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The curves show that the small-diameter coaxial cable standardized by
C.C.I.T.T. with the diameter ratio 1.18/4.43 mm has an optimal system size
of N ~ 900 channels when the annual costs are considered. On the basis
solely of first cost, the nearest optimal system size will be about 2700
channels.
For purposes of conversion a 2700-channel system will under all circumstances
be economically advantageous compared with the installation of new
cable.
The saving in per cent of the first cost for three sizes of system through the
use of small-diameter coaxial cable instead of the heavier 2.6/9.5 mm cable has
been calculated below. The costs of laying of the cable have not been included
since they are virtually independent of the diameter of the coaxial tubes.
System
Saving
300
50%
960
20%
1700
0
Another factor which makes small-diameter coaxial cable attractive also
on heavy-traffic routes is that the cost of standby equipment in a multitube
small-diameter coaxial system is small compared with the conditions in a
normal coaxial cable system with small number of tubes.
Fig. 2
Structure of small-diameter
type A, with balloon insulation
coaxial tube,
Small-Diameter Coaxial Tube
Two coaxial tubes have been standardized by C.C.I.T.T., both having the
dimensions 1.18/4.43 mm, which gives an impedance of 75 ohms. The difference
lies mainly in the structure of the outer conductor. Type A has an
outer conductor consisting of a 0.15 mm thick copper tape, formed to a tube
with a longitudinal slit, surrounded by two layers of copper-plated steel strip
(fig. 2). Type B has an outer conductor thickness of 0.18 mm and steel strip
without copper-plating.
Owing to their different designs of outer conductor, the two types have
slightly differing attenuation curves below about 500 kc/s. The insulation of
type A consists of a polythene tube of outside diameter equal to the inside
diameter of the coaxial tube, the polythene tube being crimped on to the
inner conductor at intervals of about 20 mm (fig. 2). This procedure makes
for simple manufacture and the inner conductor is well centered, which
ensures high homogeneity. The cable also has a high dielectric strength and
a minimum of dielectric adjacent to the inner conductor. Type B is supplied
with different forms of insulation and may have a slightly higher attenuation
constant in the high frequency range.
Fig. 3
Attenuation constant y. as function of the
frequency for small-diameter coaxial tube,
type A, at + 10°C
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