Return loss is the ratio, expressed in decibels, of the fractional amount of signal reflection caused by an impedance mismatch (eg: at the input to a transformer with respect to its source impedance).

General Return Loss Equation:
Return Loss = 20 log₁₀dB where:
Z_R= the reference impedance
Z_M= the measured impedance

Expanded:
Return Loss = 20 log₁₀dB where:
R<_R= reference or generator series resistance
X_R<= reference or generator series reactance
R_R= equipment under test series resistance
X_M= equipment under test series reactance

Reflections in a long communication circuit may result in echoes of the transmitted signal. If the reflections are severe, this may cause the transmission path to go into a sustained oscillatory condition referred to as singing. When reflections impair voice circuits they may sound like phantom talkers on the line. Slower speech on voice facilities, and reduced data rates on digital facilities are the result of even moderate reflections. This reflection phenomena is particularly aggravated by the very long transmission distances associated with geosynchronous satellite connections.

Return loss figures are a function of the impedances of the circuits involved and are therefore frequency dependent. The Figure at right shows the impedance plot of a simplified transformer model. At high frequencies the high magnetizing inductive reactance causes the network to look essentially like a resistor and results in a high (good) return loss figure.

As the frequency is decreased, the network reactance becomes more inductive and the return loss figure degrades. Using impedance correcting techniques, it is sometimes possible to improve the return loss of the transformer network by adding proper values of compensating capacitance.