[0001] The present invention relates to a thin communication cable which is non-sensitive
to moisture from an electrical point of view. A communication cable is especially
intended for use in frequencies up to and including the MHz-region. The cable is provided
with at least two electric conductors and a surrounding sheath.
[0002] It has been shown that a conventional field cable can get an electric line attenuation
increase of more than 100% at for instance drizzle at the frequency 1.024 MHz which
is considerably more than previously used frequencies. The purpose of the invention
is therefore in the first place to provide a communication cable, the electric line
attenuation of which holds a low and continuous level also when the cable is subjected
to moisture or varying external environment with respect to the weather conditions
when using one and the same frequency. This is provided in connection with a communication
cable of small diameter of the kind mentioned in the introductory portion first of
all thereby that the sheath is composed of three different, mutually combined layers
- an internal layer, an external layer and an intermediate layer - and that the intermediate
layer consists of a semiconductor material. Said electrical line attenuation can be
of the magnitude ≦ 14 dB/km at 1.024 MHz.
[0003] Further advantages and characterizing features concerning the new communication cable
are apparent from the following description and from the claims.
[0004] The Fig. shown on the enclosed drawing discloses a section of a communication cable
which is formed according to the principles of the invention.
[0005] The communication cable 10 in said Fig. is a so-called two-wire-cable but the invention
can, of course, be applicable to a cable with two or more pairs of wires. The communication
cable comprises two similar separate electric conductors 12, each consisting of a
polyethylene insulated tinned copper wire 14. Moreover, the cable also comprises two
tension relieve elements 16, which in this case each consists of a polyethylene insulated
stainless steel wire 18. The two insulated conductors 12 and the two insulated steel
wires 16 are twisted to a compact quad in a manner known per se. The wires 16 and
the conductors 12 are surrounded by a sheath which has been fill extruded around the
whole quad.
[0006] As is apparent from the Fig. of the drawing the sheath is composed of three different
mutually combined layers. The layers consist of plastic, preferably polyethylene.
The internal layer 20 and the external layer 22 are non-conducting but the intermediate
layer 24 is semi-conducting.
[0007] The assembly has been made by melting the three layers in one single moment but,
of course, the use of glue or some other assembly method may be imagined.
[0008] The internal layer 20 of polyethylene can have a low density, whilst the external
layer 22 shall consist of a durable plastic with a high density or another durable
plastic, for instance polyamide. The intermediate layer 24 should have a volume resistivity
suitable for the line attenuation level, preferably of the magnitude 10²Ωcm, at at
temperature of 20°. For the purpose intended it is suitable that the internal layer
20 is between four and six times thicker than the intermediate layer 24.
[0009] It should be noticed that the sheath on the whole must be proportionately thin, for
the exemplified two-wire-cable about 0.6 mm, in which case the intermediate layer
24 should have a thickness ≦ 0.10 mm.
[0010] A communication cable of the kind stated above which has an external diameter of
nominally 4.7 mm, gives a line attenuation of 12.8 ± 0.5 dB/km at the temperature
20°C, irrespective of the outer weather conditions.
1. A thin communication cable (10) intended for use at frequences up to and including
the MHz-region, which cable from an electrical point of view is non-sensitive to moisture
and has a small diameter and is provided with at least two electric conductors (12)
with surrounding sheath (20,22,24) characterized in that the sheath is composed of
three different mutually assembled layers - one internal layer (20), one external
layer (22) and one intermediate layer (24) - and in that the intermediate layer (24)
consists of a semiconductor material.
2. Communication cable according to claim 1, characterized in that the layers (20,
22, 24) consist of plastic, preferably of polyethylene.
3. Communication cable according to claim 1 or 2, characterized that the three layers
(20,22,24) are joined, preferably by melting at extrusion in one single step.
4. Communication cable according to any of claims 1-3, characterized in that the internal
layer (20) consists of polyethylene with a low density, that the external layer (23),
consists of polyethylene with a high density or of polyamide and that the intermediate
layer (24) consists of semiconductive polyethylene with a suitable volume resistivity.
5. Communication cable according to claim 4, characterized in that the volume resistivity
of the intermediate layer (24) is of the magnitude 10²Ω cm.
6. Communication cable according to any of claims 1-5, characterized in that the internal
layer (20) is between 4 and 6 times the thickness of the intermediate layer (24).
7. Communication cable according to any of claims 1-6, characterized in that both
the internal and the external layer (20 and 22, respectively) consist of insulating
material.
8. Communication cable according to any of claims 1-7, characterized in that the external
layer consists of durable plastic.