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Ionospheric propagation is normally limited to the HF bands, below 30 MHz. Occasional exceptions to that occur on 6 meters, and even more rarely on 2 meters, both of which are VHF bands. It almost never happens at UHF and above.
The reason for that is that the energy contained in a radio wave increases with frequency. Planck's equation describes this.
When the energy in the wave is higher than the ionosphere can refract for a given level of ionization, it passes through the ionosphere rather than being refracted back to earth.
Generally speaking VHF, UHF, and higher frequency radio waves possess too much energy for the ionosphere to refract.
The curvature of the earth does little to hinder the propagation of electromagnetic waves, unless the altitude of a particular geographic feature literally "gets in the way of line of sight".
Radio waves usually will not propagate through dirt, and they are often blocked my man-made structures. Especially those made of metal, or reinforced by metal (think rebar in concrete).
Structures made of non-conductive materials, such as a wood-sided house, have little effect.