Sec. 73.313  Prediction of coverage.

    (a) All predictions of coverage made pursuant to this section shall 
be made without regard to interference and shall be made only on the 
basis of estimated field strengths.
    (b) Predictions of coverage shall be made only for the same purposes 
as relate to the use of field strength contours as specified in 
Sec. 73.311.
    (c) In predicting the distance to the field strength contours, the 
F(50,50) field strength chart, Figure 1 of Sec. 73.333 must be used. The 
50% field strength is defined as that value exceeded for 50% of the 
time.
    (1) The F(50,50) chart gives the estimated 50% field strengths 
exceeded at 50% of the locations in dB above 1 uV/m. The chart is based 
on an effective power radiated from a half-wave dipole antenna in free 
space, that produces an unattenuated field strength at 1 kilometer of 
about 107 dB above 1 uV/m (221.4 mV/m).
    (2) To use the chart for other ERP values, convert the ordinate 
scale by the appropriate adjustment in dB. For example, the ordinate 
scale for an ERP of 50 kW (17 dBk) should be adjusted by 17 dB and, 
therefore, a field strength of 40 dBu would be converted to 57 dBu. When 
predicting the distance to field strength contours, use the maximum ERP 
of the main radiated lobe in the pertinent azimuthal direction. When 
predicting field strengths over areas not in the plane of the maximum 
main lobe, use the ERP in the direction of such areas, determined by 
considering the appropriate vertical radiation pattern.
    (d) The antenna height to be used with this chart is the height of 
the radiation center of the antenna above the average terrain along the 
radial in question. In determining the average elevation of the terrain, 
the elevations between 3 and 16 kilometers from the antenna site are 
used.
    (1) Profile graphs must be drawn for eight radials beginning at the 
antenna site and extending 16 kilometers therefrom. The radials should 
be drawn for each 45 deg. of azimuth starting with True North. At least 
one radial must include the principal community to be served even though 
it may be more than 16 kilometers from the antenna site. However, in the 
event none of the evenly spaced radials include the principal community 
to be served, and one or more such radials are drawn in addition, these 
radials must not be used in computing the antenna height above average 
terrain.
    (2) Where the 3 to 16 kilometers portion of a radial extends in 
whole or in part over a large body of water or extends over foreign 
territory but the 50 uV/m contour encompasses land area within the 
United States beyond the 16 kilometers portion of the radial, the entire 
3 to 16 kilometers portion of the radial must be included in the 
computation of antenna height above average terrain. However, where the 
50 uV/m contour does not so encompass United States land area and (i) 
the entire 3 to 16 kilometers portion of the radial extends over large 
bodies of water or foreign territory, such radial must be completely 
omitted from the computation of antenna height above average terrain, 
and (ii) where a part of the 3 to 16 kilometers portion of a radial 
extends over large bodies of water or foreign territory, only that part 
of the radial extending from the 3 kilometers sector to the outermost 
portion of land area within the United States covered by the radial must 
be used in the computation of antenna height above average terrain.
    (3) The profile graph for each radial should be plotted by contour 
intervals of from 12 to 30 meters and, where the

[[Page 132]]

data permits, at least 50 points of elevation (generally uniformly 
spaced) should be used for each radial. In instances of very rugged 
terrain where the use of contour intervals of 30 meters would result in 
several points in a short distance, 60 or 120 meter contour intervals 
may be used for such distances. On the other hand, where the terrain is 
uniform or gently sloping the smallest contour interval indicated on the 
topographic map should be used, although only relatively few points may 
be available. The profile graph should indicate the topography 
accurately for each radial, and the graphs should be plotted with the 
distance in kilometers as the abscissa and the elevation in meters above 
mean sea level as the ordinate. The profile graphs should indicate the 
source of the topographical data used. The graph should also show the 
elevation of the center of the radiating system. The graph may be 
plotted either on rectangular coordinate paper or on special paper that 
shows the curvature of the earth. It is not necessary to take the 
curvature of the earth into consideration in this procedure as this 
factor is taken care of in the charts showing signal strengths. The 
average elevation of the 13 kilometer distance between 3 and 16 
kilometers from the antenna site should then be determined from the 
profile graph for each radial. This may be obtained by averaging a large 
number of equally spaced points, by using a planimeter, or by obtaining 
the median elevation (that exceeded for 50% of the distance) in sectors 
and averaging those values.
    (4) Examples of HAAT calculations:
    (i) The heights above average terrain on the eight radials are as 
follows:

                                                                        
------------------------------------------------------------------------
                                                                 Meters 
------------------------------------------------------------------------
0 deg.........................................................       120
45 deg........................................................       255
90 deg........................................................       185
135 deg.......................................................        90
180 deg.......................................................       -10
225 deg.......................................................       -85
270 deg.......................................................        40
315 deg.......................................................        85
------------------------------------------------------------------------

    The antenna height above terrain (defined in Sec. 73.310(a)) is 
computed as follows:

(120+255+185+90-10-85+40+85)/8=85 meters.

    (ii) Same as paragraph (d)(4)(i) of this section, except the 0 deg. 
radial is entirely over sea water. The antenna height above average 
terrain is computed as follows (note that the divisor is 7 not 8):

(255+185+90-10-85+40+85)/7=80 meters.

    (iii) Same as paragraph (d)(4)(i) of this section, except that only 
the first 10 kilometers of the 90 deg. radial are in the United States; 
beyond 10 kilometers the 90 deg. radial is in a foreign country. The 
height above average terrain of the 3 to 10 kilometer portion of the 
90 deg. radial is 105 meters. The antenna height above average terrain 
is computed as follows (note that the divisor is 8 not 7.5):

(120+255+105+90-10-85+40+85)/8=75 meters.

    (e) In cases where the terrain in one or more directions from the 
antenna site departs widely from the average elevation of the 3 to 16 
kilometer sector, the prediction method may indicate contour distances 
that are different from what may be expected in practice. For example, a 
mountain ridge may indicate the practical limit of service although the 
prediction method may indicate otherwise. In such cases, the prediction 
method should be followed, but a supplemental showing may be made 
concerning the contour distances as determined by other means. Such 
supplemental showings should describe the procedure used and should 
include sample calculations. Maps of predicted coverage should include 
both the coverage as predicted by the regular method and as predicted by 
a supplemental method. When measurements of area are required, these 
should include the area obtained by the regular prediction method and 
the area obtained by the supplemental method. In directions where the 
terrain is such that antenna heights less than 30 meters for the 3 to 16 
kilometer sector are obtained, an assumed height of 30 meters must be 
used for the prediction of coverage. However, where the actual contour 
distances are critical factors, a supplemental showing of expected 
coverage must be included together with a description of the method used 
in predicting such coverage. In special cases, the

[[Page 133]]

FCC may require additional information as to terrain and coverage.
    (f) The effect of terrain roughness on the predicted field strength 
of a signal at points distant from an FM transmitting antenna is assumed 
to depend on the magnitude of a terrain roughness factor (h) which, for 
a specific propagation path, is determined by the characteristics of a 
segment of the terrain profile for that path 40 kilometers in length 
located between 10 and 50 kilometers from the antenna. The terrain 
roughness factor has a value equal to the distance, in meters, between 
elevations exceeded by all points on the profile for 10% and 90% 
respectively, of the length of the profile segment. (See Sec. 73.333, 
Figure 4.)
    (g) If the lowest field strength value of interest is initially 
predicted to occur over a particular propagation path at a distance that 
is less than 50 kilometers from the antenna, the terrain profile segment 
used in the determination of terrain roughness factor over that path 
must be that included between points 10 kilometers from the transmitter 
and such lesser distances. No terrain roughness correction need be 
applied when all field strength values of interest are predicted to 
occur 10 kilometers or less from the transmitting antenna.
    (h) Profile segments prepared for terrain roughness factor 
determinations are to be plotted in rectangular coordinates, with no 
less than 50 points evenly spaced within the segment using data obtained 
from topographic maps with contour intervals of approximately 15 meters 
(50 feet) or less if available.
    (i) The field strength charts (Sec. 73.333, Figs. 1-1a) were 
developed assuming a terrain roughness factor of 50 meters, which is 
considered to be representative of average terrain in the United States. 
Where the roughness factor for a particular propagation path is found to 
depart appreciably from this value, a terrain roughness correction 
(F) should be applied to field strength values along this path, 
as predicted with the use of these charts. The magnitude and sign of 
this correction, for any value of h, may be determined from a 
chart included in Sec. 73.333 as Figure 5.
    (j) Alternatively, the terrain roughness correction may be computed 
using the following formula:

                F=1.9-0.03(h)(1+f/300)

Where:

    F=terrain roughness correction in dB
    k=terrain roughness factor in meters
    f=frequency of signal in MHz (MHz)

(Secs. 4, 5, 303, 48 Stat., as amended, 1066, 1068, 1082 (47 U.S.C. 154, 
155, ))

[28 303 FR 13623 , Dec. 14, 1963, as amended at  40 FR 27678 , July 1, 1975;  48 FR 29507 , June 27, 1983;  52 FR 11655 , Apr. 10, 1987;  52 FR 37789 , Oct. 
9, 1987;  57 FR 48333 , Oct. 23, 1992]

    Effective Date Note: At  42 FR 25736 , May 19, 1977, the effective 
date of Sec. 73.313 paragraphs (i) and (j) was stayed indefinitely.