FMPOWER uses the FM propagation curves program to calculate the effective radiated power (ERP) needed to achieve facilities equivalent to the reference facilities for an FM station class. Only three pieces of information are required to use this program -- the U.S. state of interest, the station class, and the antenna height above average terrain (HAAT). The HAAT for a particular FM station may be found via the FM Query. More after the form.
For example, FMpower shows that 4.1 kW ERP at 122 meters HAAT is equivalent to reference Class A facilities of 6.0 kW ERP at 100 meters HAAT. Reference parameters for the various classes are specified in 47 CFR 73.211 and are listed in a table at FM Station Classes and Service Contours.
For shorter antenna heights, FMpower may display an equivalent ERP that is larger than the maximum permitted ERP for the station class. In such cases, a warning message will be posted. In practical terms, this means that coverage equivalent to the reference facilities for the station class cannot be obtained with the given HAAT. (This also illustrates the importance of antenna height on coverage for FM and TV facilities.)
Equivalence between FM facilities is made by comparing the distances to the 60 dBu (1 mV/m) F(50,50) service contour. The sole exception to this practice is for those grandfathered Class A stations covered by rule section 47 CFR 73.213(c). For these grandfathered stations, equivalence is determined by comparing the 34 dBu F(50,10) interfering contour if the HAAT is below the Class A reference HAAT of 100 meters, or by comparing the 60 dBu contours if the HAAT is 100 meters or more.