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 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) 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(b):  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. 

   (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)  Special Temporary Authority (see  Sec. 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  Sec. 73.154 . 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;  63 FR 33876 , June 22, 1998;  66 FR 20755 , Apr. 25, 2001;  67 FR 13231 , Mar. 21, 2002]