Physically, the reflection coefficient seen at port is due to an impedance step from , that of the port interface, to a new impedance consisting of the parallel combination of all other port impedances meeting at the junction. Let
Let's check this ``physical'' derivation against the formal definition Eq.(F.17) leading to in Eq.(F.19). Toward this goal, let
denote the parallel combination of all admittances connected to the junction. Then by Eq.(F.21), we have for all . Now, from Eq.(F.14),
and the result is verified.