Goto Section: 73.683 | 73.685 | Table of Contents

FCC 73.684
Revised as of November 19, 2020
Goto Year:2020 | 2022
  §  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.

   


Goto Section: 73.683 | 73.685

Goto Year: 2020 | 2022
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