47 CFR § 73.1670 - Auxiliary transmitters.
(a) A licensee of a broadcast station may, without further authority from the FCC, install and use with the main antenna system one or more auxiliary transmitters for the following purposes:
(1) The transmission of regular programs upon failure of the main transmitter.
(2) The transmission of regular programs during maintenance or modification of the main transmitter.
(3) Emergency broadcast system operation.
(5) The transmission of tests to determine the operating condition of the auxiliary transmitter or auxiliary antenna.
(6) For testing, upon the request of representatives of the FCC.
(b) Authorization to install an auxiliary transmitter for use with other than the main antenna or authorized auxiliary antenna must be obtained by filing an application for a construction permit on FCC form 301 (FCC form 340 for noncommercial educational stations).
(c) The following technical and operating standards apply to auxiliary transmitters:
(2) The carrier frequency of the auxiliary transmitter must be measured as often as necessary to ensure that it is maintained within the prescribed tolerance.
(3) When using an auxiliary transmitter, the operating power may be less than the authorized power but may not exceed the authorized power within the permitted tolerance for the type of station. If operation with an auxiliary transmitter at reduced power continues for a period exceeding 10 days, the FCC in Washington, DC must be notified. (See § 73.51, AM; § 73.267, FM; § 73.567, NCE-FM; and § 73.663, TV).
(4) Normal operator requirements apply to the operation of the auxiliary transmitter.
After January 1, 1979, new licenses will not be issued nor will existing licenses be renewed for auxiliary transmitters that are operated into the main antenna system.