NanoVNA is a unidirectional two port VNA. It makes corrected measurements of S11 and S21 of a two port DUT wrt two reference planes:
- Port 1; and
- Port 2.
The location of the reference planes is determined by the calibration process and saved data that is subsequently used to correct raw measurements. Note that the reference plane can be displaced from the calibration plane by using the e-delay feature.
The “Ports” are not connectors on the VNA (even if the labels might suggest that), they are the boundaries of the DUT.
For the purpose of this discussion about a unidirectional VNA, let’s call the VNA port that transmits signal the TX port, and the one that receives signal the RX port.
Example: setup to measure SMA(M)-SMA(F) attenuator
Above is the instrument setup. The instrument is reflection (SOL) calibrated using the parts shown on the upper SMA(F)-SMA(F) adapter which defines the DUT attachment as Port 1. Thru (T) calibration is done with the lower SMA(M) connected to the upper SMA(F)-SMA(F) adapter, so defining that SMA(M) DUT attachment as Port 2.
Measuring S22 and S12
As mentioned, the NanoVNA is a unidirectional two port VNA and measures only S11 and S21, the saved .s2p file will have zeros in the S22 and S12 columns. To measure S22 and S12 you must reverse the DUT, sweep using exactly the same frequency scan, and save this second set of measurement data which superficially appears to be measurement of S11 and S21, but you know that due to the reversal the measured / saved S11 and S21 values are actually S22 and S12. The NanoVNA has no way of knowing the DUT is reversed, no way of knowing that the DUT’s Port 2 is now known to the NanoVNA as Port 1.
You must then manually merge the two datasets. Some client software provide tools for this merge, it is a common operation. Client software may even coordinate the two sweeps and merge the results, though I have not yet seen that for NanoVNA.
Conclusions
Port 1 and Port 2 define the boundaries of the DUT, they are properties of the DUT rather than a connector on the VNA. Port 1 and Port 2 ‘belong’ to the DUT, not the VNA or fixture.
If your VNA connector is labelled Port 2, think of it as “from Port 2” or “cable Port 2 to here”.
If you cable the DUT Port 2 to the TX connector of the NanoVNA and DUT Port 1 to the RX port of the NanoVNA, you will need to reinterpret the NanoVNA measurements (recorded S11 is actually S22, recorded S21 is actually S12), note the transposition of the subscripted indexes.