Goto Section: 73.312 | 73.314 | Table of Contents
FCC 73.313
Revised as of October 1, 2010
Goto Year:2009 |
2011
§ 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 45DEG 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
0DEG 120
45DEG 255
90DEG 185
135DEG 90
180DEG -10
225DEG -85
270DEG 40
315DEG 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 0DEG
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 90DEG radial are in the United States;
beyond 10 kilometers the 90DEG radial is in a foreign country. The
height above average terrain of the 3 to 10 kilometer portion of the
90DEG 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 (D 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 D 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:
D F =1.9 -0.03(D h )(1+ f /300)
Where:
D F =terrain roughness correction in dB
D 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.
Goto Section: 73.312 | 73.314
Goto Year: 2009 |
2011
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