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In an electronic communications system, return loss is the portion of a signal that is reflected back to the transmitter. Return loss is usually caused by imperfections or impedance mismatches within the cables and cable connections. Any energy reflected back in this way reduces the overall power of the transmitted signal and can lead to problems with signal reception and clarity.
The most basic communication system consists of a cable with a transmitter and one end and a receiver at the other. A signal sent by the transmitter travels down the cable and is processed by the receiver. Even a system as basic as this has some amount of return loss. Just as a voice traveling down a tunnel will generate an echo that leads to distortion, so a signal traveling through a cable will echo and create return loss.
Return loss is caused by imperfections in the wiring of the system. These imperfections can be introduced at any time during the manufacturing or installation process. Imperfections in wiring lead to non-uniform impedance, meaning that the cable may create more or less resistance to the flow of signal at one physical point than at another. Parts of the signal can essentially get stuck and reflect back toward the transmitter if they are unable to overcome the resistance at any given point in the circuit. Individual issues might easily be ignored, but can add up over the length of the system to cause major return losses.
Non-uniform impedance can be caused by improper installation. If the wires are not trimmed correctly, if the cable is compressed in a way it was not designed to be, or if cable ends are not properly installed, impedance issues may result. Mismatched components can also cause return loss if the impedance of one component is much higher or lower than the impedance of another component in the system.
Return loss has become more of a problem as telecommunications technology has improved. Many communications systems now use bidirectional signaling, so signals are no longer simply sent from a transmitter to a receiver. More often transceivers, which both transmit and receive signals, are used.
In such systems, the problem is magnified. The echo not only affects the signal received at a single point, but all of the data in the entire system. A transceiver might have sent a signal and be listening for the response when echoes of its own signal are reflected back to it.