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?

VHF and UHF signals normally travel via line-of-sight and easily pass right through the ionosphere, which typically limits their range. However, certain weather conditions in the troposphere (the lowest layer of the atmosphere) can trap these signals and guide them along the Earth's curvature. This process, called tropospheric ducting, effectively acts like a waveguide or fiber-optic cable for radio waves. It enables reliable over-the-horizon communication on VHF and UHF bands at ranges of 300 miles or more. The D and F2 regions of the ionosphere are primarily responsible for bouncing or refracting lower-frequency HF (shortwave) signals. They do not regularly return VHF and UHF signals to Earth in this manner.