Reference distance

In broadcast engineering, the reference distance is the distance which, under normal circumstances and flat terrain, a radio station would reach with a particular level of signal strength. This distance depends on two factors: effective radiated power (ERP) and height above average terrain (HAAT). The actual distance a station's signal travels depends highly on weather, where factors like temperature inversions and heavy precipitation have a strong and highly variable influence on radio propagation. However, for purposes of broadcast law such as construction permits and broadcast licenses, fixed calculations called propagation curves are applied to determine the reference distances for all existing and proposed stations. These also take into account beam tilt, carrier frequency, and even the Earth's curvature at longer distances.