Goto Section: 73.683 | 73.685 | Table of Contents
FCC 73.684
Revised as of October 2, 2015
Goto Year:2014 |
2016
§ 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√ 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 (Δ 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 (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 § 73.699 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:
ΔF=C−0.03(Δh)(1 + f/300)
Where:
ΔF = 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
Δh = 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.
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Goto Section: 73.683 | 73.685
Goto Year: 2014 |
2016
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