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In telecommunication, 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:
Source: from Federal Standard 1037C and from MIL-STD-188
