Sec. 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 
Sec. 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 Sec. 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 
Sec. 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 Sec. 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 V/m

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(0.5 Vm) contour using a 6 dB protection factor. The distance to the 500 
V/m groundwave contour of the 1 kW station is determined by the 
use of the appropriate curve in Sec. 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 x 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 Sec. 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 Sec. 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 kilomteres.
    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]