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 (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 × 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 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]