Wim, PA3DJS wrote:
"Of course you don`t know how much power is dissipated in nearby
structures (buildings, etc)."

Yes. Standard broadcast field strengths are measured at distances along
radial paths begining at one mile from the station to ensure far field
determination. Conductivity of the earth affects received signal
strength. Over sea water, the millivolts per meter decline inversely
with distance. Over the earth, decline is more rapid depending on
conductivity. Some sites along the radials are unacceptable due to
proximity of conducting structures. The more measurements along a
radial, the better. 25 sites per radial is about the minimum. In the
USA, the FCC has published Groundwave Field vs Distance Charts, and made
them a part of its rules. One can try to fit his measured data to the
FCC curves. Groundwaves is what broadcasting is all about at these
frequencies.

Best regards, Richard Harrison, KB5WZI