FM station powers are listed as Effective Radiated Power (ERP) in Watts. ERP takes into account transmitter output power, antenna gain, and the directionality of the radiating elements. Most antennas are designed to avoid radiating along the vertical axis, since there are unlikely to be any listeners immediately above or below the transmitter site. Antenna gain will vary depending on the size of the antenna, and the number of elements. For comparison, we also list in each profile the power in decibels over a Watt (dBW), considered by some to be a more useful number to compare.
As with AM stations, FM power is not the only factor influencing signal coverage. Height is the most significant factor. The number, shape, and maintenance record of the transmitting elements (or ``bays''), and the location of the bays on the antenna itself can affect the directional pattern of the station, although most FM stations are considered to be theoretically omnidirectional in a plane parallel to the ground. Some stations also radiate different signals for horizontal and vertical polarization, which also has a directional effect. Finally, in built-up areas, interference from other sources and from reflected images of the station's own signal can significantly impair reception; this effect also occurs in rugged terrain.
FM frequency allocations are associated with a particular class of station, which defines a restriction on power and height a station is allowed. Roughly speaking, a class A station is allowed up to 6 kW at 300 m; a class B station is allowed up to 50 kW at 300 m, and a class C station is allowed up to 100 kW at effectively unlimited height. The US is divided up into three zones: the Northeast, Southern California, and everywhere else; class B stations are only found in the first two, and class C stations are only found in the last. (A ``class B''-type allocation in zone 2 is called a ``class C2''; there are also B1, B2, C1, and C3.)
Low-power stations like boosters, translators, and some grandfathered non-commercial stations are considered class D stations, and have a maximum power of 250 W. Some stations are grandfathered with higher powers than normally allowed (list). The contour of interest for FM is defined by an electric field intensity of 3.16 mV/m.