The NanoVNA compensates for the port impedance when making a shunt measurement. But port impedances that differ from 50Ω degrade series measurement accuracy since there is no compensation.
Rudy Severns, N6LF, measured the port impedances of a NanoVNA-H4 rev. 4.3 to 100 MHz with an Array Solutions VNA2180. He used an HP 908A 50Ω load for VNA2180 calibration.
Port impedances with the VNA powered. The port 1 spike is due to the VNA output signal.
Shunting port 1 with 412Ω 1%, either internally or externally, makes HF resistance close to 50Ω.
If the small HF error matters, shunt port 2 with 5.1kΩ 5%.
The equivalent parallel capacitance is 1.9 pF across port 1 and 1.4 pF across port 2.
Circuit coupling capacitors degrade port reactance below 1 MHz.
88–108 MHz