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:
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 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.
What is plotted and labelled Return Loss is actually the magnitude of S11 or |S11|.
What is RP
The parameter RP had me guessing… was it equivalent parallel resistance component? It does look like it may be the phase of S11… Return Phase? That’s a new term.
If we take the reported Z at the marker to be 61.0-j3.0, we can calculate various other values.
We can see above that |S11| is 0.1027 which in dB is -19.77dB. The phase of S11 is -13.7° which reconciles with the minVNA RP parameter at the marker.
It does look like the miniVNA world is one of an alternate nomenclature and alternate meanings to the mainstream.
Little wonder hams learn bad practice from instruments that should be an example of what is right rather than what is wrong.
PS: the same software plots TL (Transmission Loss?) though a passive device as negative… wrong sign again, probably caused by not understanding S parameters and the fundamental meaning of Loss, then labelling |S21| as TL.
Links / References
- IEEE. 1988. IEEE standard dictionary of electrical and electronic terms, IEEE Press, 4th Edition, 1988.