Reply To: Isolated ADM2582E link works without a ground wire. How, and should I trust it?

Makes sense. Measured it: GND2 to GND2 drifted around 0.6 V on the bench, so plenty of margin there and none to count on outdoors. Ordered cable with an extra pair, reference through 100 ohm at each end, shield earthed at the gateway end. Thanks Ciaran, and Tyler for the supply capacitance hint.