S21 also represents the gain of an amplifier with the input connected to
Port 1 and the output connected to Port 2 (often through attenuators). The
gain can be calculated based on all the s-parameters of the circuit,
including S12, but is predominantly related to S21.

I have used the NanoVNA to adjust several VHF preamps.

Zack W9SZ

On Wed, Aug 25, 2021 at 10:50 PM Roland Turner via groups.io <roland=
rolandturner.com@groups.io> wrote:

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


------------------------------------------------------------------------


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