If the cable is damaged, the signal will reflect at the damaged point prior to being absorbed. As signal energy travels down a cable, some of the signal energy is lost due to the resistance of the cable. This is known as cable loss. If the transmitted signal energy reaches an impedance discontinuity, some or all of the energy is reflected back up the cable. The ratio of energy transmitted to reflected is known as return loss.
Return loss is a way of measuring impedance changes in a cable.
Understanding dBRL is sometimes confusing. This is due to the fact that a large dBRL number means the reflection or fault is small and vise versa. A large return loss means that most of the transmitted pulse was lost instead of being returned as a reflection. The signal simply continued down the cable or was absorbed by a termination or load on the cable. A small return loss means that most of the transmitted pulse was reflected or returned due to an impedance change caused by a fault or the end of the cable.