47 CFR § 73.1520 - Operation for tests and maintenance.
(a) Broadcast stations may be operated for tests and maintenance of their transmitting systems on their assigned frequencies using their licensed operating power and antennas during their authorized hours of operation without specific authorization from the FCC.
(b) Licensees of AM stations may operate for tests and maintenance during the hours from 12 midnight local time to local sunrise, if no interference is caused to other stations maintaining a regular operating schedule within such period. No AM station licensed for “daytime” or “specified hours” of operation may broadcast any regular or scheduled programs during this period of test and maintenance operation.
(c) Licensees of AM stations may obtain special antenna test authorizations, and operate under the provisions described in § 73.157, to operate with nighttime facilities during daytime hours in conducting directional antenna field strength and antenna proof of performance measurements.