Return Loss - Sign

Sign

Properly, loss quantities, when expressed in decibels, should be positive numbers. However, return loss has historically been expressed as a negative number, and this convention is still widely found in the literature.

Taking the ratio of reflected to incident power results in a negative sign for return loss;

where RL'(dB) is the negative of RL(dB).

Return loss is identical to the magnitude of Γ when expressed in decibels but of opposite sign. That is, return loss with a negative sign is more properly called reflection coefficient. The S-parameter S₁₁ from two-port network theory is frequently also called return loss, but is actually equal to Γ.

Caution is required when discussing increasing or decreasing return loss since these terms strictly have the opposite meaning when return loss is defined as a negative quantity.