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 eight or more 
radials, at intervals of approximately 0.2 kilometer up to 3 kilometers 
(1.87 miles) from the antenna, at intervals of approximately 1 kilometer 
from 3 kilometers (1.87 miles) to 10 kilometers (6.2 miles) from the 
antenna, at intervals of approximately 3 kilometers from 10 kilometers 
(6.2 miles) to 25 or 34 kilometers (15.5 miles or 20 miles) 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 as many as 18 
measurements on each radial. 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

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the character of the intervening terrain), the measurements at greater 
distances should be made at closer intervals. (It is suggested that 
``wave tilt'' measurements may be made to determine and compare 
locations for taking field strength measurements, particularly to 
determine that there are no abrupt changes in ground conductivity or 
that reflected waves are not causing abnormal strengths.
    (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 map required in paragraph (b)(2) 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 above) 
for each point of measurement.
    (2) Map showing each point of measurement numbered to agree with 
tabulation required above.
    (3) Description of method used to take field strength measurements.
    (4) The family of theoretical curves used in determining the curve 
for each radial properly identified by conductivity and dielectric 
constants.
    (5) The curves drawn for each radial and the field strength pattern.
    (6) The antenna resistance at the operating frequency.
    (7) Antenna current or currents maintained during field strength 
measurements.

[ 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]