my NanoVNA has S21 dynamic range at about 70 dB. Below 300 MHz it is usable down to 90 dB with some noise.
From 0 dB to 50 dB it has almost straight line with attenuators. 60 dB has a little deviations. 70 dB has more deviation. 80 dB a little noisy.
The problem with CH1 port is that it has not so good VSWR. It drops down to 42 ohms at 450 MHz and to 40 ohm at 900 MHz.
