications in RF design , ranging from through line power sensors to transmitter automatic levels controls. As such they are particularly useful, enabling power levels to be sensed without making a direct connection to the transmission line carrying the power.

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 is a four port device. The four ports are generally termed:

  • 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.

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 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.

RF Directional Coupler

RF Directional Coupler

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.



Term
 

Description
 
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.
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).
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.
Isolation Power level difference between Port 1 and Port 4 (related to directivity).


RF directional coupler practical considerations

there are a number of physical considerations that need to be taken into account when choosing and using RF directional couplers.

By Ian Poole


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