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.

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:

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.