I was browsing a ham forum recently when I came across a Return Loss plot apparently from a ham grade miniVNA Tiny.

Lets just remind ourselves of the meaning of the term Return Loss. (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:

20*log

_{10}|(Z_{1}+Z_{2})/(Z_{1}-Z_{2})| decibelwhere Z

_{1}and Z_{2}= 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 wrt a real reference impedance will ALWAYS be a positive number in passive networks.

## Return Loss according to the miniVNA Tiny

Above, the miniVNA Tiny presents Return Loss as a negative value. Continue reading The sign of Return Loss