Soil-conductivity Measurements

Author: Edmund A. Laport

When the soil conductivities are not known, they must be measured in some manner. The best known method is to use a test transmitter to radiate signals and to measure the field strengths with a suitable field-strength meter. If a test transmitter is used, it is best to operate at the frequency for which the data are desired. Sometimes measurements can be made on another radio station operating at some other frequency and the data converted to conductivity in the manner prescribed in detail in the FCC Standards of Good Engineering Practice Concerning Standard Broadcast Stations. This same procedure is standard with all nations that are parties to the North American Regional Broadcasting Agreement (NARBA).

For ordinary use where precision of the result is not important, and for longer distances from the transmitting site, the conductivity may be obtained by the ratio method. Measurements of field strength are made on a known frequency at large-distance intervals (such as every 5 miles or more) and a sufficient number of measurements made in each locale to establish a reliable average field strength at these distances. During the measurements the transmitting-antenna current is maintained at a constant value. Then, by taking the ratio of the measured fields at, say, 5 and 10 miles on the same radial, one can refer to the ground-wave propagation curves for that frequency and find the conductivity curve that gives the same field-strength ratio for these same distances. The conductivity curve giving the same ratio may then be taken as the value of conductivity for this interval. The same is done for other intervals of distance. The intervals may be chosen according to convenience of access and measurement and would normally include regions of special interest in coverage studies.

By this method, a few careful measurements can quickly establish a working value of conductivity to use in any subsequent studies. If the test frequency is other than that to be used for operation, the value of conductivity found is transferred to the propagation curves for the desired frequency and the field strengths calculated therefrom. If soil characteristics are obviously constant over a very large area, one ratio measurement may suffice. Where the soil or topography varies in character, the ratios and conductivities for several intervals of distance are required.

As an example of how this is applied, let us assume that measurements of field strength were made on a frequency of 1,000 kilocycles, and the result was a value of 17 millivolts per meter at 6.5 miles. At 13 miles the average value on the same radial was 4.85 millivolts per meter. The ratio is 3.5. Looking now at the propagation curves for 1,000 kilocycles (Fig. 2.2), at these same distances, it is found that a conductivity of 4·10^-14 electromagnetic unit gives this same ratio. This is taken as the conductivity for the terrain between 6.5 and 13 miles.

There is a practical precaution to observe in this process. Since the field-strength ratios must be precisely determined (because a small difference in the ratio may make a substantial error in the conductivity figure), care should be taken wherever possible to use distance intervals that will permit the two sets of measurements to be made on the same attenuator position of the field-strength meter. There is almost always a small error between attenuator positions, which is ordinarily negligible but which in this type of measurement cannot be tolerated. This error becomes inconsequential when large-distance intervals and higher frequencies are used to give rather large field-strength ratios.

A more exact method of determining conductivity is that in which a large number of field-strength measurements are made along a radial line and the complete attenuation curve is plotted from these data out to any given distance. From such a curve, the slope as a function of distance indicates the conductivity, by direct comparison with the ground-wave propagation curves. When the measured curve is plotted on exactly the same paper as used for the reference propagation curves, and to the same scales, the conductivities can be found by matching the slopes of the measured and reference curves, at the various distances.

Table 2.2 provides the basic information for the plotting of accurate ground-wave propagation curves for the frequency range 600 to 1,600 kilocycles. By interpolation between the values given, the values for all intermediate frequencies, conductivities, and distances can be obtained. For practical use, these data can be plotted on log-log coordinate graph paper, one sheet for each frequency. Other sheets should be made for intermediate frequencies, since there is enough change with frequency to require a different curve about every 50 kilocycles in this band.