It is a term that we can find in electrical characteristics of a cable or connector.
RL is a measure of the reflected energy from a transmitted signal and is expressed in “dB”, the higher the value, the better. The reflections are caused by impedance mismatch caused by connectors, improper installation such as stretching the cable or too sharp a bend radius, improper manufacturing, or improper load.
Broadcasters are very familiar with return loss, calling it by a different name, SWR (standing wave ratio) or VSWR (voltage standing wave ratio). In fact, return loss measurements can easily be converted into VSWR values, or vice versa. Return loss can be found with the equation:
RL= 20 log Difference/Sum
Difference= is the difference (absolute value) between the desired impedance and the actual measured impedance.
Sum= is the desired impedance and the actual measured impedance added together.
The desired impedance for all UTP data cables 5E, 6 6A… is 100 Ω.
The desired impedance for all passive video, HD, SD, 3G-SDI is 75 Ω.
The desired impedance for all digital audio twisted pair cable is 110 Ω.
The desired impedance for all digital audio on coaxial cable is 75 Ω.
With 1000Base-T systems, the pairs simultaneously transmit and receive. As the transmitter sends data, it is also listening for data being sent from the opposite end of the same pair. Any reflected signal from the sending end that reflects back to the sending end mixes with the sending signal from the far end, reducing intelligibility.
With 10Base-T or 100Base-T data networks, one pair transmits while another receives, so reflections (RL, return loss) are not a major consideration and were not required to be measured. Now, with pairs simultaneously transmitting and receiving, called duplex mode, RL is a critical measurement for data applications.
The return loss value required by the SMPTE 424M standard for connectors and cable is > 10 dB at 3 GHz.
Handbook for sound engineers. Fourth edition. Glen Ballou. Focal Press.
SMPTE 424M-2006 SMPTE STANDARD, for Television- 3Gb/s Signal/Data Serial Interface