# Dictionary:Line of sight

The straight-line distance from an object to an observer. High-frequency (short-wavelength) radio waves travel such raypaths so that the curvature of the Earth limits the range that can be achieved. Line-of-sight range *R* is given in terms of the height of transmitting and receiving antennas *h*_{t} and *h*_{r}:

The constant *k* is 1.22 nautical miles when *h* is given in feet, 4.08 km when in meters. Refraction increases the effective range of radio waves depending on their wavelength; the sensitivity of the detecting system also affects the range; see *radio earth*.