Scattering parameter say, S11, can be defined as the ratio of the reflected wave from port 1 to the incident wave on port 1. If it is a 2-port network, we imply the condition that the load at port 2 is matched to the characteristic impedance of the line. Take this excerpt from another website: "If the output port 2 is terminated, that is, the transmission line is connected to a matched load impedance giving rise to no reflections, then there is no input wave on port 2." My confusion it the "no input wave on port 2" part. A matched load impedance at port 2 will give rise to no reflections. Fine, I believe that. That can be seen from the definition of the reflection coefficient, Gamma. However, there is no input wave on port 2? If there is no incident wave on port 2 to begin with, then why does the load impedance need to be matched? There is no danger of reflecting anything anyway, so why match the load (at port-2) to the line?