I have a NanoVNA V2 Plus4. I started replacing the included calibration standards with a decent quality ones (Radiall, Huber&Suhner, Spinner, R&S) without going down the VNA metrology rabbit hole. Joeqsmith has very educational YouTube videos on NanoVNA and calibration standards where he explains the necessities of good standards in order to make comparable and meaningful measurements.
From my experience...
- Paying 10 bucks for a (2nd hand market) Huber&Suhner 50 Ohm SMA terminator doesn't hurt much and it has some tighter specs like 50 Ohm +/- 1% over a wide frequency range
- I measured the resistance of an unbranded 50 Ohm BNC terminator which was within 50 Ohm +/- 2% spec. Unfortunately, it was equipped with a NTC resistor so temperature changes (+/- 5K) affected the resistance significantly. A comparison to a Huber&Suhner 50 Ohm BNC showed a stable resistance over a temperature range of approx. +/- 5 K
- One should stick to one cable/connector "ecosystem" if possible. Mating cheap connectors with quality ones may result in damage and performance deterioration of the quality equipment
- You can protect the NanoVNA port connectors by using short pieces of SMA cables between DUT and NanoVNA. Performing calibrations puts some stresses on the connectors so it's better to sacrifice a replaceable cable than risking port wear-out (although they can be replaced...)
- NanoVNA V2 Plus4 works fine for me. I was able to perform simple measurements such as SWR/Transmission/Reflection/Impedance without difficulties. It's a very useful addition to the lab