47 CFR § 73.507 - Minimum distance separations between stations.
(a) Minimum distance separations. No application for a new station, or change in channel or transmitter site or increase in facilities of an existing station, will be granted unless the proposed facilities will be located so as to meet the adjacent channel distance separations specified in § 73.207(a) for the class of station involved with respect to assignment on Channels 221, 222, and 223 listed in § 73.201 (except where in the case of an existing station the proposed facilities fall within the provisions of § 73.207(b)), or where a Class D station is changing frequency to comply with the requirements of § 73.512.
(b) Stations authorized as of September 10, 1962, which do not meet the requirements of paragraph (a) of this section and § 73.511, may continue to operate as authorized; but any application to change facilities will be subject to the provisions of this section.
(1) Stations separated in frequency by 10.6 or 10.8 MHz (53 or 54 channels) from allotments or assignments on non-reserved channels will not be authorized unless they conform to the separations in Table A given in § 73.207.
(2) Under the United States-Mexican FM Broadcasting Agreement, for stations and assignments differing in frequency by 10.6 to 10.8 MHz (53 or 54 channels), U.S. noncommercial educational FM allotments and assignments must meet the separations given in Table C of § 73.207 to Mexican allotments or assignments in the border area.