47 CFR § 73.69 - Antenna monitors.
(b) In the event that the antenna monitor sampling system is temporarily out of service for repair or replacement, the station may be operated, pending completion of repairs or replacement, for a period not exceeding 120 days without further authority from the FCC if all other operating parameters, and the field monitoring point values are within the limits specified on the station authorization.
(c) If conditions beyond the control of the licensee prevent the restoration of the monitor to service within the allowed period, an informal letter request in accordance with § 73.3549 of the Commission's rules must be filed with the FCC, Attention: Audio Division, Media Bureau in Washington, DC for such additional time as may be required to complete repairs of the defective instrument.
(d) If an authorized antenna monitor is replaced by another antenna monitor, the following procedure shall be followed:
(1) Temporary authority shall be requested and obtained from the Commission in Washington to operate with parameters at variance with licensed values, pending issuance of a modified license specifying new parameters.
(2) Immediately before the replacement of the antenna monitor, after a verification that all monitoring point values and the common point current reading are within the limits or tolerances specified in the rules, the following indications must be recorded for each radiation pattern: Final plate current and plate voltage, common point current, antenna monitor phase and current indications, and the field strength at each monitoring point.
(3) With the new monitor substituted for the old, all indications specified in paragraph (d)(2) of this section, again must be read. If no change has occurred in the indication for any parameter other than the indications of the antenna monitor, the new antenna monitor indications must be deemed to be those reflecting correct array adjustments.
(4) If it cannot be established by the observations required in paragraph (d)(2) of this section that the common point current reading and the monitoring point values are within the tolerances or limits prescribed by the rules and the instrument of authorization, or if the substitution of the new antenna monitor for the old results in changes in these parameters, a partial proof of performance shall be executed and analyzed in accordance with § 73.154.
(5) An informal letter request for modification of license shall be submitted to the FCC, Attention: Audio Division, Media Bureau in Washington, DC within 30 days of the date of monitor replacement. Such request shall specify the make, type, and serial number of the replacement monitor, phase and sample current indications, and other data obtained pursuant to this paragraph (d).
(e) The antenna monitor must be calibrated according to the manufacturer's instructions as often as necessary to ensure its proper operation.