Goto Section: 73.182 | 73.184 | Table of Contents
FCC 73.183
Revised as of October 2, 2015
Goto Year:2014 |
2016
§ 73.183 Groundwave signals.
(a) Interference that may be caused by a proposed assignment or an existing
assignment during daytime hours should be determined, when possible, by
measurements on the frequency involved or on another frequency over the same
terrain and by means for the curves in § 73.184 entitled “Ground Wave Field
Strength versus Distance.”
Note: Groundwave field strength measurements will not be accepted or
considered for the purpose of establishing that interference to a station in
a foreign country other than Canada, or that the field strength at the
border thereof, would be less than indicated by the use of the ground
conductivity maps and engineering standards contained in this part and
applicable international agreements. Satisfactory groundwave measurements
offered for the purpose of demonstrating values of conductivity other than
those shown by Figure M3 in problems involving protection of Canadian
stations will be considered only if, after review thereof, the appropriate
agency of the Canadian government notifies the Commission that they are
acceptable for such purpose.
(b)(1) In all cases where measurements taken in accordance with the
requirements are not available, the groundwave strength must be determined
by means of the pertinent map of ground conductivity and the groundwave
curves of field strength versus distance. The conductivity of a given
terrain may be determined by measurements of any broadcast signal traversing
the terrain involved. Figure M3 (See Note 1) shows the conductivity
throughout the United States by general areas of reasonably uniform
conductivity. When it is clear that only one conductivity value is involved,
Figure R3 of § 73.190, may be used. It is a replica of Figure M3, and is
contained in these standards. In all other situations Figure M3 must be
employed. It is recognized that in areas of limited size or over a
particular path, the conductivity may vary widely from the values given;
therefore, these maps are to be used only when accurate and acceptable
measurements have not been made.
(2) For determinations of interference and service requiring a knowledge of
ground conductivities in other countries, the ground conductivity maps
comprising Appendix 1 to Annex 2 of each of the following international
agreements may be used:
(i) For Canada, the U.S.-Canada AM Agreement, 1984;
(ii) For Mexico, the U.S.-Mexico AM Agreement, 1986; and
(iii) For other Western Hemisphere countries, the Regional Agreement for the
Medium Frequency Broadcasting Service in Region 2.
Where different conductivities appear in the maps of two countries on
opposite sides of the border, such differences are to be considered as real,
even if they are not explained by geophysical cleavages.
(c) Example of determining interference by the graphs in § 73.184:
It is desired to determine whether objectionable interference exists between
a proposed 5 kW Class B station on 990 kHz and an existing 1 kW Class B
station on first adjacent channel, 1000 kHz. The distance between the two
stations is 260 kilometers and both stations operate nondirectionally with
antenna systems that produce a horizontal effective field of 282 in mV/m at
one kilometer. (See § 73.185 regarding use of directional antennas.) The
ground conductivity at the site of each station and along the intervening
terrain is 6 mS/m. The protection to Class B stations during daytime is to
the 500 µV/m (0.5 Vm) contour using a 6 dB protection factor. The distance
to the 500 µV/m groundwave contour of the 1 kW station is determined by the
use of the appropriate curve in § 73.184. Since the curve is plotted for 100
mV/m at a 1 kilometer, to find the distance of the 0.5 mV/m contour of the 1
kw station, it is necessary to determine the distance to the 0.1773 m/Vm
contour.
(100 × 0.5 / 282 = 0.1773)
Using the 6 mS/m curve, the estimated radius of the 0.5 mV/m contour is 62.5
kilometers. Subtracting this distance from the distance between the two
stations leaves 197.5 kilometers. Using the same propagation curve, the
signal from the 5 kW station at this distance is seen to be 0.059 mV/m.
Since a protection ratio of 6 dB, desired to undesired signal, applies to
stations separated by 10 kHz, the undesired signal could have had a value of
up to 0.25 mV/m without causing objectionable interference. For co-channel
studies, a desired to undesired signal ratio of no less than 20:1 (26 dB) is
required to avoid causing objectionable interference.
(d) Where a signal traverses a path over which different conductivities
exist, the distance to a particular groundwave field strength contour shall
be determined by the use of the equivalent distance method. Reasonably
accurate results may be expected in determining field strengths at a
distance from the antenna by application of the equivalent distance method
when the unattenuated field of the antenna, the various ground
conductivities and the location of discontinuities are known. This method
considers a wave to be propagated across a given conductivity according to
the curve for a homogeneous earth of that conductivity. When the wave
crosses from a region of one conductivity into a region of a second
conductivity, the equivalent distance of the receiving point from the
transmitter changes abruptly but the field strength does not. From a point
just inside the second region the transmitter appears to be at that distance
where, on the curve for a homogeneous earth of the second conductivity, the
field strength equals the value that occurred just across the boundary in
the first region. Thus the equivalent distance from the receiving point to
the transmitter may be either greater or less than the actual distance. An
imaginary transmitter is considered to exist at that equivalent distance.
This technique is not intended to be used as a means of evaluating
unattenuated field or ground conductivity by the analysis of measured data.
The method to be employed for such determinations is set out in § 73.186.
(e) Example of the use of the equivalent distance method;
It is desired to determine the distance to the 0.5 mV/m and 0.025 mV/m
contours of a station on a frequency of 1000 kHz with an inverse distance
field of 100 mV/m at one kilometer being radiated over a path having a
conductivity of 10 mS/m for a distance of 20 kilometers, 5 mS/m for the next
30 kilometers and 15 mS/m thereafter. Using the appropriate curve in
§ 73.184, Graph 12, at a distance of 20 kilometers on the curve for 10 mS/m,
the field strength is found to be 2.84 mV/m. On the 5mS/m curve, the
equivalent distance to this field strength is 14.92 kilometers, which is
5.08 (20−14.92 kilometers nearer to the transmitter. Continuing on the
propagation curve, the distance to a field strength of 0.5 mV/m is found to
be 36.11 kilometers.
The actual length of the path travelled, however, is 41.19 (36.11 + 5.08)
kilometers. Continuing on this propagation curve to the conductivity change
at 44.92 (50.00−5.08) kilometers, the field strength is found to be 0.304
mV/m. On the 15 mS/m propagation curve, the equivalent distance to this
field strength is 82.94 kilometers, which changes the effective path length
by 38.02 (82.94−44.92) kilometers. Continuing on this propagation curve, the
distance to a field strength of 0.025 mV/m is seen to be 224.4 kilometers.
The actual length of the path travelled, however, is 191.46 (224.4 +
5.08−38.02) kilometers.
[ 28 FR 13574 , Dec. 14, 1963, as amended at 44 FR 36037 , June 20, 1979; 48 FR 9011 , Mar. 3, 1983; 50 FR 18822 , May 2, 1985; 50 FR 24522 , June 11, 1985; 51 FR 9965 , Mar. 24, 1986; 54 FR 39736 , Sept. 28, 1989; 56 FR 64866 , Dec. 12,
1991; 57 FR 43290 , Sept. 18, 1992]
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Goto Section: 73.182 | 73.184
Goto Year: 2014 |
2016
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