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
This is in turn used to define most broadcast classes for FM stations in North America. Each class (except D) is defined as having a maximum ERP and HAAT. If the HAAT of a station's radio antenna exceeds that specified for the class, it must reduce ERP so that its signal does not exceed the reference distance. The signal strength used differs by class, but generally the value is 1.0mV/m (millivolt per meter) or 60dBu (decibels over one microvolt per meter) for most of the United States, and 0.5mV/m or 54dBu in Canada, and for some U.S. stations in parts of certain areas including California, the Great Lakes region, and the Northeast. This is considered the service contour of a station by the Federal Communications Commission (FCC) and the Canadian Radio-television and Telecommunications Commission (CRTC), and by COFETEL in Mexico, according to NARBA and other international agreements among the three. The reference distances are in turn used to create mandatory minimum spacing distances among co-channel stations, and certain adjacent channels as well.