Following two graphs show the actual electrical properties of 3dB Coupler Line (CL-358/3) and 20dB Coupler Line (CL358/20) at 1GHz center frequency.
The insertion loss of Coupler Line is deducting total output power from input power, which is less than 0.3dB.
CL-358/3 coupling chart
CL-358/20 coupling chart
※A click of a picture looks at an enlargement.