RADIO HORIZON

The locus of points at which direct rays from an antenna are tangential to the surface of the Earth. ( 188 ) Note: If the Earth were a perfect sphere and there were no atmospheric anomalies, the radio horizon would be a circle. In practice, the distance to the radio horizon is affected by the height of the transmitting antenna, the height of the receiving antenna, atmospheric conditions, and the presence of obstructions, e.g., mountains.