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I have found in my experience that AM signals do not want to go uphill from the antenna.
Not to question such an experience, but could the reason(s) for this experience be mis-diagnosed?
Regardless of their radiated powers, the electrically short monopoles used by Part 15 AM operators radiate about 71% of their peak field at an elevation angle of 45 degrees above the horizon that they radiate in the horizontal plane.
At elevation angles less than 45 degrees they radiate closer to 100% of their maximum field — which maximum field always occurs in the horizontal plane.
In most cases within a few thousand feet from such a transmit system operating optimally and legally under §15.219, a ~29% reduction in received field would not be highly significant to most AM broadcast receivers and listeners — unless the peak r-f power radiated by such a system was sub-optimal.
Such sub-optimal operation could be the result of several factors including low output power from the transmitter, and/or a non-resonant antenna system, and/or high resistive losses in the loading coil and r-f ground connection in use.