Hi Mike,

I suspect that you've misunderstood the terms S11 and S21. They're not ports, they're characteristics of a DUT (specifically the scattering parameters <https://en.wikipedia.org/wiki/Scattering_parameters>) that the instrument can be used to measure. Roughly speaking:

* S21 is the fraction of the power injected at port 1 that is present
at port 2. This is typically used for examining two-port filters, of
which attenuators are a special case. Formally what is measured is
gain, but the usual parameter of interest is insertion loss.
* S11 is the fraction of the power injected at port 1 that is
reflected at port 1. This is typically used for examining things
like terminators and antennas which only have one port. Formally
what is measured is the reflection coefficient, but the usual
parameter of interest is either of return loss or VSWR.
* Similarly for S12 and S22 with respect to power injected at port 2,
however NanoVNA designs don't inject power at port 2. If you wish to
measure these parameters, you need to swap the connectors.

To make matters just slightly more confusing, most NanoVNA designs use "programmer numbering" (perhaps because of port numbers in the underlying firmware/OS?) and therefore label the first port as port 0 and the second as port 1, meaning that S21 is the fraction and phase angle of the power injected at port 1 that is present at port 0, and analogously for S11.

- Roland 9V1RT


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