Schematic, not to scale, with the Earth's curvature exaggerated. A VHF transmitter mast on the left sends a signal along a straight line to a light aircraft. As the aircraft moves further away the straight path is first blocked by a hill and then by the curve of the Earth; the shaded area behind them is out of line of sight. A low cloud on the path does not block the signal. A higher receiver keeps line of sight to a greater distance. This illustrates the principle only; it does not predict real radio coverage.
