Sec. 73.684  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. The peak power of the visual signal 
is used in making predictions of coverage.
    (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. 73.683(c).
    (c) In predicting the distance to the field strength contours, the F 
(50,50) field strength charts (Figures 9 and 10 of Sec. 73.699) shall be 
used. If the 50% field

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strength is defined as that value exceeded for 50% of the time, these F 
(50,50) charts give the estimated 50% field strengths exceeded at 50% of 
the locations in dB above 1 uV/m. The charts are based on an effective 
power of 1 kW radiated form a half-wave dipole in free space, which 
produces an unattenuated field strength at 1.61 kilometers (1 mile) of 
about 103 dB above 1 uV/m. To use the charts to predict the distance to 
a given contour, the following procedure is used: Convert the effective 
radiated power in kilowatts for the appropriate azimuth into decibel 
value referenced to 1 kW (dBu). If necessary, convert the selected 
contour to the decibel value (dBu) above 1 microvolt per meter (1 uV/m). 
Subtract the power value in dBk from the contour value in dBu. Note that 
for power less than 1 kW, the difference value will be greater than the 
contour value because the power in dBk is negative. Locate the 
difference value obtained on the vertical scale at the left edge of the 
chart. Follow the horizontal line for that value into the chart to the 
point of intersection with the vertical line above the height of the 
antenna above average terrain for the appropriate azimuth located on the 
scale at the bottom of the chart. If the point of intersection does not 
fall exactly on a distance curve, interpolate between the distance 
curves below and above the intersection point. The distance values for 
the curves are located along the right edge of the chart.
    (1) In predicting the distance to the Grade A and Grade B field 
strength contours, the effective radiated power to be used is that 
radiated at the vertical angle corresponding to the depression angle 
between the transmitting antenna center of radiation and the radio 
horizon as determined individually for each azimuthal direction 
concerned. The depression angle is based on the difference in elevation 
of the antenna center of radiation above the average terrain and the 
radio horizon, assuming a smooth sperical earth with a radius of 8,495.5 
kilometers (5,280 miles) and shall be determined by the following 
equation:

A = 0.0277 H

Where:

A is the depression angle in degrees.
H is the height in meters of the transmitting antenna radiation center 
          above average terrain of the 3.2--16.1 kilometers (2-10 miles) 
          sector of the pertinent radial.

    This formula is empirically derived for the limited purpose 
specified here. Its use for any other purpose may be inappropriate.

    (2) In case where the relative field strength at the depression 
angle determined by the above formula is 90% or more of the maximum 
field strength developed in the vertical plane containing the pertaining 
radial, the maximum radiation shall be used.
    (3) In predicting field strengths for other than the Grade A and 
Grade B contours, the effective radiated power to be used is to be based 
on the appropriate antenna vertical plane radiation pattern for the 
azimuthal direction concerned.
    (4) Applicants for new TV stations or changes in the facilities of 
existing TV stations must submit to the FCC a showing as to the location 
of their stations' or proposed stations' predicted Grade A and Grade B 
contours, determined in accordance with Sec. 73.684. This showing is to 
include maps showing these contours, except where applicants have 
previously submitted material to the FCC containing such information and 
it is found upon careful examination that the contour locations 
indicated therein would not change, on any radial, when the locations 
are determined under this Section. In the latter cases, a statement by a 
qualified engineer to this effect will satisfy this requirement and no 
contour maps need be submitted.
    (d) The antenna height to be used with these charts 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 3.2-16.1 kilometers (2-10 miles) from the antenna 
site are employed. Profile graphs shall be drawn for 8 radials beginning 
at the antenna site and extending 16.1 kilometers (10 miles) therefrom. 
The radials should be drawn for each 45 degrees of azimuth starting with 
the True North. At least one radial must include the principal community 
to be served even though such community may be more than 16.1 kilometers 
(10 miles) from the antenna

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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 to the 8 evenly spaced radials, such additional 
radials shall not be employed in computing the antenna height above 
average terrain. Where the 3.2-16.1 kilometers (2-10 mile) portion of a 
radial extends in whole or in part over large bodies of water as 
specified in paragraph (e) of this section or extends over foreign 
territory but the Grade B strength contour encompasses land area within 
the United States beyond the 16.1 kilometers (10 mile) portion of the 
radial, the entire 3.2-16.1 kilometers (2-10 mile) portion of the radial 
shall be included in the computation of antenna height above average 
terrian. However, where the Grade B contour does not so encompass United 
States land area and (1) the entire 3.2-16.1 kilometers (2-10 mile) 
portion of the radial extends over large bodies of water of foreign 
territory, such radial shall be completely omitted from the computation 
of antenna height above average terrain, and (2) where a part of the 
3.2-16.1 kilometers (2-10 mile) portion of a radial extends over large 
bodies of water or over foreign territory, only that part of the radial 
extending from the 3.2 kilometer (2 mile) sector to the outermost 
portion of land area within the United States covered by the radial 
shall be employed in the computation of antenna height above average 
terrian. The profile graph for each radial should be plotted by contour 
intervals of from 12.2-30.5 meters (40-100 feet) and, where the 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 30.5 meters (100 feet) would 
result in several points in a short distance, 61.0-122.0 meter (200-400 
foot) 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 topograhic may (see paragraph (g) of 
this section) should be used, although only relatively few points may be 
available. The profile graphs 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 employed. 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 which 
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 12.9 kilometer (8 miles) distance between 3.2-
16.1 kilometers (2-10 miles) 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.

    Note: The Commission will, upon a proper showing by an existing 
station that the application of this rule will result in an unreasonable 
power reduction in relation to other stations in close proximity, 
consider requests for adjustment in power on the basis of a common 
average terrain figure for the stations in question as determined by the 
FCC.

    (e) In instance where it is desired to determine the area in square 
kilometers within the Grade A and Grade B field strength contours, the 
area may be determined from the coverage map by planimeter or other 
approximate means; in computing such areas, exclued (1) areas beyond the 
borders of the United States, and (2) large bodies of water, such as 
ocean areas, gulfs sounds, bays, large lakes, etc., but not rivers.
    (f) In cases where terrain in one or more directions from the 
antenna site departs widely from the average elevation of the 3.2 to 
16.1 kilometers (2 to 10 mile) sector, the prediction method may 
indicate contour distances that are different from what may be expected 
in practice. For example, a

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mountain ridge may indicate the practical limit of service although the 
prediction method may indicate otherwise. In such case the prediction 
method should be followed, but a supplemental showing may be made 
concerning the contour distances as determined by other means. Such 
supplemental showing should describe the procedure employed 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 predicted method and the 
area obtained by the supplemental method. In directions where the 
terrain is such that negative antenna heights or heights below 30.5 
meters (100 feet) for the 3.2 to 16.1 kilometers (2 to 10 mile) sector 
are obtained, an assumed height of 30.5 meters (100 feet) shall 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 
employed in predicting such coverage. In special cases, the Commission 
may require additional information as to terrain and coverage.
    (g) In the preparation of the profile graph previously described, 
and in determining the location and height above sea level of the 
antenna site, the elevation or contour intervals shall be taken from the 
United States Geological Survey Topographic Quadrangle Maps, United 
States Army Corps of Engineers' maps or Tennessee Valley Authority maps, 
whichever is the latest, for all areas for which such maps are 
available. If such maps are not published for the area in question, the 
next best topographic information should be used. Topographic data may 
sometimes be obtained from State and Municipal agencies. Data from 
Sectional Aeronautical Charts (including bench marks) or railroad depot 
elevations and highway elevations from road maps may be used where no 
better information is available. In cases where limited topographic data 
is available, use may be made of an altimeter in a car driven along 
roads extending generally radially from the transmitter site. Ordinarily 
the Commission will not require the submission of topographical maps for 
areas beyond 24.1 kilometers (15 miles) from the antenna site, but the 
maps must include the principal community to be served. If it appears 
necessary, additional data may be requested. United States Geological 
Survey Topographic Quadrangle Maps may be obtained from the United 
States Geological Survey, Department of the Interior, Washington, DC 
20240. Sectional Aeronautical Charts are available from the United 
States Coast and Geodetic Survey, Department of Commerce, Washington, DC 
20235. In lieu of maps, the average terrain elevation may be computer 
generated, except in the cases of dispute, using elevations from a 30 
second point or better topographic data file. The file must be 
identified and the data processed for intermediate points along each 
radial using linear interpolation techniques. The height above mean sea 
level of the antenna site must be obtained manually using appropriate 
topographic maps.
    (h) The effect of terrain roughness on the predicted field strength 
of a signal at points distant from a television broadcast station 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 
40.2 kilometers (25 miles) in length, located between 9.7 and 49.9 
kilometers (6 and 31 miles) from the transmitter. The terrain roughness 
factor has a value equal to the difference, in meters, between 
elevations exceeded by all points on the profile for 10 percent and 90 
percent, respectively, of the length of the profile segment (see 
Sec. 73.699, Fig. 10d).
    (i) If the lowest field strength value of interest is initially 
predicted to occur over a particular propagation path at a distance 
which is less than 49.9 kilometers (31 miles) from the transmitter, the 
terrain profile segment used in the determination of the terrain 
roughness factor over that path shall be that included between points 
9.7 kilometers (6 miles) from the transmitter and such lesser distance. 
No terrain roughness correction need be applied when all field strength 
values of

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interest are predicted to occur 9.7 kilometers (6 miles) or less from 
the transmitter.
    (j) Profile segments prepared for terrain roughness factor 
determinations should be plotted in rectangular coordinates, with no 
less than 50 points evenly spaced within the segment, using data 
obtained from topographic maps, if available, with contour intervals of 
15.2 meters (50 feet), or less.
    (k) The field strength charts (Sec. 73.699, Figs. 9-10c) 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. 73.699 as Figure 10e, with linear interpolation 
as necessary, for the frequency of the UHF signal under consideration.
    (l) Alternatively, the terrain roughness correction may be computed 
using the following formula:

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

Where:

F=terrain roughness correction in dB
C=a constant having a specific value for use with each set of field 
          strength charts:
1.9 for TV Channels 2-6
2.5 for TV Channels 7-13
4.8 for TV Channels 14-69
h=terrain roughness factor in meters
f=frequency of signal in megahertz (MHz)

[ 28 FR 13660 , Dec. 13, 1963, as amended at  40 FR 27683 , July 1, 1975;  44 FR 36039 , June 20, 1979;  48 FR 44807 , Sept. 30, 1983;  50 FR 23699 , June 
5, 1985;  51 FR 26251 , July 22, 1986;  52 FR 36879 , Oct. 1, 1987]

    Effective Date Note:  At  42 FR 25736 , May 19, 1977, in Sec. 73.684, 
(k) and (l) are stayed indefinitely.