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With regards to RF directional couplers, I'm trying to understand the precise usage/definition of the "directivity" specification especially with its relationship to "coupling factor".

The "coupling factor" is seemingly straightforward. A 20dB coupling factor for a device means the coupled port will output a signal that is 20dB lower than the signal presented at the device's input port. (This would be useful for, say, monitoring the characteristics of the input signal)

The "directivity" seems to be an indication of how much signal will come out of that same coupled port relative to signals present at the device's output port. So a device with a 20dB directivity spec means that the coupled port would have a signal 20dB down from any signal coming into the output port.

These example specs are from http://www.minicircuits.com/pdfs/ZFDC-20-33+.pdf. Note that the device's "directivity" spec and "coupling factor" spec are nominally identical.

So just how "directional" is a coupler that allows the same relative signal (-20dB) at the coupled port? It seems not "very".

-Mike

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Directional coupler from here: -

enter image description here

Coupling Loss - Amount of power lost to the coupled port (3) and to the isolated port (4). Assuming a reasonable directivity, the power transferred unintentionally to the isolated port will be negligible compared to that transferred intentionally to coupled port.

Main line loss - Resistive loss due to heating (separate from coupling loss). This value is added to the theoretical reduction in power that is transferred to the coupled and isolated ports (coupling loss).

Directivity - Power level difference between Port 3 and Port 4 (related to isolation). This is a measure of how independent the coupled and isolated ports are. Because it is impossible to build a perfect coupler, there will always be some amount of unintended coupling between all the signal paths.

Isolation - Power level difference between Port 1 and Port 4 (related to directivity).

So, if the coupled port (3) receives -20 dB of the power from the input port (1), the power fed to the isolated port (4) would be another 20 dB lower.

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  • \$\begingroup\$ I'm not interested in the "Isolated port" at all (many couplers do not even provide that on a connector, instead having an internal termination). From here: minicircuits.com/app/COUP7-2.pdf - "Directivity - The difference in dB of the power output at a coupled port, when power is transmitted in the desired direction, to the power output at the same coupled port when the same amount of power is transmitted in the opposite direction. " \$\endgroup\$ – mike65535 Oct 28 '16 at 15:20
  • \$\begingroup\$ The isolated port is internal to your device and if a reflected wave came back from the load it would internally couple to the isolated port at 20 dB down and be therefore 40 dB down at the coupled port. Hence it's a directional coupler - forward signals couple at -20 dB and reflected signals couple only at -40 dB. \$\endgroup\$ – Andy aka Oct 28 '16 at 15:25
  • \$\begingroup\$ Andy, Got it. That's the answer I was hoping for. Seems all the online definitions were lacking in clarity (at least to me) and now that I've (re)read the the Minicircuits description for the n-th time, I can see what was wrong with my interpretation. (Essentially, the coupling factor and directivity "stack" in my scenario) Thanks! \$\endgroup\$ – mike65535 Oct 28 '16 at 15:55
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That is a reasonable interpretation, but more detail is provided in this application note https://www.minicircuits.com/app/COUP7-2.pdf which defines directivity thus:

The difference in dB of the power output at a coupled port, when power is transmitted in the desired direction, to the power output at the same coupled port when the same amount of power is transmitted in the opposite direction.

So S21 is -20db, S23 is -40dB in your example.

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