The Decibel

The usual method of microwave power measurement is by sampling a portion of power incident on a device like a directional coupler that taps off a very small portion (perhaps one thousandth) while passing the rest.

The picture is a manufacturer's depiction of a technique of using a directional coupler to sample a portion of the microwave power from a source, and also a portion of the power that is reflected back from the termination.