GB2067824A

GB2067824A - A Flexible Coaxial Cable

Info

Publication number: GB2067824A
Application number: GB8038981A
Authority: GB
Original assignee: Junkosha Co Ltd
Current assignee: Junkosha Co Ltd
Priority date: 1979-12-28
Filing date: 1980-12-04
Publication date: 1981-07-30
Also published as: GB2067824B; DE3048781A1; JPS5694802A; FR2472849A1; FR2472849B1

Abstract

A flexible coaxial cable is provided having a portion bent to a rigid desired radius and angle at a particular desired location. Transmission of high frequency signals through the bend is achieved without deterioration of the electrical characteristics of the cable. To make the bent cable portion, the outer jacket 4 of the coaxial cable is stripped at the desired bend location and a hardening material 6 is applied around the bent portion to firmly affix the desired bend radius and angle. The hardening material is preferable solder or a resin based adhesive. <IMAGE>

Description

SPECIFICATION A Flexible Coaxial Cable The present invention relates to a flexible coaxial cable having a fixed, bent portion and, more particularly, a flexible coaxial cable with a bent portion through which high frequency signals can be transmitted without the electrical properties of the signal being impaired.

Coaxial cables for the transmission of high frequency electromagnetic waves, for example microwaves ranging from 1 GHz to 40 GHz, are classified as either flexible or non-flexible. The flexible coaxial cables consist of a center conductor, insulation dielectric, outside conductor or shield, and a jacket, all of which are concentrically assembled. Where improved high frequency characteristics are required, a dielectric with a lower dielectric constant, a conductor having high electro-conductivity, and an outside conductor having a stable construction, e.g., double braiding or metal tape wrapping, or combination of these, are used. The non-flexible coaxial cables are called either rigid or semi-rigid coaxial cables and are made of high quality materials, particularly the outer conductor which usually consists of a copper tube.

In actual applications, the coaxial cables are not only used straight, but are also bent to the desired angle in the desired place according to the routing and space. When a bend is used the high frequency coaxial cables produce various mechanical and electrical problems. These problems are contradictory in the flexible and nonflexible coaxial cables. That is, the flexible cables are easy to bend, but unstable when bends are used, because the bent portion is not stabilized, and hence, is unstable electrically. For example, when the flexible coaxial cable is bent less than 900, reflection of the signal takes place at the bend, and gets worse as time passes.

Accordingly, in applications where the cable must be bent near the connector at a small angle and a small bend radius, the cable is usually not bent, but instead an angled connector, for example, a right angle connector, is used. In case the cable must be bent in the middle, two pieces of cable are connected by an adaptor having the desired bend. However, angled connectors and adaptors are more expensive than straight ones, and exhibit -even worse electrical characteristics. These connectors appear as a defect in the transmission system, particularly in the transmission of high frequency waves, e.g., 10 GHz-1 8 GHz range.

Therefore, it is better to select a connector as simple in construction as possible, or to improve the wiring environment so as to remove the necessity of a connector. However, there is a limit to improving the wiring environment.

According to the present invention there is provided a flexible coaxial cable for the transmission of electrical signals wherein a desired portion of the cable is fixed in a rigid bend to a desired bend radius and bend angle, said bend being fixed in place by a hardened material, said cable comprising: (a) at least one center conductor, (b) a dielectric material surrounding said center conductor, (c) at least one outer conductor surrounding said dielectric, and (d) an outer covering surrounding said outer conductor, and said rigid bend comprising said hardened material applied to said outer conductor at a bent portion thereof wherein said outer covering is stripped away therefrom.

The invention will now be particularly described, by way of example, with reference to the accompanying drawings in which: Figures 1(A) and 1(B) show examples of cross sections of conventional flexible coaxial cables; Figures 2(A) and 2(B) are side views of coaxial cables with the outer jacket stripped off; Figures 3(A) and 3(B) show the bending of the stripped portions of Figure 2; Figures 4(A) through (E) show an example of the procedure involved in the installation of a connector to the cable; Figure 5 is another example of the connector installation; Figures 6(A) and 6(B) are graphs of insertion loss and Voltage Standing Wave Ratio (VSWR) measurements taken on a coaxial cable assembly according to this invention;; Figures 7(A) and 7(B) are graphs of the same measurement tests taken on a coaxial cable assembly terminated with a conventional rightangled connector, and Figures 8(A) and 8(B) are graphs of the same measurement tests taken on an alternate embodiment of the coaxial cable assembly of this invention.

A flexible coaxial cable is provided having a portion bent to a rigid desired radius and angle at a particular desired location. Transmission of high frequency signals through the bend is achieved without deterioration of the electrical characteristics of the cable. To make the bent cable of this invention, the outer jacket of the coaxial cable is stripped at the desired bend location and a hardening material is applied around the bent portion to firmly affix the desired bend radius and angle. The hardening material is preferably solder or a resin based adhesive.

Figures 1(A) and 1 (B) comprise a center conductor 1, an insulating dielectric 2, an outer conductor 3 or 3a, 3b, and an outer jacket.

The material of the inner and outer conductors, 1 and 3 is selected from a group of metals having good electric conductivity such as copper, copper alloy, copper clad steel, or the foregoing plated with tin or silver. Center conductor 1 may be solid or stranded, and outer conductor 3 may be a single layer as shown in Figure 1(A) or a double or multiple layer 3a, 3b as shown in Figure 1(B) depending on its purpose. The single layered outer conductor 3 consists of braiding, serving or tape-wrapping, and the double or multiple layered outer conductor 3a, 3b consists of an optional combination of the above.Among such combinations, a double layer comprised of a helical wrapping of silver plated copper tape as the inside layer 3a and a close braiding of silver plated copper wires as the outside layer 3b of the outer conductor provides the best electrical properties. An electro-conductive plastic material may also be used as the outside conductor layer 3b.

The insulating dielectric 2 filling the space between the inside and outside conductors is selected from various plastics. For improved high frequency transmission capability and stabilized electrical properties, polyolefins (such as polyethylene) and fluorinated resins such as PTFE, FEP, PFA, ETFE and PVDF are suitable. These resin materials are formed as a solid or porous body around the center conductor 1. Where high frequency performance is required dielectric 2 is made porous in order to produce a lower dielectric constant. Of the porous dielectric materials having low dielectric constants, expanded PTFE (obtained by the method described in Japanese Patent Publication JPP Sho 51-18991) is, in many respects such as dielectric constant, heat resistance, etc., the best material available as a dielectric for high frequency coaxial cables.The method of producing expanded PTFE is, in short, comprised of conventionally preparing a formed article from a mixture of PTFE fine power using a liquid lubricant, removing the lubricant, stretching the formed article at a certain condition at an elevated temperature below 3270C, and heating the stretched article to a temperature a little below or above 3270C while retaining the article in the stretched condition.

Outer jacket material 4 is selected from plastics that will not affect the transmission performance of the coaxial cable and are suitable for the specified application. If solder is used as the hardening material, fiuorinated resins are preferably used.

In one embodiment of the present invention, the desired length, generally 1.5-3.5 cm, of the outer jacket 4 on the portion of the cable to be bent is stripped, by using a sharp blade, as shown in Figure 2(A) at an end portion of the cable, or Figure 2(B) in a middle portion of the cable, to expose the outer conductor 3. Numeral 5 shows the portion with the jacket stripped off. The coaxial cable is then bent at the portion 5 at a prescribed bend radius and angle as shown in Figure 3(A) and Figure 3(B). When bending, care should be taken to maximize the uniformity of the bent portion 5' so that the concentricity of the conductor and dielectric is not lost. When a sharper bend angle with a small bend radius is required, a desired number of primary strands constituting the braid of outer conductor 3b may be cut in order to bend the cable uniformly.The bent state of the cable, as obtained above, is held in place temporarily by using clamps (not shown).

Subsequently, a hardening material 6 is applied around the bent portion 5' of the exposed outer conductor 3 which has been bent to a specified bend radius and bend angle and held with clamps. Solder and hardening adhesives such as epoxy-, urea-, phenol-, and vinylacetatebased adhesives can be used as the hardening material 6. The hardening material should be applicable to the bent portion 5' of the exposed outer conductor surface 3, and be capable of being hardened there to form hardened coat 6.

The rigidity of the hardened coat must be such that the bent portion 5' of the cable remains firmly and permanently bent and adequately resists any tendency to return to its original state: Of the various types of hardening material 6, solder is preferred. When applying solder to the exposed outer conductor 3 which has been stripped of the outer jacket 4, fabrication should be made with care so that the molten solder impregnates, by wicking, the void spaces in the texture of outer conductor 3. For example, a soldering iron is brought into contact with the braided conductor in order to heat it adequately so that the braid will absorb the molten solder However, when using solder, the dielectric and the outer jacket should both be highly heat resistant because the solder, a metallic material, must be applied while heated to a high temperature.This is particularly important when the outer jacket 4 covers the bent portion 5' as shown in Figure 5, and the molten solder impregnates the outer conductor 3 up into the bent portion 5' beneath the outer jacket 4 by wicking up (capillary action) from the exposed portion of the outer conductor 3 where the solder is applied under heat. Fluorocarbon resins, such as PTFE, FEP, PFA, etc., are such heat resistant resins.

In cases where the cable is bent at an end portion and a conductor is attached to the end, it is preferred that the above-mentioned attachment step including soldering be done first since a lathe is used for the installation of the connector, followed by the bending step. That is, the desired length of the outer jacket 4 at the top portion 5 of the cable is removed as shown in Figure 4(A); solder 6 is then impregnated and hardened into only the shorter portion 51 of the exposed conductor 3 necessary to attach the connector Figure 4(B); and a proper length at the top of the hardened outer conductor is cut by a lathe or the like to provide a flat cross section of the outer conductor 3 and dielectric 2, while leaving the center conductor 1 intact. The dielectric 2 is then removed in line with the outer conductor cross section to expose the center conductor 1 at the top which is later used as a center pin of the connector as shown in Figure 4(C). The top of the exposed center conductor 1 should be machined to a cone shape according to the need. The shell 7 of connector 10 is fitted and soldered onto the remaining top portion of the solder-hardened outer conductor by means of soldering 11 as shown in Figure 4(D). The connecting nut 9 is mounted onto the shell 7 via ring 8 such that it can rotate around the shell. This assembly constitutes connector 10 in which, according to this example, the center conductor 1 becomes the center pin. Then the exposed outer conductor portion 3 between the back end of the shell 7 and the top end of the outer jacket 4 is bent at a prescribed bend angle and radius 5'.The bent portion 5' is heated to impregnate molten solder into the exposed outer conductor 3. The solder which has been impregnated into the exposed outer conductor 3 is allowed to cool in order to fix the bent state 5' permanently as shown in Figure 4(E).

In the above fabrication sequence, it appears possible that the solder-hardening of the exposed outer conductor 3 in Figure 4(B) can be made over the entire length of the exposed portion and the bending of the cable can then be made after process steps (C) and (D) of Figure 4. However, in such a cable bending step, since the exposed outer conductor 3 is hard due to the applied solder 6, the concentricity of the outer conductor 3, dielectric 2 and center conductor 1 is liable to be lost. The electrical characteristics can be greatly damaged particularly in the connector as shown in Figure 4 where the center conductor 1 serves also as the center pin. Thus, the solder hardening of the exposed conductor 3 is to be made to the minimal length still able to withstand the lathing, while avoiding the portion to be bent.

After connector installation and cable bending, the exposed, bent portion 3 is heated and impregnated smoothly with solder.

In an alternative embodiment, as shown in Figure 5, the outer jacket 4 at the top portion of the cable is stripped off to a shorter length than in the embodiment shown in Figure 4, but a little longer than the length of shell 7. The exposed outer conductor is inserted into connector shell 7.

The cable portion with jacket 4 behind the shell is bent at a prescribed bend radius and angle 5' as in Figure 5. While being held in this bent state, the shell and the cable-portion are heated, for example, by bringing a heated soldering iron into coritact with the top section of the shell.

Meanwhile molten solder 6 is applied to the outer conductor 3 between the back end of shell 7 and the end of the outer jacket 4 in order to impregnate the solder into the outer conductor beneath the shell and the bent portion 5' due to capillary action. By this treatment, the shell 7, or connector 10, is bonded to the outer conductor, whereas the bent state 5' is fixed due to the solidified solder in the outer conductor below the outer jacket 4.

Besides solder, resin based commercial adhesives can be effectively used as the hardening material. There are no particular problems concerning the method of application of such adhesives, or the reactivity of such adhesives to the dielectric and the outer jacket material. Such resin adhesives should be used so that the outer conductor of the bent portion 5' is sufficiently held in place.

It is preferred that the bent portion of the cable fixed by the hardening material 6 be covered with a protective layer, for example, tape wrapping or heat shrinkable tubing.

In the flexible coaxial cable having a fixed bent portion 5' utilizing hardening material 6, high frequency signals are transmitted effectively without reflection and attenuation.

The following examples are provided for illustration only and are not to be limitative of the claims below in any way.

Example 1 A flexible coaxial cable (characteristic impedance 50 ohms) of the following construction was prepared: Center conductor 1: silver plated annealed copper wire, 0.912 mm O.D.

Dielectric 2: expanded, porous PTFE (dielectric constant 1.54) Outer conductor 3: inside layer: silver plated over-lapping copper foil tape 3a outside layer: silver plated braided copper wire 3b, 3.55 mm Outer jacket 4: extruded FEP SMA male connector 10 (conforming to MIL-C- 39012/92) was installed to the end of the cable following the procedure A-D of Figure 4 described above. The cable portion behind connector 10 was bent at a bend radius of 20 mm (inside radius) and an angle of 900 as shown in Figure 4(E). The bent portion 5' was heated, and molten solder 6 was impregnated into the exposed outer conductor 3, and allowed to cool.

The bent state 5' was permanently fixed.

The coaxial cable assembly so obtained (1 m long) was tested using a Time Domain Reflectometer (TDR) with a pulse of 35 picosecond rise time to show no alteration of the characteristic impedance. The insertion loss test showed a good result and no adverse effect due to bending as shown in Figure 6(A). VSWR (reflection attenuation) was 1.17 at 18.5 GHz, showing no adverse effects due to bending [see Figure 6(B).

If the bent portion 5 is not fixed, the electrical properties of the cable,lwhen tested under a 1 kgf bending load, are so affected that the cable cannot be used at 18.5 GHz.

For the purpose of comparison, insertion loss and VSWR were measured on a cable (1 m long) terminated with a conventional rigid-angled connector, and the results are shown in Figure 7(A) and 7(B), respectively. The insertion loss and VSWR are larger than those of Figure 6(A) and 6(B).

Example 2 The above Example 1 was duplicated with the exception that the fixing of the bent portion 5' of the cable was done by the impregnation/hardening of epoxy resin instead of solder.

Taking 1 m of the cable assembly, insertion loss and reflection attenuation (VSWR) were measured as in Example 1. The results are shown in Figure 8(A) and 8(B). These results are similar to those obtained in Example 1 where solder was used on the bent cable assembly, and are better than those obtained on the cable terminated with right angled connectors, as shown in Figure 7(A) and 7(B).

While the invention has been disclosed herein in connection with certain embodiments and detailed descriptions, it will be clear to one skilled in the art that modifications or variations of such details can be made within the scope of the claims hereinbelow.

Claims

1. A flexible coaxial cable for the transmission of electrical signals wherein a desired portion of the cable is fixed in a rigid bend to a desired bend radius and bend angle, said bend being fixed in place by a hardened material, said cable comprising: (a) at least one center conductor, (b) a dielectric material surrounding said center conductor, (c) at least one outer conductor surrounding said dielectric, and (d) an outer covering surrounding said outer conductor, and said rigid bend comprising said hardened material applied to said outer conductor at a bent portion thereof wherein said outer covering is stripped away therefrom.

2. The cable of claim 1 wherein said hardened material is hardened solder.

3. The cable of claim 1 wherein said hardened material is a hardened, resin based adhesive.

4. A flexible coaxial cable substantially as herein described with reference to Figure 4(E) or Figure 5 of the accompanying drawings.