It is the loss of power in the signal returned/reflected by a discontinuity in a transmission line or optical fiber.
Return loss is the difference in power (expressed in dB) between the incident power and the power reflected back by the load due to a mismatch. It can be measured directly in dB with a Spectrum Analyzer or Network Analyzer along with a few key components. VSWR is defined as the ratio of the maximum voltage to the minimum voltage in the standing wave. The larger the impedance mismatch, the larger the amplitude of the standing wave. A perfect impedance match would cause no voltage standing wave, so the ratio of the maximum voltage to the minimum would be 1 (1:1). VSWR stands for Voltage Standing Wave Ratio, and is also referred to as Standing Wave Ratio (SWR). VSWR is a function of the reflection coefficient, which describes the power reflected from the antenna. If the reflection coefficient is given by, then the VSWR is defined by the following formula: The reflection coefficient is also known as s11 or return loss. See the vswr table below to see a numerical mapping between reflected power, s11 and VSWR. VSWR is the ratio of the peak amplitude of a standing wave to the minimum amplitude of a standing wave, as seen in the following Figure: Figure 1. Voltage Measured Along a Transmission Line. 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 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, where the vertical bars indicate magnitude. Definition of Return Loss. In technical terms, RL is the ratio of the light reflected back from a device under test, P out, to the light launched into that device, P in, usually expressed as a negative number in dB. RL = 10 log 10 (P out /P in).  Impedance mismatches result in standing waves along the transmission line, and SWR is defined as the ratio of the partial standing wave 's amplitude at an antinode (maximum) to the amplitude at a node (minimum) along the line. The term power standing wave ratio (PSWR) is sometimes referred to, and defined as the square of the voltage standing wave ratio. The term is widely cited as misleading.. In the words of Gridley: The reflection coefficient is a voltage ratio and must be squared to be used for power calculations. It is sometimes easier to think of reflected power in terms of reflection coefficient than in return loss. Reflected power is equal to the incident power multiplied by the reflection coefficient squared. VSWR is defined as the ratio of the maximum voltage to the minimum voltage in the standing wave. The larger the impedance mismatch, the larger the amplitude of the standing wave. A perfect impedance match would cause no voltage standing wave, so the ratio of the maximum voltage to the minimum would be 1 (1:1). Return Loss is generated by any change in impedance of a cable (due to changes in physical dimensions, or type of insulation), or for that matter, even coaxial cable connectors used in the cable system, not to mention the actual termination provided by the terminal equipment. An example of this condition occurs when there is a nearby lightning strike strong enough to burn out or change the value of terminating resistors. This can decrease the Return Loss enough in that cable section to induce data errors into the system. By definition, Return Loss refers to that portion of a signal that cannot be absorbed by the end of line termination, or cannot cross an impedance change at some point in the transmission system. An example of this condition occurs when there is a nearby lightning strike strong enough to burn out or change the value of terminating resistors. This can decrease the Return Loss enough in that cable section to induce data errors into the system. In telecommunications, return loss is the loss of power in the signal returned/reflected by 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. 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, where the vertical bars indicate magnitude.
http://www.vtimicrowave.com/File/mw-vswr-def.html http://www.antenna-theory.com/definitions/vswr.php https://en.wikipedia.org/wiki/Return_loss http://lunainc.com/return-loss-measure-it/ https://en.wikipedia.org/wiki/VSWR http://www.vtimicrowave.com/File/mw-vswr-def.html http://www.fmsystems-inc.com/manuals/RETURN%20LOSSart.pdf http://www.fmsystems-inc.com/manuals/RETURN%20LOSSart.pdf https://en.wikipedia.org/wiki/Return_loss
return loss definition