I do a 'loss' measurement rather than an impedance measurement. To do this, do a 'through' measurement (S21), either on a single wire, or both wires together, connected to the center pins of the nanovna leads - either measurement method should show you the same results. The ground (shield) of the leads to the nanovna should be connected together. A good choke should give >20dB of S21 loss across the frequency range of interest - the more loss (attenuation) the better.
