Goto Section: 73.182 | 73.184 | Table of Contents
FCC 73.183
Revised as of October 1, 2010
Goto Year:2009 |
2011
§ 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 uV/m (0.5
Vm) contour using a 6 dB protection factor. The distance to the 500
uV/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]
Goto Section: 73.182 | 73.184
Goto Year: 2009 |
2011
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