Goto Section: 73.312 | 73.314 | Table of Contents
FCC 73.313
Revised as of October 1, 2008
Goto Year:2007 |
2009
Sec. 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 Sec. 73.311.
(c) In predicting the distance to the field strength contours, the F(50,50)
field strength chart, Figure 1 of Sec. 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 Sec. 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 Sec. 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 ( Sec. 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 Sec. 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
Sec. 73.313 paragraphs (i) and (j) was stayed indefinitely.
Goto Section: 73.312 | 73.314
Goto Year: 2007 |
2009
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