§  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]