@John. Interesting. That 2.5Mhz graph is still exponential. I guess that's how ground waves behave.
Out in free space (i.e., no ground), they would probably behave similarly to FM frequencies.
Am I mistaken? I thought this discussion was based on part 15 fm
@richpowers When taking measurements it is best to do it not over concrete or indoors with a floor or walls or objects. Outside over earth ground/grass, AM or FM, is most accurate as ground absorbs, doesn't reflect RF like concrete or pavement. Then you get just direct through space readings. In the test lab that I was at the testing room walls and ceiling are covered in RF absorbing coverings, this room has a special name, anechoic chamber, and the floor is wood. So is the table the transmitter is on.