Why are simplex UHF signals rarely heard beyond their radio horizon?
They are too weak to go very far
FCC regulations prohibit them from going more than 50 miles
UHF signals are usually not propagated by the ionosphere✓Bonne réponse
UHF signals are absorbed by the ionospheric D region
Explication
UHF signals are too high in frequency for the ionized upper atmosphere to bend them back to earth, so they normally pass straight through into space. That leaves line-of-sight propagation, which stops at the radio horizon; it is a matter of physics rather than power or regulation.