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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.
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.
Last Update: 2011-03-19