§  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 § 73.311.

   (c) In predicting the distance to the field strength contours, the
   F(50,50) field strength chart, Figure 1 of § 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 should be adjusted by 17 dB [10 log (50 kW) = 17
   dBk], and therefore a field strength of 60 dBu would correspond to the
   field strength value at (60−17 =) 44 dBu on the chart. When predicting
   the distance to field strength contours, use the maximum ERP of the
   main radiated lobe in the pertinent azimuthal direction (do not account
   for beam tilt). 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° 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 (34 dBu) 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 (34 dBu) 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 over 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 3 kilometers to
   the outermost portion of land in the United States covered by the
   radial used 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 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°      120
   45°     255
   90°     185
   135°     90
   180°    −10
   225°    −85
   270°     40
   315°     85

   The antenna height above terrain (defined in § 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° 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° radial are in the United States; beyond
   10 kilometers the 90° radial is in a foreign country. The height above
   average terrain of the 3 to 10 kilometer portion of the 90° 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 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 § 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 (§ 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 (Δ 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 Δ h, may be determined from a chart
   included in § 73.333 as Figure 5.

   (j) Alternatively, the terrain roughness correction may be computed
   using the following formula:

   Δ F =1.9−0.03(Δ h )(1+ f /300)

   Where:

   Δ F =terrain roughness correction in dB

   Δ 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, 303))

   [ 28 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;  63 FR 33877 , June 22, 1998]

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