, return loss
is the ratio, at the junction of a transmission line
and a terminating impedance
or other discontinuity, of the amplitude
of the reflected wave to the amplitude of the incident wave. The return loss value describes the reduction in the amplitude of the reflected energy
, as compared to the forward energy. For example, if a device has 15dB
of return loss, the reflected energy from that device is always 15dB lower than the energy presented. For all devices that are not perfect transmission lines, or purely resistive loads (perfect black-bodies), the return loss value varies with frequency
Note 1: Return loss is usually expressed in dB.
Note 2: Return loss is a measure of the dissimilarity between impedances in metallic transmission lines and loads, or between refractive indices in dielectric media, e.g optical fibers.
Note 3: In a metallic transmission line, return loss is given by the following equation, where Z1 is the impedance toward the source and Z2 is the impedance toward the load, and the vertical bars indicate magnitude:
For dielectric media, e.g. optical fibers, see reflection loss (def #2).
Source: from Federal Standard 1037C and from MIL-STD-188