- noun a line along which direct rays from a radio frequency transmitter become tangential to the Earth’s surface
- A horizon as seen from an antenna transmitting site. Such a horizon is the furthest distance that a direct wave may travel. It is the line that is formed by the points at which the direct waves from a radio transmitter become tangential to the surface of the earth. Due to atmospheric refraction, this horizon extends beyond the apparent horizon. Also called horizon (2).