A characteristic of certain radio signals that causes them to act in the same way as light. These signals travel in straight lines and cannot bend around obstructions. Very-high-frequency (VHF) and ultra-high-frequency (UHF) communications are line-of-sight transmissions which require a clear path between the transmitting and receiving antennas. The reception distance of VHF and UHF signals is dependent upon the altitude of the aircraft above the transmitting antenna.