Return Loss is a term frequently encountered in literature and discussions on transmission lines, antennas, and networks.
(IEEE 1988) defines Return Loss as:
(1) (data transmission) (A) At a discontinuity in a transmission system the difference between the power incident upon the discontinuity. (B) The ratio in decibels of the power incident upon the discontinuity to the power reflected from the discontinuity. Note: This ratio is also the square of the reciprocal to the magnitude of the reflection coefficient. (C) More broadly, the return loss is a measure of the dissimilarity between two impedances, being equal to the number of decibels that corresponds to the scalar value of the reciprocal of the reflection coefficient, and hence being expressed by the following formula:
where Z1 and Z2 = the two impedances.
(2) (or gain) (waveguide). The ratio of incident to reflected power at a reference plane of a network.
Return Loss expressed in dB will ALWAYS be a positive number in passive networks.
(Bird 2009a, b) raises his concern as Editor-in-Chief of IEEE Transactions at the growing misuse of Return Loss. It is perhaps caused by practitioners who think that Return loss is equivalent to S11 displayed on VNAs, but in fact Return Loss is the reciprocal of |S11| so that while |S11| expressed in dB is negative, Return Loss expressed in dB is positive.
So, when you see a manufacturer, seller or author talking about Return Loss as a negative dB quantity, you might properly consider their credibility compromised. Is it correct because it is commonly done? No, only in the ham world perhaps. What other terms have they redefined?