Antennas located on high structures can “look” over the horizon and “see” the receiving antennas.
Because refraction is involved, direct waves travel 20% farther than light waves due to scattering of
radio waves by the environment. Trees and other foliage are invisible to HF radio waves. Direct wave
propagation is possible at all frequencies, but this mode of propagation is seldom used on our high
frequency bands, but it is the usual propagation mode used by repeaters and others on VHF and UHF. If
you watch TV on an outside antenna or on a “rabbit ears antenna,” you are receiving the signal by
direct wave propagation.