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Matthias, F. T. (ed.) / The Wisconsin engineer
Volume 33, Number V (February 1929)

Huntley, H. R.
Intercity toll cables,   pp. 159-161


Page 159


The WISCONSIN ENGINEER
Intercity Toll Cables
                By H. R. HUNTLEY, e'21
       Wisconsin Telephone Company, Milwaukee
IN situations where the long distance telephone traffic
along     a route has grown      to the extent that it is
difficult to place the necessary number of open wire or
carrier telephone circuits on existing pole lines, the use
of intercity  toll cables is  often  considered.   This is
particularly true in those sections of the country where
large open   wire lines are frequently   subject to  storm
damage. By the use of toll cables, a relatively large
number of telephone circuits can be carried on one route
and are so enclosed and protected as to be reasonably free
from damage due to external causes. In a single toll
cable, for instance, it is practicable to carry as many cir-
cuits as could be supported by 8 or 10 heavily loaded
pole lines on an open wire basis. Where the growth in
circuits is expected to be very rapid so that it is likely
that a number of cables will be required within a reason-
able time or local conditions make it advisable, under-
  FIG. 1. Approximate Transmission
    Loss of 19 and 16 Gauge Non,
      loaded Cable Side Circuits.
and the inductance per unit length
ground conduit con-
struction proves in.
   When circuits are
placed in toll cable,
they, of course, em-
ploy conductors hav-
ing very much smal-
ler cross-section which
are much nearer to-
gether than in open
wires.  Due to    the
small copper cross-sec-
tion employed, the
resistance of the cir-
cuits is relatively high.
Likewise, due to the
small spacing between
conductors, the capac-
ities between them
are relatively great
of circuit is relatively
small. In the general case, the inductance of a non-loaded
cable circuit can be neglected at voice frequencies and the
circuit can be considered electrically to consist only of
series resistance and shunt capacity.
  Due to the relatively high resistance of the conductors
and the high mutual capacity between the conductors, the
attenuation per unit length of non-loaded cable circuit is
relatively  high.  In  addition, the attenuation  per unit
length varies approximately as the square root of the fre-
quency within the audio frequency range so that uniform
efficiency over the audio frequency range is not provided
unless some means of correction is employed. In Figure 1,
the attenuation at various frequencies for typical 16 and
19 gauge non-loaded cable circuits is shown.
  In the curves shown in Figure 1, the unit of attenuation
employed is the "decibel" (abbreviated "db"). The decibel
is defined as follows:
FIG. 2. Schematic Diagram of Two-way Telephone Repeater.
  If the ratio of two powers (such as the powers at
  the beginning and end of a section of an infinitely
  long cable circuit) is 10 l, the attenuation is said to
  be 1 decibel. The power ratio for "n" decibles is, of
  course, (lO-i)n or 10-1n.
  In order to improve the characteristics of the cable cir-
cuits, loading is generally used. Loading consists essentially
of adding inductance in series with the conductors. When
inductance is added in the proper manner, the attenuation
is reduced and, within limits, as described later, the attenu-
ation becomes more nearly constant with frequency varia-
tion over a limited range than for the case of the non-
loaded circuit. One method of applying loading is to
enclose the conductor in a sheath or wrapping of magnetic
material. This has been used both in telephone and
telegraph deep sea cables.   In land cables, however, the
use of "lumped" loading in which the inductance is in-
serted by means of coils spaced at regular intervals is
generally more economical and practicable and is very
widely used.
  At the present time the spacing between successive coils
on toll cables is generally 6,000 feet. In one of the gen-
erally used types of loading, coils having an inductance of
172 millihenrys are used on the side circuits and coils
having 63 millihenrys inductance are used on the phantom
circuits. In another commonly used type of loading, coils
having 44 millihenrys inductance are used on the side
I
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159
FEBRUARY, 1929
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