Return loss is the name for the reflection from a Device Under Test, or DUT. As a specific example, an RF signal is launched into a transmission line connected to the DUT. If the input impedance of the DUT is not equal to the characteristic impedance of the transmission line, there will be a reflection. Reflected energy is not available to pass into the DUT and is returned to the source and lost. The impedance mismatch determines the reflection coefficient (see above), and the return loss may be calculated for it.

Return loss is normally expressed in dB and as a positive number. If a VNA measurement of the DUT is performed with Port 1 connected to the DUT input, then S₁₁, as displayed in Log-Mag mode, would be the return loss. S₁₁ for a passive device will always be negative but, by convention, return loss is expressed as a positive number. If S₁₁= -20 dB, then the return loss would be expressed as 20 dB.

See this and more important radio frequency charts and formulas in this comprehensive application note.