EP2525371A1

EP2525371A1 - Cable for transmitting radio frequency signals

Info

Publication number: EP2525371A1
Application number: EP11305613A
Authority: EP
Inventors: Ekkehard Schomburg; Michael Kammer
Original assignee: Alcatel Lucent SAS
Current assignee: Alcatel Lucent SAS
Priority date: 2011-05-20
Filing date: 2011-05-20
Publication date: 2012-11-21

Abstract

The invention relates to a cable (CAB1) for transmitting radio frequency signals above a minimum transmission frequency. The cable (CAB1) comprises at least one tubular electrical conductor (IC). A thickness of the at least one tubular electrical conductor (IC) perpendicular to a longitudinal direction (LA) of the cable (CAB1) is adapted to an order of magnitude of the depth of the skin effect of the minimum transmission frequency. The cable (CAB1) further comprises means (DM2) for reducing mechanical forces acting on the at least one tubular electrical conductor (IC).

Description

FIELD OF THE INVENTION

The invention relates to the field of electric lines and more particularly of cables for transmitting radio frequency signals.

BACKGROUND

Cables such as coaxial cables for transmitting radio frequency signals are used for example to connect transmitters or receivers of a base station of a radio communication system with external antennas.
Such cables must provide a low attenuation for the radio frequency signals to allow for a high energy efficiency of the base station and must provide sufficient mechanical stability for outdoor use.
The attenuation of the cable is given by cross-sectional dimensions of the cable, by a conductivity of a material used as a conductor of the cable and by an attenuation loss of a dielectric medium applied within the cable. Normally highly conductive materials such as copper or silver are applied to keep the attenuation losses within the conductor as small as possible. Such highly conductive materials are costly and are based on limited natural resources.

SUMMARY

An inner structure of a cable effects manufacturing costs and effects mechanical stability of the cable.
Therefore, it is an object of the invention to reduce manufacturing costs and save natural resource by maintaining at a same time a mechanical stability of the cable.
The object is achieved by a cable for transmitting radio frequency signals above a minimum transmission frequency. The cable comprises at least one tubular electrical conductor. A thickness of the at least one tubular electrical conductor perpendicular to a longitudinal direction of the cable is adapted to an order of magnitude of the depth of the skin effect of the minimum transmission frequency. The cable further comprises means for reducing mechanical forces acting on the at least one tubular electrical conductor. The cable may be for example a coaxial cable and the at least one tubular electrical conductor may be an inner conductor and/or an outer conductor of the coaxial cable.
Preferably, the order of magnitude and thereby the thickness of the at least one tubular electrical conductor is between 2 times and 10 times the depth of the skin effect and with a thickness in a range between 10 and 100 micrometer. More preferably, the thickness is between 10 and 20 micrometer.
The invention provides a benefit of requiring less material for the at least one tubular electrical conductor in comparison to a conventional coaxial cable having a thickness of 0.2 mm for the outer conductor and a thickness of 0.5 mm for the inner conductor by maintaining mechanical properties of the conventional cable such as a specific so-called crush resistance or a specific bending resistance depending on the operating conditions. This reduces manufacturing costs for the cable due to the high costs for highly conductive materials. Furthermore, an overall weight of the cable can be reduced making an installation of the cable easier. In additional, natural resources of metallic elements can be saved.
According to an embodiment, the means for reducing mechanical forces on the at least one tubular electrical conductor may comprise for example by one or several support layers, which are located directly adjacent to the at least one tubular electrical conductor. The at least one support layer may be for example a cable jacket, a layer between the at least one tubular electrical conductor or a material located within the at least one tubular electrical conductor. Thereby it can be avoided, that the thin layer of the at least one tubular electrical conductor obtains a sharp bending, if for example the cable is stocked or shipped or must be installed with a bending. Furthermore, the support layer improves the crush resistance of the thin tubular electrical conductor and thereby avoids a change of or a deviation from the circular geometrical form of the cross section of the tubular electrical conductor. Thereby, unfavourable electrical effects with respect to the transmitted radio frequency signals such as local impedance changes and subsequent structural return loss can be avoided.
According to a preferred embodiment, the at least one support layer may be for example an elastic layer such as a dielectric foam. This allows maintaining a cross section of the tubular electrical conductor even if the cable is bent by a swelling effect of the elastic layer, which presses the tubular electrical conductor to a further layer of the cable such as a dielectric layer or a cable jacket.
According to an additional embodiment the means for reducing the mechanical forces on the at least one tubular electrical conductor may comprise a surface of the at least one electrical conductor with a first embossed pattern such as a first corrugation or a first ribbing. The first embossed pattern has the effect, that the tubular electrical conductor comprises an expansion component perpendicular to the longitudinal axis of the cable. The expansion component can be used to prolong the tubular electrical conductor on the side of the cable, which is on the opposite side to a direction of a bending of the cable. Thereby, the crush resistance and a flexibility of the cable can be increased.
According to a further preferred embodiment, a surface of the at least one support layer facing towards the at least one tubular electrical conductor comprises a second embossed pattern such as a second corrugation or a second ribbing. Similar to the first embossed pattern, the second embossed pattern has the effect, that the support layer comprises an expansion component perpendicular to the longitudinal axis of the cable. The expansion component can be used to prolong the support layer on the side of the cable, which is on the opposite side to a direction of a bending of the cable. Thereby, mechanical forces in a longitudinal direction of the cable affecting the support layer can be reduced and also the crush resistance and the flexibility of the cable can be increased.
According to an even further preferred embodiment, the first embossed pattern fits to the second embossed pattern. The equal geometrical shapes of the surfaces of the tubular electrical conductor and the support layer allows obtaining a contact between the tubular electrical conductor and the support layer across a whole length of the cable. Furthermore, similar to plane surfaces of the tubular electrical conductor and the support layer in a longitudinal direction of the cable, it can be avoided, that the thin layer of the at least one tubular electrical conductor obtains irreversible sharp bendings such as folds or kinks, if for example the cable must be installed with a bending. In addition, a change of or a deviation from the circular geometrical form of the cross section of the tubular electrical conductor can be avoided or at least largely limited. Thereby, the above mentioned unfavourable electrical effects with respect to the transmitted radio frequency signals can be avoided.
According to an additional embodiment, the tubular electrical conductor is a surface coating such as a metal coating on the at least one support layer. Thereby, a possibility of cracks within the tubular electrical conductor can be considerably reduced, because a large amount of the mechanical forces taking effect on a single part of the cable comprising the tubular electrical conductor and the support layer can be mostly absorbed by the support layer.
According to another embodiment, the at least one tubular electrical conductor is a foil of a material such as copper, silver or copper plated aluminium.
Further advantageous features of the invention are defined and are described in the following detailed description of the invention.

BRIEF DESCRIPTION OF THE FIGURES

The embodiments of the invention will become apparent in the following detailed description and will be illustrated by accompanying figures given by way of non-limiting illustrations.

Figure 1 shows schematically a block diagram in a cross-sectional view of a cable according to a first embodiment.
Figure 2 shows schematically a block diagram of an inner part of a cable according to second embodiments in a cross-sectional view.
Figure 3 shows schematically a block diagram of an inner part of a cable according to a third embodiment in a cross-sectional view.
Figure 4 shows schematically a block diagram of an inner part of a cable according to a fourth embodiment in a cross-sectional view.
Figure 5 shows schematically a block diagram of an inner part of a cable according to a fifth embodiment in a cross-sectional view.

DESCRIPTION OF THE EMBODIMENTS

Figure 1 shows schematically an electrical cable CAB1, which comprises an inner tubular electrical conductor IC, a first insulating layer DM1 surrounding the inner tubular electrical conductor IC, an outer tubular electrical conductor OC surrounding the first insulating layer DM1 and a cable jacket CJ surrounding the outer tubular electrical conductor OC. The cable jacket CJ is for example a plastic sheath such as polyethylene (PE) or polyvinyl chloride (PVC). Such a cable is well-known as a coaxial cable.
The first insulating layer DM1 may be for example a solid dielectric such as polytetrafluoroethylene (PTFE) or polyethylene or a dielectric foam such as a polyethylene foam.
Alternatively, the electrical cable may comprise a single tubular electrical conductor, an insulating layer surrounding the tubular electrical conductor and a cable jacket surrounding the insulating layer.
In a further alternative, the electrical cable may comprise more than two tubular electrical conductors. For example the electrical cable may comprise an inner tubular electrical conductor, a first insulating layer surrounding the inner tubular electrical conductor, a first outer tubular electrical conductor surrounding the first insulating layer, a second insulating layer surrounding the first outer tubular electrical conductor, a second outer tubular electrical conductor surrounding the second insulating layer and a cable jacket surrounding the second outer tubular electrical conductor OC. Such a cable is well-known as a triaxial cable.
The inner tubular electrical conductor IC may have a form of a pipe with a flat surface (see Figure 1).
The outer tubular electrical conductor OC may also have a form of a pipe with a corrugated surface (see Figure 1). The pipe can be made as a seamlessly drawn tube or formed and welded from metal strips.
A material of the inner tubular electrical conductor IC may be a conductive material such as copper, silver or gold.
A material of the outer tubular electrical conductor OC may be a conductive material such as copper, silver, gold or aluminium.
The cable CAB1 may be used for example for transmitting radio frequency signals via one or two jumper cables and a feeder cable between a transceiver of a base station of a radio communication system and an antenna system of the base station.
A thickness of the inner tubular electrical conductor IC perpendicular to a longitudinal axis LA of the cable CAB1 is adapted to an order of magnitude of the depth of the skin effect of a minimum transmission frequency ν _MIN of the radio frequency signals to be transmitted over the cable CAB1.
The skin depth δ can be approximated by following equation: $δ = \sqrt{\frac{2 ρ}{ωμ}}$

where
δ : resistivity of the conductor material
ω = 2πν : angular frequency of current (ν: frequency)
µ = µ₀ · µ _r : absolute permeability of the conductor material (µ₀ : permeability of free space; µ _r : relative permeability of the conductor material)
Due to the skin effect over 98 % of an alternating electric current (AC) flows within a layer of 4 times the skin depth from the surface of the conductor material.
The skin depth decreases with increasing frequency according to equation (1). Therefore, the thickness of the inner tubular electrical conductor IC is designed for the minimum transmission frequency ν _MiN and the cable CAB1 is able to transmit all radio frequency signals with a frequency equal to or above the minimum transmission frequency ν _MIN without perceptible attenuation. For radio frequency signals with a frequency below the minimum transmission frequency ν _MIN transmission losses will increase with decreasing frequency.
The thickness of the inner tubular electrical conductor IC may be also adapted to the resistivity and the permeability of the conductor material and with respect to the minimum transmission frequency ν _MIN.
The minimum transmission frequency ν _MIN of the radio frequency signals may be in a range between 100 MHz to 10 GHz.
The order of magnitude of the depth of the skin effect may be in a range between 2 times to 10 times the depth of the skin effect. A thickness of the inner tubular electrical conductor IC of 2 times the depth of the skin effect may be used in short cables (e.g. length of the cable up to 5 m), a thickness of 5 times the depth of the skin effect may be used in mid-length cables (e.g. length of the cable between 5 m and 20 m), and a thickness of 10 times the depth of the skin effect may be used in longer cables (e.g. length of the cable equal to or above 20 m). The thickness of the inner tubular electrical conductor IC may be in a range between 10 and 100 micrometer, and the thickness of the inner tubular electrical conductor IC preferably depends on a length of the cable CAB1 and/or on operating conditions of the cable CAB1 such as indoor use or outdoor use (e.g. wind action on the cable CAB1) or protecting the cable CAB1 by a cable channel in comparison to hauling the cable CAB1 in a free space.
Preferably, the thickness of the inner tubular electrical conductor IC is in a range between 10 micrometer and 50 micrometer and even more preferably in a range between 10 micrometer and 20 micrometer.
The inner tubular electrical conductor IC is for example a thin foil.
The cable CAB1 further comprises means for reducing mechanical forces acting on the inner tubular electrical conductor IC. The means for reducing the mechanical forces are provided according to the embodiment of Figure 1 by a second cylindrical insulating layer DM2 located within the inner tubular electrical conductor IC and filling as a bulk material a hollow space of the inner tubular electrical conductor IC. Preferably, the second insulating layer DM2 is in direct contact to the inner tubular electrical conductor IC across the whole inner surface of the inner tubular electrical conductor IC.
The second insulating layer DM2 acts as a support layer for the inner tubular electrical conductor IC. Preferably, the second insulating layer DM2 is an elastic layer. More preferably, a material of the elastic layer comprises a dielectric foam such as a polyethylene foam.
In Figure 1 an image section IS of the transition between the second insulating layer DM2, the inner tubular electrical conductor IC and the first insulating layer DM1 is highlighted. Figures 2 to 4 show further alternatives for the transition between the second insulating layer DM2, the inner tubular electrical conductor IC and the first insulating layer DM1 according to further embodiments of the invention.
Figure 2a shows schematically an inner part of an electrical cable CAB2 in a cross sectional view, which comprises an inner tubular electrical conductor IC-1 and a first insulating layer DM1-1 surrounding the inner tubular electrical conductor IC-1.
The first insulating layer DM1-1 may be for example a solid dielectric such as PTFE, PE or a dielectric foam such as a polyethylene foam.
A thickness of the inner tubular electrical conductor IC-1 is preferably in a range between 50 micrometer and 100 micrometer.
The electrical cable CAB2 further comprises a second cylindrical insulating layer DM2-1 as a bulk material surrounded by the inner tubular electrical conductor IC-1 for reducing the mechanical forces on the inner tubular electrical conductor IC-1. Preferably, the second insulating layer DM2-1 is an elastic layer and comprises a material as a dielectric foam or a polyethylene foam.
The electrical cable CAB2 may be a cable with a single electrical conductor, with two electrical conductors for example equally to a coaxial cable, with three electrical conductors for example equally to a triaxial cable or with more than three electrical conductors. The inner tubular electrical conductor IC-1 is for example a thin foil.
The means for reducing the mechanical forces according to the embodiment of Figure 2a are further provided by a first embossed pattern of a surface of the inner tubular electrical conductor IC-1. The first embossed pattern comprises a first corrugation or a first ribbing with a smoothed wave form EP1 such as a sine wave as shown in Figure 2b i). Wave toughs of the first corrugation are preferably in contact to a surface of the second cylindrical insulating layer DM2-1 and wave crests of the first corrugation are preferably in contact to an inner surface of the first insulating layer DM1 -1. A distance between the wave toughs and the wave crests may be in a range between 0.5% and 5% of the cable diameter. A periodicity of the waves of the wave form may be in a range between 1 % and 20% of the cable diameter.
In further alternatives, the electrical cable CAB2 may comprise the inner tubular electrical conductor IC-1 with a triangle wave form EP2 (see Figure 2b ii)) or with a wave form of an isosceles trapezium EP3 (see Figure 2b iii)).
Figure 3 shows schematically an inner part of an electrical cable CAB3 in a cross sectional view, which comprises an inner tubular electrical conductor IC-2 and the first insulating layer DM1-1 surrounding the inner tubular electrical conductor IC-2. The electrical cable CAB3 further comprises a second cylindrical insulating layer DM2-2 as a bulk material and surrounded by the inner tubular electrical conductor IC-2 for reducing the mechanical forces on the inner tubular electrical conductor IC-2. Preferably, the second cylindrical insulating layer DM2-2 is an elastic layer and comprises a material as a dielectric foam or a polyethylene foam.
The electrical cable CAB3 may be a cable with a single electrical conductor, with two electrical conductors for example equally to a coaxial cable, with three electrical conductors for example equally to a triaxial cable or with more than three electrical conductors.
The means for reducing the mechanical forces according to the embodiment of Figure 3 are further provided by a first embossed pattern of a surface of the inner tubular electrical conductor IC-2 with one of the alternative wave forms EP1, EP2, EP3 as discussed according to the inner tubular electrical conductor IC-1.
In comparison to the embodiments of Figure 2a and 2b, a surface of the second cylindrical insulating layer DM2-2 facing towards the inner tubular electrical conductors IC-2 comprises a second embossed pattern. The second embossed pattern of the second cylindrical insulating layer DM2-2 preferably fits to the first embossed pattern of the inner tubular electrical conductor IC-2 by having an equal form. This means, that the inner surface of the inner tubular electrical conductor IC-2 is preferably in direct contact to the lateral surface of the second cylindrical insulating layer DM2-2 across a whole length of the electrical cable CAB3.
The inner tubular electrical conductor IC-2 is for example a thin foil. In a further alternative, the inner tubular electrical conductor IC-2 is a surface coating on the lateral surface of the second cylindrical insulating layer DM2-2 produced for example by sputter deposition or physical vapour deposition.
Figure 4 shows schematically an inner part of an electrical cable CAB4 in a cross sectional view, which comprises the inner tubular electrical conductor IC-2 and a first insulating layer DM1-2 surrounding the inner tubular electrical conductor IC-2.
The first insulating layer DM1-2 may be for example a solid dielectric such as PTFE, PE or a dielectric foam such as a polyethylene foam.
The electrical cable CAB4 further comprises the second cylindrical insulating layer DM2-2 as a bulk material and surrounded by the inner tubular electrical conductor IC-2 for reducing the mechanical forces on the inner tubular electrical conductor IC-2.
The electrical cable CAB4 may be a cable with a single electrical conductor, with two electrical conductors for example equally to a coaxial cable, with three electrical conductors for example equally to a triaxial cable or with more than three electrical conductors.
The means for reducing the mechanical forces according to the embodiment of Figure 4 are further provided by a first embossed pattern of a surface of the inner tubular electrical conductor IC-2 with one of the alternative wave forms EP1, EP2, EP3 as discussed according to the inner tubular electrical conductor IC-1.
In comparison to the embodiments of Figure 3, a surface of the first insulating layer DM1 -2 facing towards the inner tubular electrical conductors IC-2 comprises a third embossed pattern. The third embossed pattern of the first insulating layer DM1-2 preferably fits to the first embossed pattern of the inner tubular electrical conductor IC-2 by having an equal form EP1, EP2, EP3. This means, that the outer surface of the inner tubular electrical conductor IC-2 is preferably in direct contact to the lateral inner surface of the first insulating layer DM1-2 over a whole length of the electrical cable CAB4 for further reducing the mechanical forces on the inner tubular electrical conductor IC-2.
Figure 5 shows schematically an inner part of an electrical cable CAB5 in a cross sectional view, which comprises the inner tubular electrical conductor IC-1 and the first insulating layer DM1 -1 surrounding the inner tubular electrical conductor IC-1.
The electrical cable CAB5 may be a cable with a single electrical conductor, with two electrical conductors for example equally to a coaxial cable, with three electrical conductors for example equally to a triaxial cable or with more than three electrical conductors.
The electrical cable CAB5 further comprises a second cylindrical insulating layer DM2-3 surrounded by the inner tubular electrical conductor IC-1 for reducing the mechanical forces on the inner tubular electrical conductor IC-1. The cylindrical insulating layer DM2-3 is a tube with a hollow space HS inside the tube. A thickness T-DM2-4 of the tube may be in a range between 2 - 12 % of the cable diameter.
A material of the tube may be an elastic material such as a polyethylene foam.
The means for reducing the mechanical forces according to the embodiment of Figure 5 may be further provided by the first embossed pattern of the surface of the inner tubular electrical conductor IC-1 with the one of the alternative wave forms as discussed according to the inner tubular electrical conductors IC-1, IC-1a and IC-1b shown in Figure 2a and 2b.
In an alternative, instead of the inner tubular electrical conductor IC-1 the inner tubular electrical conductor IC of Figure 1 with a flat surface in a longitudinal direction of the electrical cable CAB5 and with a circular surface perpendicular to the longitudinal direction of the electrical cable CAB5 may be applied with the first insulating layer DM1-1 and the second cylindrical insulating layer DM2-3 staying in contact to opposite surfaces of the inner tubular electrical conductor IC across a whole length and a whole perimeter of the inner tubular electrical conductor IC.
The embodiments of Fig. 3 und Fig. 4 according to the inner tubular electrical conductors IC-2, IC-3 may be also applied to one or several of the outer tubular electrical conductors of the electrical cables CAB3, CAB4. In such cases, a similar supporting functionality of the second cylindrical insulating layers DM2-2, DM2-3 may be taken over by the first insulating layers DM1-2, DM1-3 and a similar supporting functionality of the first insulating layers DM1-2, DM1-3 may be taken over by the cable jacket CJ. In further alternatives, only one or several of the outer tubular electrical conductors of the electrical cables CAB3, CAB4 comprise a thickness perpendicular to the longitudinal direction of the cable CAB3, CAB4 adapted to an order of magnitude of the depth of the skin effect of the minimum transmission frequency for the which the electrical cable CAB3, CAB4 is to be designed and the cable CAB3, CAB4 only comprises means for reducing the mechanical forces on the one or several of the outer tubular electrical conductors. This means, that the cable CAB3, CAB4 may comprise an inner tubular electrical conductor of a well-known type with a well-known thickness of about 0.5 millimeter.

Claims

A cable (CAB1, CAB2, CAB3, CAB4, CAB5) for transmitting radio frequency signals above a minimum transmission frequency, said cable (CAB1, CAB2, CAB3, CAB4, CAB5) comprising at least one tubular electrical conductor (IC, IC-1, IC-2), a thickness of said at least one tubular electrical conductor (IC, IC-1, IC-2) perpendicular to a longitudinal direction of said cable (CAB1, CAB2, CAB3, CAB4, CAB5) is adapted to an order of magnitude of the depth of the skin effect of said minimum transmission frequency and wherein said cable (CAB1, CAB2, CAB3, CAB4, CAB5) further comprises means for reducing mechanical forces acting on said at least one tubular electrical conductor (IC, IC-1, IC-2).
Cable (CAB1, CAB2, CAB3, CAB4, CAB5) according to claim 1,
wherein said order of magnitude is from 2 times to 10 times the depth of the skin effect and wherein said minimum transmission frequency is in a range between 100 MHz to 10 GHz.
Cable (CAB1, CAB2, CAB3, CAB4, CAB5) according to any of the preceding claims, wherein said means for reducing said mechanical forces on said at least one tubular electrical conductor (IC, IC-1, IC-2) comprise at least one support layer adjacent to said at least one tubular electrical conductor (IC, IC-1, IC-2).
Cable (CAB1, CAB2, CAB3, CAB4, CAB5) according to claim 3,
wherein said at least one support layer is either of the following: a cable jacket (CJ), a layer (DM1, DM1-1, DM1-2) between said at least one tubular electrical conductor (IC, IC-1, IC-2) and a further electrical conductor (OC) or a material (DM2, DM2-1, DM2-2, DM2-3) located within said at least one tubular electrical conductor (IC, IC-1, IC-2).
Cable (CAB1, CAB2, CAB3, CAB4, CAB5) according to any of the preceding claims 3 or 4, wherein said at least one support layer is an elastic layer.
Cable (CAB1, CAB2, CAB3, CAB4, CAB5) according to claim 5,
wherein a material of said elastic layer comprises a dielectric foam.
Cable (CAB2, CAB3, CAB4, CAB5) according to any of the preceding claims, wherein said means for reducing said mechanical forces on said at least one tubular electrical conductor (IC, IC-1, IC-2) comprise a surface of said at least one electrical conductor (IC, IC-1, IC-2) with a first embossed pattern (EP1, EP2, EP3).
Cable (CAB2, CAB3, CAB4, CAB5) according to claim 7, wherein said first embossed pattern (EP1, EP2, EP3) is a first corrugation or a first ribbing.
Cable (CAB3, CAB4) according to any of the preceding claims 3 to 8, wherein a surface of said at least one support layer (DM2-2, DM1-2) facing towards said at least one tubular electrical conductor (IC-2) comprises a second embossed pattern (EP1, EP2, EP3).
cable (CAB3, CAB4) according to claim 9, wherein said second embossed pattern (EP1, EP2, EP3) is a second corrugation or a second ribbing and wherein said first embossed pattern fits to said second embossed pattern.
Cable (CAB3, CAB4) according to any of the preceding claims 3 to 10, wherein said at least one tubular electrical conductor (IC-2) is a surface coating on said at least one support layer (DM2-2).
Cable (CAB1, CAB2, CAB3, CAB4, CAB5) according to any of the preceding claims, wherein said at least one tubular electrical conductor (IC, IC-1, IC-2) is a foil.
Cable (CAB1, CAB2, CAB3, CAB4, CAB5) according to any of the preceding claims, wherein said cable (CAB1, CAB2, CAB3, CAB4, CAB5) is a coaxial cable and wherein said at least one electrical conductor is an inner conductor (IC, IC-1, IC-2) and/or an outer conductor (OC) of said coaxial cable.
Cable (CAB1, CAB2, CAB3, CAB4, CAB5) according to claim 13,
wherein said coaxial cable is a feeder cable or a jumper cable for transmitting radio frequency signals between a base station of a radio communication system and an antenna system of the base station.