EP2351051A1

EP2351051A1 - Coaxial cable

Info

Publication number: EP2351051A1
Application number: EP09744363A
Authority: EP
Inventors: Stefan Metz; Stefan Schaelle
Original assignee: Huber and Suhner AG
Current assignee: Huber and Suhner AG
Priority date: 2008-10-30
Filing date: 2009-10-20
Publication date: 2011-08-03
Also published as: WO2010049315A1; IL212446A0; CN102197442A; JP2012507128A; CH699805A2; US20110209892A1

Abstract

The invention relates to a coaxial cable (10) comprising a central inner conductor (11) that is concentrically surrounded by an outer conductor (12) at a specified distance, wherein the intermediate space between the inner conductor (11) and outer conductor (12) is filled with a dielectric (13). A coaxial cable having especially good mechanical and electrical properties is achieved by the dielectric (13) comprising a plurality of hollow fibers (14, 15, 19) that extend in the longitudinal direction of the cable and are composed of a glass, and by at least the inner conductor (11) having a thermal expansion coefficient that is matched to the dielectric (13) with regard to a least possible temperature-related phase change in the coaxial cable (10).

Description

coaxial

Coaxial cables have a central inner conductor, which is surrounded concentrically at a predetermined distance from an outer conductor. The gap between the central inner conductor and the concentric outer conductor is filled with a dielectric. In demanding applications, such as in satellite technology in space, for coaxial cable in addition to the suitability for the highest frequencies in the GHz range, especially a very low attenuation (in dB / m) and a linear and / or going to zero Dependence of the phase shift on the temperature over a wide temperature range of eg -55 ⁰ C to +1 25 ⁰ C of particular importance.

A very low damping can be achieved, for example, by filling the intermediate space between the central inner conductor as completely as possible with air (relative dielectric constant ε _r = 1). On the other hand, the dielectric in many cases at the same time takes over the task of stabilizing and fixing the central inner conductor in the center, so that the concentric structure of the cable is maintained even during bending or twisting of the cable.

A high proportion of air in the space or dielectric can be achieved in various ways:

1 . By locating spacers or elements that support and fix the inner conductor to the outer conductor (see, for example, EP-A2-0 899 750 or US-A-5,742,002).

2. By low density foamed or porous material (see e.g.

34 1 5 746 or WO-Al -2007/147271 with further references). Known technologies are paste extrusion, melt extrusion, powder sintering or plastic banding. 3. By tightly packed bundles of pipes or hoses running in the cable longitudinal direction.

The variant (1) is very complex to manufacture and is - especially with small cable diameters - to realize only with difficulty.

Variant (2) is used in a variety of forms but is limited in either porosity or mechanical strength.

With regard to variant (3) of the above list, various solutions have been proposed in the past:

The document DE-Al -I 440 771 describes a coaxial cable, in particular a method for producing such a cable, in which hoses or tubes are placed in the longitudinal direction parallel to the central axis of the cable in a single layer around the inner conductor as spacers. The tubes or tubes have relatively thin walls, so that the volume between the inner conductor and the outer conductor or -mantel is filled primarily with air or possibly with a gas. The hoses or pipes are made of a foam plastic or elastomer. Hoses are understood to mean both filled and unfilled pipes as well as rod shapes, regardless of whether they are hollow or not.The hoses thus defined can also be produced from glass fibers impregnated or reinforced with various materials, which also have a reinforcement made of silicone The same applies to the related document GB-909,343.

From the document DE-AS-I 059 522 a coaxial cable is known in which the conductors are supported by means of one or more tubular insulating elements against each other and isolated from each other. The insulating elements are single-wall helical to isolate the th inner conductor wound or arranged parallel to the inner conductor. The insulating elements are constructed in two parts, with an inner element with helical slot for improving the bending behavior of the cable and a thin sheath of the inner member. These are preferably made of polystyrene.

Document CH-257 548 discloses a coaxial cable (Figure 2) in which a hollow inner conductor is spaced and insulated from the outer conductor by a plurality of single-ply spirally wound tubes of ethylene polymer. The diameter of the tubes is chosen as large as possible in order to provide as little solid material between the inner and outer conductors. In order to provide the necessary support for the inner conductor, the tubes are produced with oversize and their cross section is deformed so far from the circular cross section that the desired diameter ratio is obtained and the necessary pressure is exerted on the inner conductor.

DE-A1-1 99 56 641 describes a coaxial cable in which the inner conductor is separated from the outer conductor by a plurality of strands, preferably monofilaments, and supported against it. The strands have as a dielectric material polyetheretherketones, polyaryletherketones or polyetherimides. Only the spaces between the strands are filled with air.

The document DE-PS-902 865 discloses a coaxial cable, in which around the inner conductor spacers hoses of insulating material, such as polyethylene, are herumges around, on which still a harness made of plastic is applied. Hoses of originally round cross-section are used, which are compressed in the tension occurring during stranding and by the subsequent banding in the radial direction on the inner conductor so that they lie very close to each other and thereby assume an approximately sector-shaped cross section. Another document, the CB-535,743, is directed to a coaxial cable in which the inner conductor is surrounded by a plurality of hoses or cables of a low dielectric loss material such as "polythene" or rubber.

Document CB-A-2 374 721 discloses a coaxial cable in which a hollow inner conductor is surrounded by a plurality of filamentary insulating strands aligned parallel to the inner conductor. The strands, which are preferably made of methylene, are characterized by a high elastic load under tension in the longitudinal direction, and serve to improve the mechanical properties of the cable.

Finally, from JP-A-71 69341 a radiation-resistant coaxial cable is known, in welhern the space between the inner conductor and outer conductor is filled with glass fibers or ceramic fibers.

With regard to a minimal temperature-induced phase change, solutions with a special structure of the dielectric are known (US Pat. No. 4,287,384), as well as solutions (US Pat. No. 3,909,555 or US Pat. No. 3,971,880) in which the inner conductor consists of a consists of a metal having a good conductive layer and a low thermal expansion. The dielectric in the latter case consists of finely divided quartz, magnesium oxide or aluminum oxide.

A known high quality coaxial cable is the SUCOFLEX 404 coaxial cable offered by the Applicant, this coaxial cable with an impedance of 500hm, an operating frequency of 26.5GHz and an outside diameter of 5.5mm has an attenuation of about 25.6GHz l, 1 5 dB / m. The (non-linear) phase change is 750ppm in the temperature range between -55 ⁰ C and +1 25 ⁰ C only. The consists of silver plated copper center conductor is here of an extruded PTFE dielectric with ultra-small Density, which has a relative dielectric constant of 1.26. has. The non-linear course of the phase change with temperature results from the non-linear temperature dependence of the portion of the phase change caused by the dielectric constant of the PTFE. According to the findings of the inventors, the small absolute value of the phase change results from the fact that the negative temperature response of the portion of the phase change caused by the dielectric constant of the PTFE is largely compensated by the positive temperature characteristic of the portion of the phase change caused by the thermal expansion of the inner conductor and outer conductor.

Although the described "SUCOFLEX 404" coaxial cable has exceptionally good attenuation, temperature phase change and field of application properties, it still desires to further improve such a cable in all respects, and more particularly, such an improved cable should be sufficiently flexible, comparatively simple and inexpensive can produce and even with small diameters in the millimeter range have a high quality.

The invention is therefore based on the object, a flexible or semi-flexible Koaxialka- to create, which avoids the disadvantages of known coaxial cable. The cable should be characterized in particular by a minimal attenuation. Furthermore, it should have a minimal temperature response of the phase shift. In addition, the dependence of the phase shift of the temperature over a wide temperature range should be as linear as possible. Finally, the cable is to be used without any problems to +1 25 ⁰ C and down to small outside diameters of a few (eg 6) millimeters, even under difficult conditions, in particular in space and in a wide temperature range of at least -55 ⁰ C a comparatively simple let produce.

The object is solved by the entirety of the features of claim 1. It is essential for the coaxial cable according to the invention that the dielectric between the inner conductor and the concentric outer conductor is made up of a plurality of hollow fibers running in the longitudinal direction of the cable, and at least the inner conductor has a coefficient of thermal expansion which limits the dielectric with respect to the lowest possible temperature Phase change in the coaxial cable is adjusted.

Hollow fibers drawn from glass can be made or drawn very uniformly down to outside diameters of less than 1 mm and wall thicknesses of less than 0.05 mm. Such glass hollow fibers or glass capillaries have a high mechanical stability in these dimensions and are also bendable within wide limits without breaking. The glass hollow fibers are tensile and can be stranded without difficulty and can thus be integrated into conventional cable manufacturing processes. In particular, coaxial cables equipped with glass hollow fibers as dielectrics are insensitive to high and low temperatures, vibrations and other mechanical effects. The voids in the hollow fibers and between the hollow fibers may optionally be evacuated over the entire cable length or filled with special gases or gas mixtures, if desired or required in certain applications.

The glass hollow fibers are good electrical insulators and chemically neutral or comparatively insensitive to external influences. Because of the low possible wall thicknesses, they build up a dielectric that has a high porosity and therefore a high proportion of air. In particular, the proportion caused by the glass hollow fibers in the temperature variation of the phase change is linear and can be close to zero, depending on the type of glass.

An embodiment of the coaxial cable according to the invention is characterized in that the portion of the phase change caused by the dielectric has an approximately decreasing temperature response, and that the thermal expansion coefficient of the Inner conductor corresponding to approximately zero. Preferably, the thermal expansion coefficient of the outer conductor is approximately zero. As a result, the temperature-induced phase changes over a wide temperature range can be kept very low, the remaining temperature dependence is largely linear.

Another embodiment of the invention is characterized in that the hollow fibers are arranged in a plurality of concentric layers. In this case, an intermediate layer, in particular of an electrically insulating material, preferably a plastic, is preferably provided between the concentric layers of the hollow fibers, which increases the stability of the cable construction. The mechanical properties are further improved if the hollow fibers of the individual concentric layers are stranded in each case, wherein the stranding can be in opposite directions or in the same direction from layer to layer.

Basically, the entire dielectric can be constructed with hollow fibers of one type. However, a higher flexibility in the design of the cable is achieved in that the hollow fibers of different concentric layers are designed differently in their construction and / or material and / or their dimensions.

Hollow fibers which consist essentially of quartz glass or SiO 2 have proven to be particularly favorable with regard to the electrical and mechanical and processing properties, with hollow fibers having a circular cross section and an outside diameter between 0.01 mm and 4 mm, in particular between 0.01 mm and 1 mm, to be preferred. The hollow fibers have a wall thickness between 0.001 mm and 2 mm, in particular between 0.001 mm and 0.05 mm.

A particularly good long-term stability of the coaxial cable can be achieved in that the hollow fibers are externally provided with a protective cover layer, preferably consists of an acrylate or silicone or a ceramic or a fluoroethylene-propylene (FEP) or polyethylene and has a layer thickness in the range of 10 .mu.m.

Since the effect of the hollow glass fiber dielectric on the temperature-dependent phase change over conventional dielectrics, e.g. PTFE, is comparatively low, an inner conductor is preferably used in the coaxial cable, which has a thermal expansion coefficient of less than or equal to 5ppm / K. In particular, the inner conductor consists of FeNi36Ag ("Invar") or Kovar or glass and is externally provided with an electrically highly conductive cladding layer, in particular of Ag.The inner conductor advantageously has an outer diameter of less than 2 mm.

In contrast, the outer conductor can consist of a wound CuAg band.

The invention will be explained in more detail below in connection with the drawing with reference to embodiments. Show it

Fig. 1 in cross section the basic structure of a coaxial cable according to an embodiment of the invention;

Fig. 2 is a perspective view of the stranding of the hollow fiber layers of

Coaxial cable according to Fig. 1;

Fig. 3 in a photo the cross section of a prototype of the coaxial cable according to

Fig. 1;

4 shows in a simplified representation the construction of the inner conductor of the coaxial cable from FIG. 1 as provided with a conductive coating layer

Core with adapted thermal expansion; 5 shows in a simplified representation the construction of a hollow fiber of the coaxial cable from FIG. 1 as a glass hollow fiber provided with a protective covering layer;

6 shows a diagram of the temperature dependence of the phase change Δp in a coaxial cable with a portion caused by a dielectric of acrylate-coated SiO 2 hollow fibers (curve al) and a portion caused by a CuAg inner conductor (made of silver-plated copper) (curve b 1); and

7 shows a diagram of the temperature dependence of the phase change Δp in a coaxial cable with a portion caused by a dielectric of acrylate-coated SiO 2 hollow fibers (curve al) and a portion caused by a silver-plated FeNi36Ag inner conductor (from "Invar") (curve b2 ).

The large porosity required for small damping with the resulting high proportion of air is known in the art, e.g. achieved by a high degree of foaming. At present, PTFE is predominantly used to achieve this property, as is the case with the commercially available coaxial cable of the type "SUCOFLEX 404." Due to the high degree of foaming or the associated porosity, however, the mechanical stability decreases With the lowest possible proportion of material or a high proportion of air, professional extrusion can be achieved today, but there are limits to porosity (60-80%) and mechanical stability.

In the present invention, the required properties are achieved by the choice of hollow fibers from a glass. By using glass hollow fibers, the Air ratio are increased significantly, while the mechanical properties are particularly good when the appropriate glass is used. Such a technology is also economical to produce compared to extrusion because it can be processed faster. Through the use of hollow fibers, preferably of quartz or silicon dioxide or SiO 2, a dielectric with good strength and a very high porosity of about 92% can be achieved. SiO2 has very good properties in terms of thermal expansion and electrical values.

In Fig. 1, the basic structure of a coaxial cable according to an embodiment of the invention is shown in cross section; FIG. 2 shows a perspective view of the stranding of the hollow fiber layers of the coaxial cable according to FIG. 1. The coaxial cable 1 0 of Fig. 1 and 2 comprises a central inner conductor 1 1, which is surrounded concentrically at a predetermined distance from an outer conductor 1 2. The annular gap between inner conductor 1 1 and outer conductor 1 2 is characterized by a high porosity dielectric, i. a high proportion of air-filled cavities H l (within the hollow fibers) and H2 (between the hollow fibers) filled. The conductor arrangement is externally surrounded by an insulating, protective jacket 22 (indicated by dashed lines in FIG. 1).

The dielectric consists of stranded hollow fibers 14, 1 5, which are arranged in two concentrically arranged layers. The hollow fibers 14 of the inner layer and the hollow fibers 1 5 of the outer layer are stranded by themselves. The stranding according to FIG. 2 is a so-called SZ stranding (ie opposing layers of stranded hollow fibers). However, it is also possible to use a "Unilay" stranding (same orientation in layers), and it is also possible to use a different fiber type (fiber material, diameter, wall thickness) for each layer plane The number of layers is not limited It may also be advantageous to vary the slope in the stranding operation with +/- 1 0% of the strand length arbitrarily. Condition is that the dimension of the dielectric, consisting of N layers of stranded hollow fibers, a defined impedance value (typically 50 or 75 ohms) results.

For stranding, it is also important to keep an eye on the end product requirements: e.g. Torsional stability, temperature stability, electrical properties of the cable (phase stability, damping, power transmission).

Between the individual layers of the stranded hollow fibers 14, 15, (concentric) intermediate layers 21 can be introduced, which likewise consist of insulating material (for example plastic). The intermediate layer 21 can be introduced by extrusion, transverse banding, longitudinal banding, or dip coating.

The diameter for the individual hollow fiber 14, 15 (d2 in FIG. 5) is in the range of 0.01 mm to 4 mm, in particular in the range between 0.01 mm and 1 mm. An exemplary diameter is 650μm. The wall thickness of the hollow fibers 14, 1 5 is in the range between 0.001 mm and 2mm, in particular between 0.001 mm and 0.05 mm. An exemplary wall thickness is 27μm. The individual hollow fibers 14, 15 can be fixed to one another or not solidified. This affects the mobility of the entire cable construction and may be advantageous in some applications after installation of the cable. The outer diameter of the dielectric is in the range between 0.03mm and 1 2mm. The outer diameter of the coaxial cable 1 0 results from the structure of the outer conductor 1 2 and is between 0.05mm and 1 6mm.

The materials from which the hollow fibers 14, 15 could be made are insulating, non-conductive materials. Although plastics (fluoropolymers, polyethylene, polypropylene, COC, TPX, COP, PVC) can be used as hollow fiber materials. In principle, however, the hollow fibers consist of a glass, in particular quartz glass or silicon dioxide, but also ruby or other gemstone glasses or materials. Are conceivable but also combinations of all these materials. In addition, it may be advantageous to produce the hollow fibers 14, 15 as shown in FIG. 5 from a plurality of concentric layers 18, 19 of the two above-mentioned classes of materials, for example a hollow quartz glass fiber or hollow fiber 19 on the outside with a thin (eg l Oμm thick), protective cover layer 1 8, for example, from a fluoropolymer (in particular FEP) is provided. Another possibility is the use of porous hollow fibers or microstructured fibers or hollow fibers with non-round cross sections or special cross-sectional contours.

Important in the context of the invention is also the suitable choice of material for inner conductor 1 1 and outer conductor 1 2 with a respect to the dielectric 1 3 adapted thermal expansion coefficient. Depending on the temperature response of the portion of the phase shift caused by the dielectric, the coefficients of expansion for compensation are both selected to be positive or both negative, or equal to or approximately equal to zero. This results in an optimal phase behavior over the temperature, because:

1 . a change in temperature leads to a change in the (relative) dielectric constant (ε _r ) in the dielectric 1 3 and thus to a change in the electrical length, resulting in a phase shift when the temperature changes, and

2. a temperature change in the inner conductor 1 1 and outer conductor 1 2 results in an associated with the change in length opposite phase shift compared to the effect of the dielectric 1 3 results.

The sum of both effects gives the total (optimal) phase change with the smallest attenuation values (because of the high porosity). If the dielectric 1 3 contributes only slightly to the temperature-induced phase change Δp, as is the case with glass hollow fibers and is characterized in FIGS. 6 and 7 by the curve a, compensation by inner and outer conductors is not necessary. Rather, then especially the inner conductor 1 1 ₁ but also the outer conductor 1 2, the smallest possible or vanishing coefficient of thermal expansion have (curve b2 in Fig. 7), so for example, from Invar, Kovar or regarding the thermal expansion similar materials be constructed. In order to achieve the required high electrical conductivity, the inner conductor 1 1 according to FIG. 4 consists of a core 1 6 of this material and is provided with a cladding layer 1 7 of Ag or the like. By superimposing both curves a1 and b2, the net effect is curve c, which practically symbolizes a temperature-independent phase shift. On the other hand, if a silver-plated Cu conductor (CuAg) is used instead of the FeNi36Ag (invar) as the material for the inner conductor 11, a proportion of the temperature-induced phase change, as shown in FIG is large and leads to a correspondingly high temperature response overall. In both cases, however - unlike the PTFE as a dielectric - at least a linear temperature response is achieved.

The outer conductor 1 2 may consist of a transversely wound metal strip or a metallized strip or a metallized film. It may also be formed as a longitudinal foil, consisting of metal or metallized tape / foil, a braid of wire, strands or metallized fibers of insulating material. But it can also include a combination of all these types of outer conductors.

The jacket 22 may consist of a layer of insulating material or plastic, a cross-wound tape or film of insulating material, a longitudinal film of insulating tape, braid of fibers of insulating material, or combinations of these types of coats ,

example

A prototype (20) of the coaxial cable according to the invention shown in FIG. 3 was produced, which has the following data: Inner conductor: Ag-coated copper conductor (diameter dl = 1, 7mm)

Hollow fiber: quartz glass, 27 x 650 μm; 25 fibers in two layers

Outer conductor: wound CuAg tape

DK _(r ε): 1, 22

Attenuation @ 1 8CHz: 3.1

Damping @ 26.5CHZ: 4.5

Phase change: 3600ppm (linear) in the temperature range -55 ° C to +1 25 ° C

However, the comparatively high temperature-induced phase change in this prototype can be drastically reduced (to 90 ppm) if the inner conductor 1 1 consists of silver-plated Fe-Ni36Ag (Invar) and thus practically has a vanishing thermal expansion.

List of reference numerals 10,20 coaxial cable 1 1 inner conductor

1 2 outer conductor

1 3 dielectric (e.g., SiO 2 hollow fibers) 14.1 5 hollow fiber (e.g., SiO 2)

1 6 core 1 7 enveloping layer

18 covering layer

1 9 hollow fiber

21 intermediate layer

22 Sheath al Curve (phase change due to dielectric made of glass hollow fibers) bl Curve (phase change through inner conductor made of CuAg) b2 Curve (phase change through FeNi36Ag inner conductor) c Curve (superimposition of al and b2) dl Diameter (inner conductor) d2 Diameter (hollow fiber) H l ₁ H2 Cavity

Claims

claims

1 . Coaxial cable (1 0, 20) with a central inner conductor (1 1), which is concentrically surrounded by a predetermined distance from an outer conductor (1 2), wherein the intermediate space between the inner conductor (1 1) and outer conductor (1 2) with a dielectric

(1 3) is filled, characterized in that the dielectric (1 3) of a plurality consisting of a glass, extending in the cable longitudinal direction hollow fibers (14, 1 5, 1 9) is constructed, and that at least the inner conductor (1 1 ) has a thermal expansion coefficient, which the dielectric (1 3) with regard to the lowest possible temperature-induced phase change in the coaxial cable (1 0,

20) is adjusted.

2. Coaxial cable according to claim I _1, characterized in that the by the dielectric (1 3) caused proportion of the phase change (curve al) has an almost vanishing temperature response, and that the thermal expansion coefficient of the inner conductor (1 1) is correspondingly approximately zero (curve b2 ).

3. Coaxial cable according to claim 2, characterized in that also the thermal expansion coefficient of the outer conductor (1 2) is approximately zero.

4. Coaxial cable according to one of claims 1 to 3, characterized in that the hollow fibers (14, 1 5, 1 9) are arranged in a plurality of concentric layers.

5. Coaxial cable according to claim 4, characterized in that between the concentric layers of the hollow fibers (14, 1 5) in each case an intermediate layer (21), in particular of an electrically insulating material, preferably a plastic, is attached.

6. Coaxial cable according to claim 4 or 5, characterized in that the hollow fibers (14, 1 5, 1 9) of the individual concentric layers are stranded in each case.

7. Coaxial cable according to one of claims 4 to 6, characterized in that the hollow fibers (14, 1 5) of different concentric layers in their construction and / or material and / or their dimensions are designed differently.

8. Coaxial cable according to one of claims 1 to 7, characterized in that the hollow fibers (14, 1 5, 1 9) consist essentially of quartz glass or SiO 2, respectively.

9. coaxial cable according to claim 8, characterized in that the hollow fibers (14, 1 5, 1 9) have a circular cross section and an outer diameter between 0.01 mm and 4mm, in particular between 0.01 mm and 1 mm.

10. Coaxial cable according to claim 8 or 9, characterized in that the hollow fibers (14, 1 5, 1 9) have a wall thickness between 0.001 mm and 2 mm, in particular between 0.001 mm and 0.05 mm.

1 1. Coaxial cable according to one of claims 8 to 1 0, characterized in that the hollow fibers (14, 1 5, 1 9) outside with a protective cover layer (1 8) are provided.

12. The coaxial cable according to claim 1 1 _1, characterized in that the cover layer (18) of an acrylate or silicone, or a ceramic or a fluorinated ethylene-propylene (FEP) or polyethylene (PE) and a layer thickness in the range of l Oμm up comprises ,

13. Coaxial cable according to one of claims 1 to 1 2, characterized in that the inner conductor (1 1) has a thermal expansion coefficient of less than or equal to 5ppm / K.

14. Coaxial cable according to claim 1 3, characterized in that the inner conductor (1 1) consists of FeNi36Ag or Kovar or glass and the outside with a highly electrically conductive cladding layer (1 7), in particular made of Ag, and that the inner conductor ( 1 1) has an outer diameter smaller than 2mm.

1 5. Coaxial cable according to one of claims 1 to 14, characterized in that the outer conductor (1 2) consists of a wound CuAg band.