In telecommunications, return loss is the loss of signal power resulting from the reflection caused at a discontinuity in a transmission line or optical fiber. This discontinuity can be a mismatch with the terminating load or with a device inserted in the line. It is usually expressed as a ratio in decibels (dB);

Failed to parse (Missing texvc executable; please see math/README to configure.): RL(\mathrm{dB}) = 10 \log_{10} {P_\mathrm i \over P_\mathrm r}

where RL(dB) is the return loss in dB, P_i is the incident power and P_r is the reflected power.

Return loss is related to both standing wave ratio (SWR) and reflection coefficient (Γ). Increasing return loss corresponds to lower SWR. Return loss is a measure of how well devices or lines are matched. A match is good if the return loss is high. A high return loss is desirable and results in a lower insertion loss.

Return loss is used in modern practice in preference to SWR because it has better resolution for small values of reflected wave.^[1]

Sign[link]

Properly, loss quantities, when expressed in decibels, should be positive numbers.^{[note 1]} However, return loss has historically been expressed as a negative number, and this convention is still widely found in the literature.^[1]

Taking the ratio of reflected to incident power results in a negative sign for return loss;

Failed to parse (Missing texvc executable; please see math/README to configure.): RL'(\mathrm{dB}) = 10 \log_{10} {P_\mathrm r \over P_\mathrm i}

where RL'(dB) is the negative of RL(dB).

Return loss is identical to the magnitude of Γ when expressed in decibels but of opposite sign. That is, return loss with a negative sign is more properly called reflection coefficient.^[1] The S-parameter S₁₁ from two-port network theory is frequently also called return loss,^[2] but is actually equal to Γ.

Caution is required when discussing increasing or decreasing return loss since these terms strictly have the opposite meaning when return loss is defined as a negative quantity.

Electrical[link]

In metallic conductor systems, reflections of a signal traveling down a conductor can occur at a discontinuity or impedance mismatch. The ratio of the amplitude of the reflected wave V_r to the amplitude of the incident wave V_i is known as the reflection coefficient Failed to parse (Missing texvc executable; please see math/README to configure.): \Gamma .

Failed to parse (Missing texvc executable; please see math/README to configure.): \mathit \Gamma = {V_\mathrm r \over V_\mathrm i}

When the source and load impedances are known values, the reflection coefficient is given by

Failed to parse (Missing texvc executable; please see math/README to configure.): \mathit \Gamma = { {Z_\mathrm L - Z_\mathrm S} \over {Z_\mathrm L + Z_\mathrm S} }

where Z_S is the impedance toward the source and Z_L is the impedance toward the load.

Return loss is the negative of the magnitude of the reflection coefficient in dB. Since power is proportional to the square of the voltage, return loss is given by,

Failed to parse (Missing texvc executable; please see math/README to configure.): RL(\mathrm{dB}) = -20 \log_{10} \left| \mathit \Gamma \right|

where the vertical bars indicate magnitude. Thus, a large positive return loss indicates the reflected power is small relative to the incident power, which indicates good impedance match from source to load.

When the actual transmitted (incident) power and the reflected power are known (i.e. through measurements and/or calculations), then the return loss in dB can be calculated as the difference between the incident power P_i (in dBm) and the reflected power P_r (in dBm),

Failed to parse (Missing texvc executable; please see math/README to configure.): RL(\mathrm{dB}) = P_\mathrm i(\mathrm{dBm}) - P_\mathrm r(\mathrm{dBm})\,

Optical[link]

In an optical fiber, the loss that takes place at any discontinuity of refractive index, especially at an air-glass interface such as a fiber endface, at which a fraction of the optical signal is reflected back toward the source. This reflection phenomenon is also called "Fresnel reflection loss," or simply "Fresnel loss."

Fiber optic transmission systems use lasers to transmit signals over optical fiber, and a high optical return loss (ORL) can cause the laser to stop transmitting correctly. The measurement of ORL is becoming more important in the characterization of optical networks as the use of wavelength-division multiplexing increases. These systems use lasers that have a lower tolerance for ORL, and introduce elements into the network that are located in close proximity to the laser.

Failed to parse (Missing texvc executable; please see math/README to configure.): ORL(\mathrm{dB}) = 10 \log_{10} {P_\mathrm i \over P_\mathrm r}

where Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle P_\mathrm r

is the reflected power and Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle P_\mathrm i
is the incident, or input, power.

See also[link]

• Hybrid balance
• Signal reflection
• Time-domain reflectometer
  • Optical time domain reflectometer
• Mismatch loss

Notes[link]

References[link]