Return loss

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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:

$L_r = 20 \log_{10} \left| { {Z_1 + Z_2} \over {Z_1 - Z_2} } \right|$

Note 4: For dielectric media, e.g. optical fibers, see reflection loss (def #2).

Note 5: Return loss will always be a loss, and therefore negative dB, however by convention it is often reported as a positive number. Therefore, the above equation gives large positive values for very small losses, and very small positive values for very large losses.

[edit] See also

The equation for return loss has a typing error. Z₅ in the denominator should read Z₂.Iqbal Syed 13:46, 11 November 2006 (UTC)

Other sources give the return loss = -20log(reflection coefficient).
Where the reflection coefficient = ZL - Z0 / ZL + Z0, the inverse of what is shown here.