[99-25 NPRM] Appendix A: FM Radio Service Areas and Interference Protection Criteria
1. The current FM Broadcast Service consists of seven classes of licensed stations, Classes A, B1, B, C3, C2, C1, and C. (An eighth class, Class D, was discontinued in 1980, although applications for renewal and modification of these existing licenses are still accepted). Each class has specific technical and operational characteristics, such as effective radiated power ("ERP"), antenna height, and protected signal coverage area. Class A stations, the least powerful and shortest range of all the classes, have a maximum power limit of 6 kW and, when using maximum power, a Class A station may not use an antenna at a height greater than 100 meters (328 feet). (All heights are referenced to the height of the antenna above the average height of the terrain surrounding the antenna support structure ("height above average terrain" or "HAAT")). Class C stations, the most powerful and longest range of all the classes, have a minimum required ERP of 100 kW and, at that power, may not use an antenna at a height greater than 600 meters (1968 feet). The reference coverage area of each class of FM station is considered to be the area bounded by the 1 millivolt-per-meter (1 mV/m) signal strength contour from the station. For Class A stations using maximum facilities, the 1 mV/m contour is a circle with a radius of 28 kilometers (17 miles) around the station antenna. For Class C stations using maximum facilities, the 1 mV/m contour is a circle with a radius of 92 kilometers (57 miles) around the station antenna. For Classes B1 and C3, the 1 mV/m contour has a radius of 39 kilometers (24 miles); for Classes B and C2, the 1 mV/m contour has a radius of 52 kilometers (32 miles); and for Class C1, the 1 mV/m contour has a radius of 72 kilometers (45 miles). (These values are derived from the Commission's F(50,50) R-6602 propagation curves. See 47 C.F.R. ' 73.333, Figure 1; see also 47 C.F.R. ' 73.211).
2. Each current class of FM station is afforded protection from several types of harmful interference, specifically: interference from co-channel stations, i.e., stations operating on the same frequency as the protected station; interference from first-adjacent channel stations, i.e., stations operating one channel higher or lower in frequency than the protected station; interference from second-adjacent channel stations, i.e., stations operating two channels higher or lower in frequency than the protected station; interference from third-adjacent channel stations, i.e., stations operating three channels higher or lower in frequency from the protected station; and intermediate frequency ("IF") interference, i.e. signals from stations offset in frequency by 10.6 and 10.8 MHz from the protected station. (FM channels are each 200 kHz wide, resulting in 1st, 2nd, and 3rd adjacency spacings of 200 kHz, 400 kHz and 600 kHz, respectively.) The 1 mV/m signal contours of Class A, C3, C2, C1, and C stations must receive 20 dB co-channel protection, 6 dB first-adjacent channel protection, and -40 dB second- and third-adjacent channel protection. (In terms of millivolts per meter, co-channel interfering signals must be no greater than 0.1 mV/m, 1st-adjacent channel signals must be no greater than 0.5 mV/m, and 2nd/3rd adjacent signals must be no greater than 100 mV/m at the service contour of the station receiving protection). The 0.7 mV/m signal contours of Class B1 stations and the 0.5 mV/m signal contours of Class B stations must receive these same degrees of protection. For IF protection, the 36 mV/m contours of all station classes are protected against the 36 mV/m interfering contours of all stations spaced +/- 10.6 and 10.8 MHz. (See Section 73.207 of the Commission's rules, 47 C.F.R. ' 73.207, for a table of minimum permissible station-to-station separation distances generally based on these protection ratios). For full-power services, Table A of Section 73.207 (a)(1) provides minimum distance separations for same and different class channel stations and for first-, second-, and third-adjacent channel stations as well. For example, a co-channel same class distance separation ranges from 290 kilometers (180 miles) for Class C stations, the most powerful FM stations, to 71 kilometers (115 miles) for Class A stations. Where the classes are different (i.e., Class A to B), the cochannel separations range from 270 kilometers (168 miles) for Class C1 to Class C, to 142 kilometers (88 miles) for Class A to Class C3. For adjacent channels, the required separations range from 241 kilometers (150 miles) for first adjacent Class C to Class C operation, to 6 kilometers (10 miles) for third adjacent Class A to Class A operation. The Commission established this distance separation method for channel assignment in 1962. It decided that using the distance separations would be most appropriate for the optimum development of the FM service. See First Report and Order in Docket 14184, 40 FCC 662, 685 (1962). In developing this method, and creating a table of minimum distance separations, the Commission took an approach that would allow it to make numerous assignments, while affording stations reasonably adequate protection from harmful interference. Based on the technology available at that time, it determined by class of station the distance required to provide protected service radii. It used field strength contours and based the original separations on these. It noted that some interference was to be expected, and that the receivers of that time could operate satisfactorily in such an environment. Id. at 686.