In a closed thread of this forum, on Dec 10, 2017 "pratto" was interested in an answer to this topic for a 50 kW transmitter.
At a distance of 1 km, the surface-wave field intensity at a horizontal distance of 1 km from a λ/4 vertical monopole antenna system such as used by typical AM broadcast stations is about 2 volts/meter.
If all of that field was collected by a receive antenna, the received power would be about 53 milliwatts -- which is about 17.2 dB greater than 1 mW.
At a distance of 1 km, the surface-wave field intensity at a horizontal distance of 1 km from a λ/4 vertical monopole antenna system such as used by typical AM broadcast stations is about 2 volts/meter.
If all of that field was collected by a receive antenna, the received power would be about 53 milliwatts -- which is about 17.2 dB greater than 1 mW.