§ 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 (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 § 73.333 as Figure 5.

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

   ΔF = 1.9−0.03(Δh)(1 + f/300)

   Where:

   ΔF = terrain roughness correction in dB

   Δk = 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.

   return arrow Back to Top