What type of propagation is responsible for allowing over-the-horizon VHF and UHF communications to ranges of approximately 300 miles on a regular basis?
Why Tropospheric ducting happens in the lowest few thousand feet of the atmosphere, where a temperature inversion (warm air over cooler, moist air) creates a sharp change in refractive index. VHF and UHF signals get trapped between that boundary and the ground or sea surface and travel along the duct with little loss, routinely reaching 300 miles or more. It is most common over water and along coastlines, especially in stable high-pressure weather.
Watch out F2 region refraction is an ionospheric mode that mainly works at HF and only rarely reaches into the low VHF range at solar maximum, and the D region absorbs rather than refracts at these frequencies. Faraday rotation is a twisting of signal polarization, usually discussed with satellite paths, not a propagation range extender.
Troposphere = weather. Weather inversions build VHF/UHF ducts good for about 300 miles.