Glossary  >  Directional Coupler

Directional Coupler


A directional coupler is a passive device that can only couple power flowing in one direction.

They are sometimes used in radio technology to sample a small amount of microwave power for measurement purposes. They consist of a 4-port waveguide junction comprising a primary main waveguide and a secondary auxiliary waveguide.

Directional Coupler

Diagram of a directional coupler

Directional couplers are made from a pair of coupled transmission lines aligned such that energy passing through one is coupled to the other.

One of the four ports in a directional coupler is isolated from the input port. But ideally, all four ports are matched, and the circuit is lossless.

The performance of a directional coupler is defined through the following parameters:

  • Coupling Factor: the input power ratio P1, delivered to P3.
  • Directivity: the ratio of forward power to back power, that is, the directional coupler’s capacity to separate waves in both directions at ports P3 and P4.
  • Isolation: the ratio of incident power delivered to the uncoupled load (P4). This defines the directive properties of the directional coupler.
  • Insertion Loss: the input power (P1) delivered to the transmitted port (P2), which is reduced by power delivered to the coupled (P3) and isolated (P4) ports.

All these parameters are measured in decibels (dB).

Directional couplers have many applications, including:

  • Providing signal samples for measurement
  • Combining feeds to and from antennas
  • Providing taps for cable-distributed systems
  • Antenna beamforming

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