Below is a link to a paper showing the effects of frequency and Earth conductivity on the propagation of groundwaves in the AM broadcast band.
The study confirms the physical reality that for long propagation paths and some sets of conditions, an AM broadcast station radiating 1 kW can produce greater groundwave field intensities than a 50 kW station.
Link
The study confirms the physical reality that for long propagation paths and some sets of conditions, an AM broadcast station radiating 1 kW can produce greater groundwave field intensities than a 50 kW station.
Link