The 60 dBu contour is a line on a map that connects points where an FM radio signal is received at 60 decibels on an open circuit. A more common definition of the contour would be the line on the map where a reliable signal can be received by an ordinary antennae and radio. The area encompassed by the 60dBu contour is important to the licensing process for the radio station.
The 60 dBu contour defines the service area for an FM radio station. The 70 dBu defines the area with what is considered a “city grade” signal from the FM station. Federal Communications Commission maps also include 54 or 57 dBu contours that indicate areas where the FM signal is not consistently receivable but may interfere with other stations.
Licensing requirements for full power FM state that at least half of the city of license must be included within the 60 dBu contour, according to the FCC website. Some stations choose to license to a city on the outskirts or suburbs of a major city in order to avoid having to construct a tower and transmitter system that reaches at least half of a major city.
Calculating 60 dBu
The distance an FM radio signal carries is dependent on the height of the tower and the power output of the transmitter. The FCC provides a calculator on its website to calculate the Effective Radiated Power (ERP) of a station based on these variables. This can be used to draw a contour based on a flat terrain. In reality, the ground terrain effects how the signal carries. The 60 dBu contour map is usually developed by engineers as part of the license application process.
The 60 dBu contour also defines the area that is considered protected from interference from other radio stations by the FCC. The FCC defines a minimum buffer zone between the 60 dBu contours of FM radio stations. This buffer zone varies from 17 to 57 miles depending on the class of the radio stations.
The 60 dBu contour is predominantly used to map an FM radio station’s coverage area and make sure proposed stations do not interfere with current stations. It is also used to define areas that are underserved by FM radio stations, according to a report prepared by the National Institute for Telecommunications Sciences prepared for National Public Radio.