c. Factors influencing attenuation. The attenuation of a pair usually is expressed in dB per loop mile of line.
One loop mile is 1 mile of two-wire line--that is, 2 miles of wire. The attenuation per loop mile of an open-wire pair
depends on the size, spacing, and material of the conductors, and on the kind and number of insulators and their condition
(wet or dry). Ice has a large effect on attenuation, particularly at high frequencies. The frequency of the current has an
important effect on attenuation. Under average conditions, an open-wire pair has less attenuation per mile than either
cable or field wires. For example, the dry weather attenuation at 1,000 hertz for 8-inch spaced wires of 165-mil, 128-mil,
and 104-mil copper is, respectively, .032, .049, and .070 dB per loop mile at 68€ F. An approximate expression for this
attenuation is as follows:
62.
Loading of Transmission Line.
Loading, as applied to a transmission line, is the method of increasing the series inductance of a line by the
addition of external inductance. Its purpose is to improve the performance of the line by reducing attenuation and
distortion. Loading may be either: lumped loading or continuous loading.
a. Lumped loading is effected by the addition of loading coils (which have relatively high inductance) at regular
intervals along the line. The loading coils used at present consist of two windings on a molybdenum permalloy dust core
(C, fig. 60). With this core material, high inductance is obtained with a rather small coil. Before the molybdenum
permalloy dust core was perfected, loading coils had iron dust or permalloy dust cores illustrated in A and B. For the
same inductance value, these coils are physically larger.
b. The insertion of heavy loading coils in submarine telephone and telegraph cables would subject them to
excessive strain at the points of insertion. For this reason, a different method of increasing series inductance, called
continuous loading, is used. Continuous loading is effected by wrapping the cable conductor with a tape or wire of
magnetic material, such as iron or permalloy. Because such loading distributes the inductance continuously along the
line, it causes the line to behave like one with distributed constants.
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