Basics of RF Directional Couplers in Various Applications

Directional couplers are typically utilized in RF design applications. Directional couplers, which are passive devices, are utilized to couple a certain proportion of power that travels in one transmission line and out through another connection. Directional couplers are used for a variety of applications in RF design, including transmitter automatic levels controls and through-line power sensors. They are highly useful, and are capable of allowing power levels to be sensed in the absence of making a direct connection to the transmission line that carries the power.

Basics of RF Directional Couplers

RF directional couplers are four-port devices that are generally named as the following:

  • Port 1 – Input
  • Port 2 – Transmitted
  • Port 3 – Coupled
  • Port 4 – Isolated

In general, the main line is between port 1 and port 2. Typically, this is more suitable for carrying high power levels and it might contain bigger RF connectors. The other ports are generally suitable for lower powers, since their intention is just to carry a minuscule proportion of the main line power. Port 3 and port 4 might have even smaller connectors to decipher them from the RF coupler’s main line ports. Many times a matched load of about 50 ohms terminates the isolated port.Though certain ports are labeled on a device, this is typically a physical constraint since certain ports will be made to carry higher powers compared to others. In actuality, any port may be the input, which will result in the directly-connected port acting as the transmitted port, the diagonal port acting as the isolated port, and the adjacent port acting as the coupled port.

Characteristics of Directional Couplers

The traits of directional couplers are vast, and include the following:

  • Output signals are unequal in amplitude. The bigger signal is at the main-line output port, while the smaller signal is at the coupled port
  • Main-line insertion loss is dependent on the coupled port’s signal level, which is determined by design
  • Between the coupled port and the output of the main-line is high isolation
  • Flat coupling over broad bandwidth
  • Low main-line loss
  • High directivity
  • Wide array of coupling values

A number of techniques can be implemented with RF directional couplers, such as coaxial feeder, stripline, and lumped or discrete elements. They can also be housed in a number of packages fromblocks, or they can be housed on a substrate carrier. In addition, they can also be made as a part of a bigger unit with other functions.