[Code of Federal Regulations]
[Title 47, Volume 2]
[Revised as of October 1, 2007]
From the U.S. Government Printing Office via GPO Access
[CITE: 47CFR73.186]

[Page 58-59]
 
                       TITLE 47--TELECOMMUNICATION
 
        CHAPTER I--FEDERAL COMMUNICATIONS COMMISSION (CONTINUED)
 
PART 73_RADIO BROADCAST SERVICES--Table of Contents
 
                     Subpart A_AM Broadcast Stations
 
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.

[[Page 59]]

    (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]