The following table lists the various classes of FM stations, the reference facilities for each station class, and the protected and city grade contours for each station class. A service (or interfering) contour may be visualized by imagining a rough circle surrounding a transmitter site at some distance, where the circle represents a certain field strength value, with greater electric field strengths inside, and lesser electric field strengths outside. The distances to the contours herein were derived using the maximum effective radiated power (ERP) and antenna height above average terrain (HAAT) combination permitted for each station class (see Section 73.211), and assumes uniform (flat) terrain in all directions. In practice, the distances to a specified contour for a given FM station in a given direction will vary from the listed value depending on the effects of terrain in that direction.
The listed values are for commercial FM stations. This data also holds true for noncommercial educational stations, except that the 70 dBu city grade requirement of 47 CFR Section 73.315(a) does not apply to those noncommercial educational stations in the portion of the FM band reserved for noncommercial educational use (the “reserved band” from 88.1 to 91.9 MHz). Such noncommercial educational stations must encompass at least 50% of the community of license with the 60 dBu service contour. Distances to reserved band noncommercial educational stations’ protected contours for Class B and Class B1 stations are the same as the distances listed for Class C2 and Class C3 stations, respectively, since the protected contour for all reserved band stations is 60 dBu ( 1.0 mV/m ).
For different combinations of ERP and HAAT, you may determine the applicable FM station class by using our FM propagation curves calculatior and comparing the result with the table below. Be sure to use the 60 dBu F(50,50) contour, since it is on the basis of that contour (not the 70 or 54 or 57 dBu contour) that equivalence between facilities is determined.
Alternatively, if you know the station class and the antenna height above average terrain (HAAT), you may use FMpower to calculate the effective radiated power (ERP) needed to achieve maximum coverage for the selected station class. This program will also advise whether the station class is valid in the selected state.
|FM Station Class||Reference (Maximum) Facilities for Station Class
(see 47 CFR Section 73.211)
ERP (in kW) / HAAT (in meters)
|FM Protected or Primary Service Contour
|Distance to Protected or Primary Service Contour (km)||Distance to 70 dBu (or 3.16 mV/m) City Grade or Principal Community Coverage Contour
(see 47 CFR Section 73.315)
|Class A||6.0 kW / 100 meters||60 dBu||1.0 mV/m||28.3 km||16.2 km|
|Class B1||25.0 kW / 100 meters||57 dBu||0.71 mV/m||44.7 km||23.2 km|
|Class B||50.0 kW / 150 meters||54 dBu||0.50 mV/m||65.1 km||32.6 km|
|Class C3||25.0 kW / 100 meters||60 dBu||1.0 mV/m||39.1 km||23.2 km|
|Class C2||50.0 kW / 150 meters||60 dBu||1.0 mV/m||52.2 km||32.6 km|
|Class C1||100.0 kW / 299 meters||60 dBu||1.0 mV/m||72.3 km||50.0 km|
|Class C0 (C-zero)||100.0 kW / 450 meters||60 dBu||1.0 mV/m||83.4 km||59.0 km|
|Class C||100.0 kW / 600 meters||60 dBu||1.0 mV/m||91.8 km||67.7 km|
Notes: Class B and B1 stations are authorized only in Zones I and I-A, which include the following states and areas: CA (south of 40° latitude), CT, DC, DE, IL, IN, MA, MD, coastal ME, MI (south of 43.5° latitude), NJ, NH (south of 43.5° latitude), NY (south of 43.5° latitude), OH, PA, PR, RI, northern & eastern VA, VI, VT (south of 43.5° latitude), southeastern WI, WV. Class C, C0 (C-zero), C1, C2, and C3 stations are not authorized in Zones I or I-A, but may be authorized elsewhere. See Section 73.205 for the exact zone boundaries. You may also use FMpower to determine whether a particular class is valid in a given state.