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Prediction of Medium-frequency Coverage

Author: Edmund A. Laport

The antenna and the power of the transmitter determine the unat-tenuated field strength at unit distance, which we take to be 1 mile (1.61 kilometers). Table 2.1 shows the theoretical maximum field strength in millivolts per meter at 1 mile with uniform-section vertical radiators for different practical heights (in electrical degrees) and different powers. These are the values one would measure on the 1-mile circle around the antenna if the earth were a perfect conductor and the antenna system 100 percent efficient. By proper design of ground system and proper choice of site, measurements corrected for attenuation within the first mile should approach these values closely.

The prediction of coverage proceeds directly from the use of ground-wave propagation curves, such as those included in the Federal Communications Commission Standards of Good Engineering Practice Concerning Standard Broadcast Stations. Plotted for reference values of 100 millivolts per meter unattenuated at 1 mile, they show field strength versus distance for soil conductivities ranging from the best existing in nature (over sea water) to values corresponding to the worst ordinarily encountered, and for different frequencies between 550 and 1,600 kilocycles. Figure 2.2 shows the curves for the range 970 to 1,030 kilocycles. If the soil conductivities are known throughout the region to be served by direct ground waves, the field strengths over a whole region can be predicted.

FIG. 2.2. Ground-wave field-strength versus distance curves for 970 to 1,030 kilocycles based on 100 millivolts per meter unattenuated at 1 mile along the ground. (Federal Communications Commission.)

When the electrical characteristics of the ground are not known, one with long experience in such propagation problems can often estimate it from an examination of the soils and geology of a region. Otherwise, soil-conductivity measurements must be made. Conductivity1 is not always the same over a large area. When it is not, a composite attenuation curve must be developed along each radial from the antenna base to all points of prime interest. The ground-wave propagation curves shown in Fig. 2.2 are adjusted for the actual field strength at 1 mile for the frequency, antenna, and power used, by proportion to the 100 millivolts per meter used for these curves. For example, if the expected field intensity at 1 mile is to be 1,100 millivolts per meter, then all field strengths will be eleven times those shown on the curves.

1) In speaking of soil conductivity it must be remembered that it is not a "constant" but is actually a function of frequency, in addition to being variable in depth as well as in area. Soil texture and composition are likely to vary greatly with depth, as will also the moisture content, which affects both conductivity and inductivity. Since the depth of penetration of earth currents tends to be greater with lowering of frequency, the characteristics of the lower subsoil become increasingly important for the lower frequencies. At higher frequencies, penetration depth may be determined by the inductivity, especially where the water table is relatively near the surface. The effective conductivity of any given soil is therefore an empirical value in any given area and cannot be measured statically by using small samples in the laboratory. When we speak of conductivity here, we refer to the actual effective value at a given frequency, taking account of the fact that the effective conductivity of a particular soil will, in general, be different for other frequencies.

Last Update: 2011-03-19