Last month the 'Engineering Radio' blog posted an interesting AM field strength experiment he undertook and the results thereof. While he's not dealing Part 15 it it seems the same applies, but probably doesn't due to the power involved (I don't know).
https://www.engineeringradio.us/blog/2025/10/am-field-strength-measurements/
I have been finishing up a project which required detuning a new monopole installed near an AM tower. One requirement was a series of field strength measurements along six evenly spaced radials around the AM tower.
The point is to see if there is any effect in the omni-directional AM signal (there should not be). For this, I used the venerable Potomac Instruments FIM-41. . ... used Google Maps to generate a set of points along each radial then noted the coordinates and .. ..
Field strength readings follow the inverse square law. Whatever the increase in the distance factor from the radiator, the electrical field will decrease inversely by the square of that factor. Thus, if the distance increases by 3, the field will decrease by a factor of 9.
This can be seen in a field strength vs distance graph which I plotted on an Excel spreadsheet:
https://www.engineeringradio.us/blog/2025/10/am-field-strength-measurements/