§  73.68   Sampling systems for antenna monitors.

   (a) Each AM station permittee authorized to construct a new directional
   antenna system which will be subject to a proof of performance based on
   field strength measurements, as described in § 73.151(a) or (b), 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 directional antennas may be of different lengths
   provided the phase difference of signals at the monitor are less than
   0.5 degrees 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) An AM station permittee authorized to construct a directional
   antenna system which will be subject to a proof of performance based on
   moment method modeling, as described in § 73.151(c), shall install a
   sampling system conforming to the requirements set forth in that
   section.

   (c) 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 letter request to the
   FCC in Washington, DC, Attention: Audio Division, Media Bureau. 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 to paragraph(c): A public notice dated December 9, 1985 giving
   additional information on approval of antenna sampling systems is
   available through the Internet at
   http://www.fcc.gov/mb/audio/decdoc/letter/1985-12-09-sample.html.

   (d) 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.

   (e) If the antenna sampling system is modified or components of the
   sampling system are replaced, the following procedure shall be
   followed:

   (1) Special Temporary Authority (see § 73.1635) shall be requested and
   obtained from the Commission's Audio Division, Media Bureau 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, and after a verification that all monitoring point
   values and operating parameters 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. Subsequent to these
   modifications or changes the procedure must be repeated.

   (3) If monitoring point field strengths or antenna monitor parameters
   exceed allowable limits following the replacement or modification of
   that portion of the sampling system above the base of the towers, a
   partial proof of performance shall be executed in accordance with
   § 73.154 . The partial proof of performance shall be accompanied by
   common point impedance measurements made in accordance with § 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.

   (f) 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;  63 FR 33876 , June 22, 1998;  66 FR 20755 , Apr. 25, 2001;  67 FR 13231 , Mar. 21, 2002;  73 FR 64561 , Oct. 30,
   2008]