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".