§  73.186   Establishment of effective field at one kilometer.

   (a) Section 73.189 provides that certain minimum field strengths are
   acceptable in lieu of the required minimum physical heights of the
   antennas proper. Also, in other situations, it may be necessary to
   determine the effective field. The following requirements shall govern
   the taking and submission of data on the field strength produced:

   (1) Beginning as near to the antenna as possible without including the
   induction field and to provide for the fact that a broadcast antenna is
   not a point source of radiation (not less than one wave length or 5
   times the vertical height in the case of a single element, i.e. ,
   nondirectional antenna or 10 times the spacing between the elements of
   a directional antenna), measurements shall be made on six or more
   radials, at intervals of approximately 0.2 kilometer up to 3 kilometers
   from the antenna, at intervals of approximately one kilometer from 3
   kilometers to 5 kilometers from the antenna, at intervals of
   approximately 2 kilometers from 5 kilometers to 15 kilometers from the
   antenna, and a few additional measurements if needed at greater
   distances from the antenna. Where the antenna is rurally located and
   unobstructed measurements can be made, there shall be at least 15
   measurements on each radial. These shall include at least 7
   measurements within 3 kilometers of the antenna. However, where the
   antenna is located in a city where unobstructed measurements are
   difficult to make, measurements shall be made on each radial at as many
   unobstructed locations as possible, even though the intervals are
   considerably less than stated above, particularly within 3 kilometers
   of the antenna. In cases where it is not possible to obtain accurate
   measurements at the closer distances (even out to 8 or 10 kilometers
   due to the character of the intervening terrain), the measurements at
   greater distances should be made at closer intervals.

   (2) The data required by paragraph (a)(1) of this section should be
   plotted for each radial in accordance with either of the two methods
   set forth below:

   (i) Using log-log coordinate paper, plot field strengths as ordinate
   and distance as abscissa.

   (ii) Using semi-log coordinate paper, plot field strength times
   distance as ordinate on the log scale and distance as abscissa on the
   linear scale.

   (3) However, regardless of which of the methods in paragraph (a)(2) of
   this section is employed, the proper curve to be drawn through the
   points plotted shall be determined by comparison with the curves in
   § 73.184 as follows: Place the sheet on which the actual points have
   been plotted over the appropriate Graph in § 73.184, hold to the light
   if necessary and adjust until the curve most closely matching the
   points is found. This curve should then be drawn on the sheet on which
   the points were plotted, together with the inverse distance curve
   corresponding to that curve. The field at 1 kilometer for the radial
   concerned shall be the ordinate on the inverse distance curve at 1
   kilometer.

   (4) When all radials have been analyzed in accordance with paragraph
   (a)(3) of this section, a curve shall be plotted on polar coordinate
   paper from the fields obtained, which gives the inverse distance field
   pattern at 1 kilometer. The radius of a circle, the area of which is
   equal to the area bounded by this pattern, is the effective field. (See
   § 73.14.)

   (5) The antenna power of the station shall be maintained at the
   authorized level during all field measurements. The power determination
   will be made using the direct method as described in § 73.51(a) with
   instruments of acceptable accuracy specified in § 73.1215.

   (b) Complete data taken in conjunction with the field strength
   measurements shall be submitted to the Commission in affidavit form
   including the following:

   (1) Tabulation by number of each point of measurement to agree with the
   maps required in paragraph (c) of this section, the date and time of
   each measurement, the field strength (E), the distance from the antenna
   (D) and the product of the field strength and distance (ED) (if data
   for each radial are plotted on semilogarithmic paper, see paragraph
   (a)(2)(ii) of this section) for each point of measurement.

   (2) Description of method used to take field strength measurements.

   (3) The family of theoretical curves used in determining the curve for
   each radial properly identified by conductivity and dielectric
   constants.

   (4) The curves drawn for each radial and the field strength pattern.

   (5) The antenna resistance at the operating frequency.

   (6) Antenna current or currents maintained during field strength
   measurements.

   (c) Maps showing each measurement point numbered to agree with the
   required tabulation shall be retained in the station records and shall
   be available to the FCC upon request.

   [ 28 FR 13574 , Dec. 14, 1963, as amended at  41 FR 44178 , Oct. 7, 1976;
    46 FR 11995 , Feb. 12, 1981;  49 FR 49851 , Dec. 24, 1984;  50 FR 18843 ,
   May 2, 1985;  50 FR 47055 , Nov. 14, 1985;  51 FR 2707 , Jan. 21, 1986;  52 FR 10570 , Apr. 2, 1987;  66 FR 20757 , Apr. 25, 2001]