The preceding chapters have discussed all the basic elements of the local- and common-battery telephone
systems except one--the transmission line. Telephone sets, switchboards, and their components were explained in detail,
but the transmission line was considered only as a metallic conductor for signals traveling between the individual
telephone sets and the interconnecting switchboard. The transmission line is a major element in all telephone systems,
however, for it presents problems which vitally affect practical operation. Chief among these problems are the power
losses along the lines, and the distortion and interference which result from interaction between adjacent lines. This
chapter explains these problems and their solutions.
Types of Transmission Lines.
Before considering the electrical
characteristics of transmission lines, this
paragraph describes the physical characteristics
of some of the types of lines in common use.
Three main classes of transmission lines are used
in military telephone installations: open-wire
lines, cables, and field wires.
a. Open-wire lines.
(1) Open-wire lines are parallel bare
mounted on the cross arms of telephone
poles, as shown in figure 50. The wires may
be made of hard-drawn copper, steel,
copper-galvanized steel, or iron. Two wires
FIGURE 50. Open-Wire Line.
constitute a line. The two wires of a line are
spaced a standard distance apart, usually