What mode is responsible for allowing over-the-horizon VHF and UHF communications to ranges of approximately 300 miles on a regular basis?

The mode that enables over-the-horizon VHF and UHF communications to distances of around 300 miles on a regular basis is tropospheric scatter. This phenomenon occurs in the lower part of the atmosphere and involves the scattering of radio waves by irregularities in the atmospheric layers.

Tropospheric scatter works well for VHF and UHF frequencies because these signals can be effectively scattered by particles and temperature variations in the troposphere. Due to the nature of this scattering, signals can bypass the line-of-sight limitations that typically affect VHF and UHF communications, allowing for reliable transmission over extended distances that would otherwise be impossible under normal conditions.

Other mechanisms, such as D layer refraction and F2 layer refraction, involve different layers of the atmosphere and are generally associated with higher frequency communications and specific conditions. Faraday rotation, on the other hand, relates to the polarization of radio waves and is not primarily responsible for enhancing the range of VHF and UHF communications in the same manner as tropospheric scatter.