Sec.  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  Sec. 73.184 as follows:
   Place  the sheet on which the actual points have been plotted over the
   appropriate Graph in  Sec. 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  Sec. 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  Sec. 73.51(a) with instruments of
   acceptable accuracy specified in  Sec. 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]