This article continues on from NanoVNA – how accurate does the LOAD need to be – part 1?
Measures such as ReturnLoss, Gamma, s11, ro |s11|, VSWR are all wrt some reference impedance, often, but not necessarily 50+j0Ω.
The process of SOL calibration of a VNA, and its subsequent correction of DUT measurements, means that raw measurements are corrected with error terms that are derived from the calibration measurements and a set of responses expected of the calibration parts.
Those responses may be ideal responses; a simple short circuit (sc), a simple open circuit (oc) and an ideal load of some nominal value (say 50+j0Ω).
More sophisticated calibration may use a more accurate model for each of the SOL components, but NanoVNA uses simple ideal models for the SOL parts, including that L is 50+j0Ω.
So, if you use for example a 55Ω resistor for LOAD, then the correction factors are calculated to correct the raw measurement for a 55+j0Ω DUT to have \(s_{11}= \frac1{\infty}+\jmath 0\) (though noise will result is a lesser value, perhaps around 1e-40), and it its further computation assumes that s11 is wrt 50Ω, so it will render that DUT impedance as 50+j0Ω, ReturnLoss in dB as a very large number (perhaps 80dB).
If you did connect a DUT that was exactly 50+j0Ω, it will ‘correct’ the raw measurement to s11=-0.04762+j0, and it will render that DUT impedance as 45.45+j0Ω, ReturnLoss in dB as 26.4dB.
Saying it renders ReturnLoss as 26.4dB for an ideal 50+j0Ω DUT is equivalent to saying it has a Directivity (wrt 50Ω) of 26.4dB.
We can return to Calculate uncertainty of ReturnLoss and VSWR given coupler directivity and calculate the uncertainty of that system when it indicates for an unknown DUT, a ReturnLoss of say 21dB (equivalent to VSWR=1.2).
So for DUT with displayed ReturnLoss=21dB, actual ReturnLoss wrt 50Ω could be anywhere from 17.3dB to 27.7dB… quite a range!
So, to obtain a reasonably low uncertainty range, the Directivity of the instrument (the ReturnLoss of the LOAD calibration part) needs to be perhaps 10dB better than the DUT… use the calculator to try different instrument Directivity values and DUT measured ReturnLoss to find a combination with acceptable uncertainty.
So if you did want to measure ReturnLoss with uncertainty of less than 3dB, you might find the following works.
So, minimum coupler directivity is 32dB, you will need a LOAD calibration part that has ReturnLoss>32dB, equivalent to VSWR<1.05. That is a pretty practical component, and will not be outrageously expensive.
An exercise for the reader: what LOAD part ReturnLoss is needed to measure ReturnLoss of 30dB (VSWR=1.065) with less than 3dB uncertainty.
So, whilst you might see |s11| displays on your NanoVNA showing -30dB, they have very wide uncertainty unless the L calibration part is one of high accuracy.