a unit length, usually a mile. The numerical values of these constants not only depend on the size of the conductors, their
spacing, and insulation, but also vary with the frequency of the transmitted signal and the weather conditions.
(1) Constants. The four line parameters mentioned above are distributed along the entire length of the line and
for this reason are called distributed constants. If the parameters had been concentrated in one place, for example, the way
a resistor concentrates resistance, they would have been called lumped constants. In the study of transmission lines, a
transmission line is shown in the form of an equivalent circuit in which the distributed constants for a given length are
shown in the form of lumped constants (fig. 53). The series resistance, R, series inductance, L, shunt capacitance, C, and
shunt conductance, G, for a unit length of 1 mile, are shown as lumped constants.
FIGURE 54. Long Line and Its Equivalent Circuit (Distributed Constants).
A long transmission line can be considered to be made up of a series of unit sections (fig. 54). In this case, five sections
are used to represent the 360-mile line so that each section represents a 72-mile length of the line. Any convenient length
can be used as the unit length, but 1 mile is the preferred length. By using this method to represent transmission lines, the
study of their behavior is simplified greatly.
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