Hello Andy, You could mount the probe at various heights above the car and see how the measured values agree with the calculated values. The bottom of the car forms a capacitor to ground. My guess is
I have a calibrated E Field antenna (Procomm AAC-1) with a quoted antenna-factor of 0.15V into 50 ohms when placed in a field of 1V/m, This is mounted in the centre of a car roof and fed via a bulk