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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On 26/8/21 4:36 am, Mike C. wrote:

Hi Don,

I posted it last night. And yes I'm now (yesterday) measuring from S11
to S21. The attenuators are mounted on S21 and connect to a short jumper
that had the SOL made through a female SMA connector back to the VNA.
See: http://www.mgte.com/vna/   for detail.

Mike C.


On 8/25/2021 2:26 PM, Donald S Brant Jr wrote:

Mike:  You ARE looking at S21 and not S11, right?  What does a thru

look like?

73, Don N2VGU