Return loss, also known as reflection loss, is a measure of the fraction of power that is not delivered by a source to a load. If the power incident on a load is Pi and the power reflected by the load is Pr, then the return loss in decibels is
The better the load is matched to the source, the lower the reflected power and hence the higher the return loss. RL is a positive quantity if the reflected power is less than the incident power. If the load has a complex reflection coefficient ρ, then
That is, the return loss is the negative of the input reflection coefficient expressed in decibels.
When generalized to a terminated two ports, the return loss is defined with respect to the input reflection coefficient of
the terminated two port. The two port has the input reflection coefficient
where ΓL is the reflection coefficient of the load. Thus the return loss of a terminated two-port is
If the load is matched, i.e. ZL=Z∗0 (the system reference impedance), then
This return loss is also called the input return loss since the reflection coefficient is calculated at Port 1. The output return loss is calculated looking into Port 2 of the two-port, where now the termination at Port 1 is just the source impedance.