Directional couplers

Updated September 27, 2009

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Directional couplers are four-port circuits where one port is isolated from the input port. Directional couplers are passive reciprocal networks, which you can read more about on our page on basic network theory. All four ports are (ideally) matched, and the circuit is (ideally) lossless. Directional couplers can be realized in microstrip, stripline, coax and waveguide. They are used for sampling a signal, sometimes both the incident and reflected waves (this application is called a reflectometer, which is an important part of a network analyzer). Directional couplers generally use distributed properties of microwave circuits, the coupling feature is generally a quarter (or multiple) quarter-wavelengths. Lumped element couplers can be constructed as well.

What do we mean by "directional"? A directional coupler has four ports, where one is regarded as the input, one is regarded as the "through" port (where most of the incident signal exits), one is regarded as the coupled port (where a fixed fraction of the input signal appears, usually expressed in dB), and an isolated port, which is usually terminated. If the signal is reversed so that it enter the "though" port, most of it exits the "input" port, but the coupled port is now the port that was previously regarded as the "isolated port". The coupled port is a function of which port is the incident port.

Looking at the generic directional coupler schematic below, if port 1 is the incident port, port 2 is the transmitted port (because it is connected with a straight line). Either port 3 or port 4 is the coupled port, and the other is the isolated port, depending on whether the coupling mode is forward or backward. How do you know which one is which? We'll talk about that in a second...

Definitions

Let's first look at some definitions using S-parameters. Let port 1 be the input port, port 2 be the "through" port. For a backward wave coupler, port 4 is the coupled port and port 3 is the isolated port. Ideally, power into port 1 will only appear at ports 2 and 4, with no power at port 3, but in real couplers some power leaks to port 3. For an incident signal at port 1 of power P1 (and output powers P2, P3 and P4 at ports 2, 3 and 4), then:

Insertion Loss (IL) = 10*log(P1/P2)=-20*log(S21)

Coupling Factor (CF) = 10*log(P1/P4)=-20*log(S41)

Isolation (I) = 10*log(P1/P3) = -20*log(S31)

Directivity (D) = 10*log(P4/P3)=-20*log(S31/S41)

Note that these numbers are positive in dB. Quite often, microwave engineers present these quantities as negative numbers, it is not a great faux pas, just look at the magnitude, Dude!

Note that directivity requires two, two-port S-parameter measurements, the other quantities require only one. Directivity is the ratio of isolation to coupling factor. In decibels, isolation is equal to coupling factor plus directivity.

Please send us any comments on the preceding statements, we are operating under a state of partial dyslexia and there is a possibility that we slipped up on a minus sign!

Forward versus backward wave couplers

Waveguide couplers couple in the forward direction (forward-wave couplers); a signal incident on port 1 will couple to port 3 (port 4 is isolated). Microstrip or stripline coupler are "backward wave" couplers. In the schematic above, that means for a signal incident on port 1, port 4 is the coupled port (port 3 is isolated).

Coupler rule of thumb

The coupled port on a microstrip or stripline directional coupler is closest to the input port because it is a backward wave coupler. On a waveguide broadwall directional coupler, the coupled port is closest to the output port because it is a forward wave coupler.

The Narda coupler below is made in stripline (you have to cut it apart to know that, but just trust us), which means it is a backward wave coupler. The input port is on the right, and the port facing up is the coupled port(the opposite port is terminated with that weird cone-shaped thingy which voids the warrantee if you remove it. Luckily Narda usually prints an arrow on the coupler to show how to use it, but the arrow is on the side that is hidden in the photo.

On the waveguide coupler below, the input is on the left, while the coupled port is on the right, pointing toward your left ear. There is a termination built into the guide opposite the coupled port, although you can't see it.

Bethe-hole coupler

This is a waveguide directional coupler, using a single hole, and is works over a narrow band. The two guides are configured to (sorry we need to finish this section!!!)

In waveguide, a two-hole coupler, two waveguides share a broad wall. Holes are 1/4 wave apart. In the foreword case the coupled signals add, in the reverse they subtract (180 apart) and disappear. Coupling factor is controlled by hole size. The "holes" are often x-shaped, and...

Bi-directional coupler
A directional coupler where the isolated port is not internally terminated. You can use such a coupler to form a reflectometer, but it is not recommended (use the dual-directional coupler you cheapskate!)

Dual-directional coupler
Here we have two couplers in series, in opposing directions, with the isolated ports internally terminated. This component is the basis for the reflectometer.

Hybrid couplers

A hybrid coupler is a special case, where a 3 dB split is desired between the through path and the coupled path. There are two types of hybrid couplers, 90 degree couplers (such as Langes or branchlines) and 180 degree hybrids (such as rat-races and magic tees). We have a separate page on this topic, click here!

Reflectometer

This is the component that allows you to measure S-parameter magnitudes using a network analyzer.

A directional coupler only does what it is supposed to if it sees a matched impedance at all four ports.

Errors due to finite directivity

Directivity can cause errors if load is not matched. 40 dB directivity will have a very small error, 20 dB may be unacceptable accuracy.