§ 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 § 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 § 73.699) shall be used. If the
   50% field 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 § 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  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
   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 (Δ 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 § 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 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 (§ 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 (ΔF) 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 Δh, may be
   determined from a chart included in § 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:

   ΔF=C−0.03(Δh)(1 + f/300)

   Where:

   ΔF = 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

   Δh = 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 § 73.684, paragraphs
   (k) and (l) were stayed indefinitely.

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