47 CFR §90.261

1. (a)Frequencies in the 450-470 MHz band as listed in §§ 90.20(c)(3) and 90.35(b)(3) may be assigned to all eligibles for fixed use on a secondary basis to land mobile operations.
2. (b)Fixed stations located 140 km (87 mi) or more from the center of any urbanized area of 600,000 or more population are limited to a transmitter output power of 75 watts. Fixed stations less than 140 km (87 mi) from the centers of these areas are limited to a transmitter output power of 20 watts. Urbanized areas of 600,000 or more population are defined in the U.S. Census of Population 1970, Vol. 1, Table 20, pages 1-74. The centers of the urbanized areas are determined from the Appendix, page 226, of the U.S. Department of Commerce publication “Airline Distance Between Cities in the United States.”
3. (c)All fixed systems are limited to one frequency pair with 5 MHz spacing and must employ directional antennas with a front-to-back ratio of 15dB, except that omnidirectional antennas having unity gain may be employed by stations communicating with a minimum of three receiving locations encompassed in a sector of at least 160° in azimuth. Stations authorized for secondary fixed operations prior to July 13, 1992, may continue to operate under the conditions of their initial authorization.
4. (d)-(e) [Reserved]
5. (f)Secondary fixed operations pursuant to paragraph (a) of this section will not be authorized on the following frequencies or on frequencies subject to § 90.267, except as provided in § 90.219(d)(3)(ii):