What Are Directional Couplers?

Directional couplers are circuits with four ports:

  • Input
  • Through
  • Coupled
  • Isolated

The input port is isolated from the other. Ideally, all four of the ports are matched, and the circuit is lossless. Directional couplers are used to sample a signal, often both the reflected and incident waves (such an application is known as a reflectometer, which is a critical component of a network analyzer).

Generally speaking, directional couplers utilize the distributed elements of microwave circuits. The coupling process typically happens within one-quarter of a wavelength or many quarter-wavelength portions of the device. The energy and fields of one structure mingle with those of another distributed structure in these distributed couplers, which couple signals from one to the other.

Hybrid couplers may also be used, as these couplers don’t depend on the fields and waves interacting among transmission structures to generate performance similar to coupled line structures. Instead, they rely on networks that are made up of un-coupled circuit elements.

On certain commercial-grade couplers, the isolated port is terminated internally or is externally with a matched load, which makes four-port devices seem somewhat like three-port devices. Should the signal reverse so it enters the “through” port, the majority of it exits the “input” port.

In the reverse coupler, energy that moves down the transmission line begins a reverse wave down the second transmission line. Single-section coupled lines of transmission are always considered backward-wave couplers. Asymmetric multi-section coupled structures offer in-phase, forward-wave response, while symmetric multi-section couplers offer quadrature, backward-wave, response.

The majority of waveguide couplers couple in the forward direction because they depend on many coupling holes. Instead, stripline or microstrip couplers are backward wave couplers since they depend on coupled lines. The coupled port on a microstrip directional coupler is nearest to the input port because it’s a backward wave coupler. The coupled port is nearest to the output port on a waveguide broadwall directional coupler because it’s a forward wave coupler.

The following are definitions of some directional couplers:

Bethe-hole coupler – These are waveguide directional couplers that use one hole, and work over a narrow band. It is a reverse coupler, which is different than the majority of waveguide couplers that are forward couplers and utilize multi-hole.

Multi-hole coupler – In a waveguide coupler, two waveguides share a broad wall and the holes are one-quarter of a wave apart. The coupling factor is controlled by the size of the hole, which is often X-shaped. It’s possible to have flat coupling over a whole waveguide band.

Hybrid coupler – These couplers are special in that a 3 dB split is ideal between the through and coupled paths. There are two variations of hybrid couplers: 90-and 180-degree hybrids.

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