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# Field-strength Contour Mapping

Author: Edmund A. Laport

To construct a field-strength contour map of a station, a number of field-strength versus distance curves are measured and plotted for several radials from the antenna out to a distance where the signal approaches the ambient-noise level. The location of various field strengths can then be transcribed on a map and the various signal strength contours drawn in. The choice of contours depends on the region, the population distribution, and the situation with regard to interference, if any, on the channel.

The usual purpose of such a map is to show service areas of different classes served by direct ground wave. These represent the daylight coverage, but not necessarily the nighttime coverage, because interference between ground waves and sky waves causes selective fading that may reduce the satisfactory service range appreciably under some conditions.

A typical example of the manner in which a composite-conductivity radial is computed is the following: A station on 1,000 kilocycles, operating with a power of 10,000 watts with a vertical radiator 60 degrees high and an optimum ground system, is situated on a plain having a conductivity of 7·10-14 electromagnetic unit. In one direction, this conductivity extends for a distance of 6 miles, then becomes fresh water for a distance of 11 miles with a conductivity of 10·10-14. From here on, there is sandy and rocky soil with an average conductivity of 2·10-14.

 TABLE 2.1. UNATTENUATED FIELD STRENGTHS AT 1 MILE FROM UNIFORM-CROSS-SECTION VERTICAL RADIATORS HAVING ESSENTIALLY SINUSOIDAL CURRENT DISTRIBUTION, AS FUNCTIONS OF ELECTRICAL HEIGHT G DEGREES AND RADIATED POWER.(To convert to the basis of 1 kilometer, multiply all values by 1.61. Field strengths in millivolts per meter) G Power radiated (watts) degrees 10 25 100 250 1,000 5,000 10,000 50,000 100,000 1,000,000 10 18.6 29.4 58.9 93 186 417 589 1,320 1,860 5,890 20 18.6 29.4 58.9 93 186 417 589 1,320 1,865 5,895 30 18.7 29.6 59.2 93.5 187 419 592 1,327 1,870 5,920 40 18.8 29.7 59.5 94.1 188 421 595 1,333 1,880 5,950 50 18.9 29.9 59.8 94.6 189 423 598 1,341 1,890 5,980 60 19.0 30.1 60.2 95.1 190 426 602 1,350 1,900 6,020 70 19.1 30.2 60.5 95.6 191 428 605 1,357 1,910 6,050 80 19.3 30.5 61.1 96.6 193 432 611 1,369 1,930 6,110 90 19.5 30.8 61.7 97.6 195 437 617 1,384 1,950 6,170 100 19.7 31.2 62.4 98.6 197 442 624 1,400 1,970 6,240 110 20.0 31.6 63.3 100. 200 448 633 1,420 2,000 6,330 120 20.3 32.1 64.3 101.6 203 455 643 1,442 2,030 6,430 130 20.7 32.8 65.5 103.7 207 464 655 1,470 2,070 6,550 140 21.1 33.4 66.8 105.6 211 473 668 1,500 2,110 6,680 150 21.6 34.2 68.4 108.1 216 484 684 1,532 2,160 6,840 160 22.1 35.0 70.0 110.7 221 495 700 1,568 2,210 7,000 170 22.8 36.1 72.2 114 228 511 722 1,620 2,280 7,220 180 23.6 37.4 74.7 118 236 529 747 1,675 2,360 7,470 190 24.5 38.8 77.6 123 245 549 776 1,740 2,450 7,760 200 25.5 40.3 80.8 128 255 571 808 1,810 2,550 8,080 210 26.5 41.9 84.0 133 265 594 840 1,880 2,650 8,400 220 27.3 43.2 86.5 136 273 612 865 1,940 2,730 8,650 230 27.6 43.7 87.5 138 276 618 875 1,958 2,760 8,750 240 26.8 42.4 84.9 134 268 600 849 1,900 2,680 8,490 250 24.4 38.6 77.3 122 244 547 773 1,733 2,440 7,730 260 20.5 32.4 65.0 103 205 459 650 1,453 2,050 6,500 300 6.0 9.5 19.0 30 60 135 190 427 600 1,900

The field at one mile, from Table 2.1, is 602 millivolts per meter. From Fig. 2.2 for 1,000 kilocycles and a conductivity of 7·10-14, we find that the field strength has fallen to 11 percent of the unattenuated value of 1 mile, or to 66 millivolts per meter at 6 miles. In passing over the fresh water a distance of 11 miles, a distance between 6 and 17 miles

from the antenna, the signal is decreased to 23 percent of 66 millivolts per meter, or to a value of 15 millivolts per meter.1 From here on, the conductivity of 2·10-14 attenuates the signal as listed:

 Distance from antenna (miles) Ratio of field strength at distance to that at water's edge 17 miles from antenna Expected field strength (millivolts per meter) 17 1.00 15 25 0.42 6.2 50 0.16 2.4 75 0.04 0.6 100 0.02 0.3 150 0.0064 0.096

If the ambient-noise level during daylight hours at a town on this radial at a distance of 150 miles averages 30 microvolts per meter, the signal-to-noise ratio average would be approximately 10 decibels.

In the same way, each radial can be computed, and the service range of the station in terms of signal-to-noise ratios or in terms of actual field strengths can be determined. The same procedure is followed if a directive antenna is used, except that in the latter case the field strength along the ground at 1 mile will vary with the azimuth angle depending upon the directive pattern of the array.

 1) This method is adequate for practical purposes when the differences in conductivities for different portions of the ground path are small. The method is subject to errors of importance when, for example, a land path with a conductivity of 2·10-14 changes to sea water. In such cases, more accurate results may be obtained by computing the same path in both directions by the method outlined, interchanging the locations of transmitter and receiver, and averaging the two curves point by Point along the radial.

Last Update: 2011-03-19