FM broadcasting
VHF Radio waves do not travel far beyond the visual horizon, so reception distances for FM stations are usually limited to 30–40 miles (50–65 km) They can also be blocked by hills or buildings.
Reference distance
Real-world calculations can also be done by including data from digital topographical maps, typically along 12 or 16 radials, and including the specifications from the radiation pattern for a directional antenna. However, this does not fit the definition of a reference in this sense, even though it determines the actual broadcast range of a station. This actual range is used in the reserved band in the U.S., while the reference distance is used for commercial radio stations.