Every FM station in the U.S. is a particular class, as defined in the FCC's table of allotments.
ERP is Effective Radiated Power. ERP is computed by factoring line loss and antenna gain into the transmitter power output, or TPO. (TPO really has no relevance outside of the transmitter building because of that line loss and antenna gain.)
HAAT is Height Above Average Terrain. A tower that's only 50m tall may actually have an HAAT of 100m because it sits on a higher elevation than surrounding terrain.
Stations may have a higher HAAT than listed above if one is lowered so as to acheive the same reference distance. For example, a class C3 station can have an antenna at 106m if its ERP is 22kW, acheiving the same coverage area as a station that is 25kW at 100m. This practice is very common among class B stations in large cities.
Reference distance is also commonly referred to as the primary community contour or primary coverage area. This distance is the radius of the primary coverage area for stations that operate at maximum ERP and HAAT.
Class C0 is a relatively new class that was added to permit upgrades in cases where a station licensed as class C has an HAAT of 450m or less. Essentially, these station's signals were being protected for much farther than they needed to be. Adding this class has permitted other stations that could not upgrade under the old rules to upgrade. A station's class is only changed from C to C0 if another station wishes to upgrade or a new allotment is made.
The country is divided into three zones. A station operating with 50kW at 150m in Minneapolis would be classified as C2, while a station operating with 50kW at 150m in Milwaukee would be classified as B. The main differences are that 100kW stations are not permitted in Zones I and I-A (the more densely populated areas of the country), and class B1 and B stations are given slightly more protection from interference from other stations.
Other classes: class D includes translators, boosters, and 10-Watt educational stations that were licensed before that service was discontinued. Class LP100 and LP10 are newer low-power FM classes.
Related information: FCC mimimum distance separations
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