The Federal Communications Commission is responsible for licensing the use of the radio spectrum. For the FM band, which spans from approximately 88 to 108 MHz, it not only licenses individual channels but also issues licenses that determine how much power a given broadcaster can use and how tall their antenna can be. These limitations exist to prevent channels from interfering with each other. While an FM station broadcasting at 98.1 in Seattle is unlikely to interfere with a station on the same frequency in San Diego, dense areas of the country like the Northeast are likely to have conflicts because media markets are relatively close together.

Class C

Stations that fall into one of the C classifications are the most powerful FM category. C, C0 and C1 broadcasters can all use 100-kilowatt transmitters and can have 600-, 450- or 299-meter tall antennas measured above the average height of the terrain. Lower power C2 and C3 stations have 50- and 25-kW transmitters and 150- and 100-meter antennas. Class C stations have relatively broad protected areas ranging from 39.1 km for a C3 transmitter to 91.8 km for a full-power C station.

Class B

The two B classes are used in denser areas where Class C stations are not authorized. B stations use 50-kW transmitters and 150-meter antennas, while a B1 station has a 25-kW maximum power and a 100-meter antenna. The primary service contour for a B class transmitter has a 65.1-km radius while a B1 transmitter has a 44.7 km area.

Class A

Class A stations represent the least powerful broadcast station class. Their transmitters are limited to six kilowatts and their antennas can only be 100 meters above the height of the average terrain in their areas. With their lower power, their protected corridor's radius is limited to 28.3 km.

Low Power FM

The FCC created a special category of FM license for community based broadcasters in 2000. Low-power FM stations are limited to just 100 watts of power with an antenna that is no more than 30 meters tall, giving them a service range of about 5.6 miles. However, in dense urban areas, this can give them a potential audience of up to a million people.

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