I found these notes i5tdj Piero sk
measure of derittivity
a source rf is connected to the input of the coupler at the desired frequency and to the output a load 50 hom that has very low ros (at least 30 db reyurn loss). From the secondary line will come out a saw proportional to the power going from the input to the load (since the load is very good, the power reflected, from the load to the input is negligible). This level is measured. The coupler is reversed, connecting the RF source and the load input to its output. low of the previous one. The relationship between the two signals expressed in db and the derittivity of the coupler
have a 436a power meter with a head of a minimum of -30 dbm + 20 dbm