[G0HZU_JMR]

I've improved the drawing to show the mainline path and which port is the coupled port. See below. Often, the REFL port is just terminated in a 50R load and left alone.

I used to use a 10:1 turns ratio to get a 20dB coupler and used this a lot when repairing CB or ham radios. This allows the radio to be used into an antenna or a dummy load and the coupled port can be fed to a scope terminated in 50R. This then shows 1/10th the voltage on the scope.

You can also use this coupler to measure return loss and with a dual channel scope (or a vector voltmeter) you can measure the reflection coefficient (magnitude and angle) to then work out the complex impedance of a load. However, you do have to add a phase offset for this coupler type. Otherwise it gets the angle of the reflection coefficient wrong.