Sec. 73.68  Sampling systems for antenna monitors.

    (a) Each AM station permittee authorized to construct a new 
directional antenna system, must install the sampling system in 
accordance with the following specifications:
    (1) Devices used to extract or sample the current and the 
transmission line connecting the sampling elements to the antenna 
monitor must provide accurate and stable signals to the monitor (e.g., 
rigidly mounted and non-rotatable loops and all system components 
protected from physical and environmental disturbances).
    (2) Sampling lines for critical directional antennas (see 
Sec. 73.14) must be of uniform length. Sampling lines for non-critical 
directional antennas may be of different lengths provided the phase 
difference of signals at the monitor are less than 0.5 deg. between the 
shortest and longest cable lengths due to temperature variations to 
which the system is exposed.
    (3) Other configurations of sampling systems may be used upon 
demonstration of stable operation to the FCC.
    (b) A station having an antenna sampling system constructed 
according to the specifications given in paragraph (a) of this section, 
may obtain approval of that system by submitting an informal request to 
the FCC in Washington, DC. The request for approval, signed by the 
licensee or authorized representative, must contain sufficient 
information to show that the sampling system is in compliance with all 
requirements of paragraph (a) of this section.

    Note: A public notice giving additional information on approval of 
antenna sampling systems is available upon request from the FCC's 
current copy contractor.

    (c) 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.
    (d) If the antenna sampling system is modified or components of the 
sampling system are replaced, 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 
parameters subsequent to modification or replacement of components.
    (2) Immediately prior to modification or replacement of components 
of the sampling system not on the towers,

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and after a verification that all monitoring point values, base current 
ratios and operating parameters are within the limits or tolerances 
specified in the instrument of authorization or the pertinent rules, the 
following indications must be read for each radiation pattern: Final 
plate current and plate voltage, common point current, base currents and 
their ratios, antenna monitor phase and current indications, and the 
field strength at each monitoring point. Subsequent to these 
modifications or changes the above procedure must be repeated.
    (3) If that portion of the sampling system above the base of the 
towers is modified or components replaced, a partial proof of 
performance shall be executed in accordance with Sec. 73.154 subsequent 
to these changes. The partial proof of performance shall be accompanied 
by common point impedance measurements made in accordance with 
Sec. 73.54.
    (4) Request for modification of license shall be submitted to the 
FCC in Washington, DC, within 30 days of the date of sampling system 
modification or replacement. Such request shall specify the transmitter 
plate voltage and plate current, common point current, base currents and 
their ratios, antenna monitor phase and current indications, and all 
other data obtained pursuant to this paragraph.
    (e) If an existing sampling system is found to be patently of 
marginal construction, or where the performance of a directional antenna 
is found to be unsatisfactory, and this deficiency reasonably may be 
attributed, in whole or in part, to inadequacies in the antenna 
monitoring system, the FCC may require the reconstruction of the 
sampling system in accordance with requirements specified above.

[ 41 FR 7405 , Feb. 18, 1976, as amended at  42 FR 24056 , May 12, 1977;  44 FR 58731 , Oct. 11, 1979;  46 FR 35462 , July 8, 1981;  48 FR 38478 , Aug. 
24, 1983;  48 FR 44805 , Sept. 30, 1983;  49 FR 32358 , Aug. 14, 1984;  50 FR 47054 , Nov. 14, 1985;  51 FR 9965 , Mar. 24, 1986;  51 FR 40435 , Nov. 7, 
1986;  56 FR 64859 , Dec. 12, 1991]