Goto Section: 73.683 | 73.685 | Table of Contents
FCC 73.684
Revised as of October 1, 2014
Goto Year:2013 |
2015
§ 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 SQRT 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 (D 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 (DF) 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 Dh, 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:
DF=C -0.03(Dh)(1+f/300)
Where:
DF=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
Dh=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.
return arrow Back to Top
Goto Section: 73.683 | 73.685
Goto Year: 2013 |
2015
CiteFind - See documents on FCC website that
cite this rule
Want to support this service?
Thanks!
Report errors in
this rule. Since these rules are converted to HTML by machine, it's possible errors have been made. Please
help us improve these rules by clicking the Report FCC Rule Errors link to report an error.
hallikainen.com
Helping make public information public