Directional Coupler Characteristics: A Short Guide

Knowing Directional Coupler Characteristics Helps

AWG Tech has over 1000 models of directional couplers. The couplers can be broadly categorized by type, frequency or coupling factor.

Types of directional coupler

We have the following types of directional coupler:

Frequency range of directional coupler

A directional coupler can be narrowband or broadband.

Narrowband couplers can further be classified according to the operational frequency:

Wideband directional coupler typically operates over one or more octaves, such as:

  • 1-4 GHz directional coupler
  • 2-8 GHz directional coupler
  • 1-6 GHz directional coupler
  • 1-18 GHz directional coupler

Coupling factor of directional coupler

Common coupling factors are:

Directional coupler: What is it?

A directional coupler is a four-port circuit where one of the port is secluded from the port that serves as the input, and directional couplers are also passive reciprocal networks. Every one of the four ports in this component is (perfectly) paired, and the circuit is (ideally) lossless. In addition, these can be realized in microstrip, stripline, coax and in waveguides.

They’re utilized for testing frequency signals, oftentimes both the incident and reflected waves (this application is referred to as the reflectometer, which is a crucial part of a network analyzer). Other directional coupler characteristics include its general utilization of the distributed properties of microwave circuits. The coupling system usually happens within a quarter-wavelength or on different quarter-wavelength portions of the device.

In these distributed couplers, the energy and fields of a single structure will interact with the energy and fields of another distributed structure, pairing frequency signals with one another. Lumped element and hybrid couplers can be manufactured as well.

In addition, these couplers don’t need the field interactions of the fields and waves between the transmission structures to generate a performance just like that of a coupled line structures; however, they instead rely on independent networks made up of un-coupled circuit aspects.

What does the “Directional” in Directional Couplers Mean?

As stated above, a directional coupler is an RF and microwave component that’s made up of four ports, where one of the four is referred to as the input, the other as the “through” port (where most of the incident signals make their exit). Adding up, the “coupled” port (where a fixed fraction of the input frequencies will appear, typically expressed in dB) and the other one to be labelled as the “isolated” port, which is the one that is typically eliminated.

On other commercialized couplers, the fourth port is internally terminated or externally with a matched load – thus making a four port device look like a three port device instead. If the frequencies are reversed so that it enters inside the “through” port, most of it will go out through the “input” port; however, the coupled port is now the port that was previously referred as the “isolated port”.

Below is a simplified version of the list of the four ports of the directional coupler:

  • Port 1 – The Input Port
  • Port 2 – The Output Port
  • Port 3 – The Coupled Signal Port
  • Port 4 – The Isolated Port

The power that passes through from port 1 to port 2 is paired with a partnered line and then exits through port 3, which is isolated from the main output port (that being port 4). Any form of reflected power from the main output is then partnered to the port that is eliminated, thus resulting into minimal reflections within the device/system. The power level of the paired frequencies is a parameter to be chosen depending on the requirements.

These are just some of the basic directional coupler characteristics one needs to know first before truly knowing how they work on every circuits. Without knowing the fundamentals, one wouldn’t know how this component would work.

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