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Yet, to discover that it’s the limited antenna height which causes the uphill limit, brings us to the same problem, i.e., “AM will not go uphill if….”
The shape of the elevation pattern of an electrically short, 3-meter monopole in the medium wave band does not differ much at all from that of a monopole that is a physical 1/4-wave in height.
So it isn’t the height of the radiator that is responsible for the described effect.
At the low radiated power from a Part 15 AM station a hill might cause a noticeable reduction in the groundwave field past the top of that hill. But that is due to obstruction loss, not because AM will not go uphill.
Receivers past the hilltop are in a “radio shadow zone” caused by the hill.
If AM receivers are located on the slope of hill having a line of sight path to the transmit antenna and still can’t receive it within a few thousand feet, then something beside the 3-m height of a §15.219 transmit monopole system is accounting for that.
