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 15
dB 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.
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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. Note 1: Return
loss is usually expressed in
dB. (
188 ) 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 where Z 1 is the impedance toward the source and Z 2 is the impedance toward the
load, and the vertical bars indicate magnitude. Note 4: For dielectric media, e.g, optical fibers, see
reflection loss (def #2).