EP0125172B1 - Process for preparing the extremity of a very high frequency flexible coaxial cable for installing a connector element, and sleeve used for carrying out the process - Google Patents

Description

The present invention relates to a method for preparing the end of a flexible very high frequency coaxial cable for the installation of a connector element.

More and more, for the transmission of energy at very high frequency, flexible coaxial cables are used to replace rigid coaxial lines which are ill-suited to many applications, in particular in aeronautics.

Such flexible coaxial cables usable at very high frequencies, usually comprise a multi-strand central conductor, in particular made of silver-plated copper, and at least one and preferably two external conductors, the first preferably formed from a wound ribbon, in particular made of silver-plated copper, the second preferably with a braid also in silvery copper on the outside of the ribbon. Between the central conductor and the first external conductor is disposed a dielectric, in particular aerated or solid polytetrafluoroethylene (PTFE). Finally, around the second external conductor is arranged an external sheath, in particular in extruded PTFE ensuring mechanical and chemical protection of the cable. This sheath also keeps the ribbon constituting the first external conductor wound at the ends of the cable.

The applicant company has already described in its European patent EP-A-14639 and in its French patent FR-A-79 02 468 a process for preparing the end of such a flexible coaxial cable at very high frequency for the installation a connector element.

According to the method described in the previous patent, the end of the cable is first punched in several notches, in particular two or four, through the external sheath so as to permanently deform the wound ribbon then it is put in place a sleeve, between the wound ribbon and a stripped part of the braid so that the end of the sleeve is substantially in line with the end of the sheath at the limit of the stripped part of the braid, the sleeve being mechanically immobilized by crimping of a ferrule on the sheath and the stripped part of the braid then welded on the rolled ribbon. Finally, the part of the ribbon is unwound beyond the end of the sleeve and the ribbon is cut then the dielectric flush with said end of the sleeve leaving a section of inner conductor stripped.

A coaxial connector produced by placing a connector element on each end of the coaxial cable thus prepared, while being very advantageous in practice, nevertheless presents an imperfection with regard to the transmission characteristics.

This drawback results from the fact that there is in a connector element thus mounted at the end of a flexible coaxial cable at very high frequency a transition of a certain length between the end of the cable sheath, which corresponds at the end of the homogeneous structure of the cable, and the beginning of the coaxial line of the connector. This transition which constitutes a disturbance zone provides additional reflection of the transmitted signal and therefore an increase in the reflection coefficient due to the connector. On the other hand, when, as is usually the case, the first external conductor is a wound ribbon, the latter loses part of its characteristic of electrical continuity between the end of the cable sheath and the rear end of the cable. muff. This disturbance causes small variations in contact resistance at high frequency between the turns of wound ribbon located in the zone of disturbance thus produced, which results in instabilities of power losses in the coaxial cord when flexions of the cable very close to its connector outlet.

The present invention proposes to provide a method for preparing the end of a flexible coaxial cable at very high frequency avoiding the presence of disturbance zones between the end of the complete coaxial cable and the start of the coaxial line of the connector.

The process according to the invention is essentially characterized by the fact that an end section of the outer sheath is cut and removed, a sleeve is fitted on the outer sheath and the stripped part of the outer conductor (s) inner recess formed by two cylindrical sections separated by a frustoconical section, so as to bring the end of the sheath to bear in said frustoconical section, the stripped end of the outer conductor (s) projecting beyond the front end of the sleeve, the sleeve is fixed, in particular by welding, to the outer conductor (s) in the area of the stripped portion of the outer conductor (s) surrounded by said sleeve, after which the outer conductor (s) are then cut and then the dielectric flush with the 'front end of the sleeve leaving bare an inner conductor section.

In the case where the or one of the external conductors is a ribbon, the end of the cable is preferably punched beforehand in several notches, in particular two or four, through the external sheath so as to permanently deform the external conductors.

Once the end of the cable has been prepared in this way, the procedure is as described in the prior mentioned patents and, for this purpose, an insulation is placed on the stripped central conductor of the end of the cable against the front end. of the sleeve, the central contact of the connector is mounted on the central conductor resting on the insulation, the central contact is welded on the central conductor, the end of the cable is mounted in the body of the connector and the assembly is immobilized by screwing in a rear plug.

The present invention also relates to a sleeve usable for the implementation of the method which, as mentioned above and as claimed in claim 3, comprises an internal recess comprising a rear cylindrical introduction section. The diameter and the length are suitable to ensure good guidance of the end of the cable introduced and to absorb the flexions to which the cable can be subjected very close to the sleeve. Preferably, the diameter of this rear cylindrical section of the recess is slightly greater, in particular 0.1 to 0.2 mm, than the outside diameter of the sheathed cable, the length of this section preferably being at least 0.6 times the outside diameter of the cable. As an extension of this cylindrical introduction section, the recess of the sleeve has a frustoconical central section, tapering towards the front of the sleeve. The angle of said cone is suitable to allow the penetration of the outer conductor (s) of the cable through the sleeve and the clamping of the end of the cable sheath on the frustoconical surface of the recess regardless of variations in the outer diameter of the cable . In practice, and depending on the nature of the cable, this angle may advantageously be between approximately 10 and 45 ° and preferably of the order of 15 °.

Beyond this frustoconical central section, the recess is extended by a cylindrical front section with a diameter less than the diameter of the rear section. This section is suitable for the passage of the outer conductor (s) of the cable without changing their structure. The length of the cylindrical section before the recess of the sleeve is determined to allow effective fixing of the sleeve, in particular by welding, on the outer conductor (s) of the cable, the length of this section preferably being in practice substantially equal to its diameter.

At its front end, the sleeve advantageously includes a flange in the form of a collar intended to allow its fixing in the connector, the front face of the flange ensuring the electrical connection between the end of the outer conductor (s) of the cable and the start of the coaxial line. of the connector.

It is understood that, although the homogeneous structure of the cable physically stops at the end of the cable sheath located in the frustoconical central section of the sleeve recess, the sleeve will in fact extend this homogeneous structure of the cable up 'at the end of the outer conductor (s) of the cable, avoiding areas of disturbance.

The method according to the invention can be implemented to produce straight connectors or bent connectors, in which case, it suffices to use a sleeve of suitable bent shape.

In order to better understand the invention, an embodiment will now be described by way of example in no way limiting, with reference to the appended drawing in which:

• la figure 2 illustre le montage d'un élément de connecteur à l'extrémité d'un câble préparé en mettant en oeuvre le procédé connu d'après le brevet français 7902468,
• la figure 3 est une vue en coupe agrandie d'un manchon utilisable dans le cadre de la présente invention,
• les figures 4 à 8 illustrent schématiquement la mise en oeuvre du procédé selon l'invention pour préparer l'extrémité d'un câble coaxial souple selon la figure 1,
• la figure 9 est une vue similaire à la figure 2 d'un élément de connecteur monté à l'extrémité d'un câble préparé par la mise en oeuvre du procédé selon l'invention,
• la figure 10 est une vue similaire à la figure 9 illustrant le montage d'un élément de connecteur en mettant en oeuvre le procédé selon l'invention utilisant un manchon coudé.

FIG. 1 represents in partial section a flexible coaxial cable at very high frequency usable for the implementation of the invention,

• FIG. 2 illustrates the mounting of a connector element at the end of a cable prepared by implementing the method known from French patent 7902468,
• FIG. 3 is an enlarged sectional view of a sleeve usable in the context of the present invention,
• FIGS. 4 to 8 schematically illustrate the implementation of the method according to the invention for preparing the end of a flexible coaxial cable according to FIG. 1,
• FIG. 9 is a view similar to FIG. 2 of a connector element mounted at the end of a cable prepared by the implementation of the method according to the invention,
• Figure 10 is a view similar to Figure 9 illustrating the mounting of a connector element by implementing the method according to the invention using an elbow sleeve.

FIG. 1 illustrates the structure of a very high frequency coaxial cable of a type commonly used and already described in said patents. This cable comprises a central conductor 1 of silver-plated copper, multi-strand, a dielectric 2, in particular of PTFE with an aerated or solid structure, a first external conductor 3 in the form of a ribbon wound in silver copper, a second external conductor in the form with a silver copper braid 4 and an outer protective sheath, for example in extruded PTFE 5.

FIG. 2 illustrates the mounting in a connector element of such a coaxial cable prepared as described in said patents.

As described in this prior document during the preparation of the end of the cable, a sleeve 6 has been put in place comprising an end flange 7, the tubular part of the sleeve being disposed between the two layers of outer conductor one in the form of a ribbon 3, the other in the form of a braid 4. The sleeve was mechanically immobilized by crimping a ferrule on the sheath and the stripped part of the braid, then welded to the wound ribbon.

As shown diagrammatically in FIG. 2, there exist in such an arrangement two disturbance zones designated respectively by A and B.

Zone A corresponds to the transition between the end of the sheath 5 of the cable, which corresponds to the end of the homogeneous structure of the cable, and the start of the connector line beyond the front face of the flange 7 of the sleeve , zone B corresponding to the transition at the level of the external conductor in the form of a braid 4 between the end of the sheath 5 and the rear end of the sleeve 6.

Is illustrated in Figure 3, in enlarged view, a sleeve used for the implementation of the method according to the present invention.

This sleeve generally designated by 9 has an interior recess 10 consisting of three successive sections 11, 12 and 13.

The rear section 11 of cylindrical shape has, in the example illustrated for a cable with a maximum diameter of 5.2 mm, a diameter of 5.3 mm and a length of 6 mm. This cylindrical section 13 which, in the example illustrated, has a length of 5 mm and a diameter of 4.7 mm.

At its front end, the sleeve 9 has a flange in the form of a collar 14 defining a front face 15 which will provide the electrical connection between the end of the outer conductors of the cable and the start of the coaxial line of the connector.

Reference is now made to FIGS. 4 to 8 to describe the process according to the invention for the preparation of the end of a very high frequency coaxial cable of the type represented in FIG. 1 using a sleeve as described in FIG. 3 .

Using a jaw punching pliers, preferably comprising in section two opposite internal bulges, the end of the cable is punched in two or preferably four notches 16 through the external sheath 5 of the cable. This has the effect of permanently deforming the wound ribbon 3 preventing it subsequently from taking place. We see in Figure 3 the end of the cable after punching. This punching phase obviously has no reason to be except in the case where the cable comprises an external conductor in the form of a wound ribbon.

As illustrated in FIG. 6, the sheath 5 of the cable is removed over a certain length at the end of the cable, revealing a section of braid 4. This stripping length must in practice be at least equal to the distance which must separate the end of the sheath from the end of the central conductor in the fully assembled state of the connector.

As seen in FIG. 7, the cable thus prepared is inserted into a sleeve 9 until the front end of the sheath 5 comes to bear against the frustoconical bearing of the section 12 of the recess of the sleeve 9.

This frustoconical bearing of the recess of the sleeve facilitates the insertion of the outer conductors of the cable and prevents strands of the braid 4 from being pushed back into the sleeve. The subsequent preparation phase consists in soldering the outer conductors 3 and 4 of the cable and the sleeve by welding at low temperature, for example with tin-lead at 180 ° C. The soldering should preferably be carried out on all the portion of the external conductors being inside the sleeve, zone diagrammed by the double arrow of length S in figure 7. This zone extends in fact between the frontal end of the sheath 5 and the end end face 15 of the sleeve.

As seen in FIG. 8, the outer conductors 3 and 4 and the dielectric 2 are then cut to the right of the front face 15 of the sleeve.

If necessary, the end of the inner conductor 1 is cut at a distance from the front face 15 suitable for mounting the central contact of the connector.

FIG. 9 shows the mounting of a connector element on a cable end thus prepared. To achieve this mounting, as already described in said patents, there is an insulator 17 on the central conductor 1 stripped from the end of the cable bearing against the front face 15 of the sleeve 9. The central contact 18 is then mounted on the central conductor 1 pressing on insulator 17 and the central contact 18 is welded on the central conductor 1, as indicated in ¹ 9.

The coaxial cable, welded by its central conductor to the central contact, is then placed in an olace in a connector body 20 and the assembly is immobilized by screwing a rear plug 21.

For the production of a bent connector element as illustrated in FIG. 10, the same process steps are implemented using the same components except for a sleeve 9 ′ which has a bent shape and a recess of which the front section has a significantly greater length than in the embodiment of the straight connector according to FIG. 9.

Although in the example illustrated, a coaxial cable with two external conductors was used, one consisting of a wound ribbon, the other consisting of a braid, the method according to the invention is not limited thereto and can be implemented with any other type of flexible cable, in particular of the type comprising a single external conductor, in the form of a braid, of ribbon wound or constituted by non-braided helical strands, or also for example of the type with two external conductors each constituted by a braid.

Finally, it is obvious that the invention is not limited to the particular embodiment described and that various modifications can be made to it without departing from its scope.

Claims (5)

1. A method of preparing the end of a flexible very high frequency coaxial cable for putting in place a connector element, the said cable exhibiting a central conductor (1), a dielectric (2), at least one outer conductor (3, 4) and an outer sheath (5), characterized by the fact that an end portion of the outer sheath is cut off and withdrawn, over the outer sheath and the bared portion of the outer conductor or conductors (3, 4) a muff (9, 9') is put in place, which comprises an inner hollow (10) formed of two cylindrical portions (11 ; 13) separated by a frustoconical portion (12) so as to bring the end of the sheath (5) to bear against the said frustoconical portion, the bared end of the outer conductor or conductors (3, 4) projecting beyond the front end of the muff, the muff is fixed, expecially by soldering, onto the outer conductor or conductors in the zone (S) of the bared portion of the outer conductor or conductors which is surrounded by the said muff, after which the outer conductor or conductors (3, 4) and then the dielectric (2) are cut off beyond the front end (15) of the muff, leaving bared a portion of inner conductor (1).

2. A method of putting a connector element in place over the end of a high frequency coaxial cable, characterized by the fact that after having prepared the end of the cable by the method as in Claim 1, there is arranged over the central conductor bared from the end of the cable an insulator (17) bearing against the face of the front end (15) of the muff, the central contact (18) of the connector is mounted on the central conductor (1) to bear against the insulator (17), the central contact is soldered onto the central conductor, the end of the cable is mounted in the connector body (20) and the whole is fixed in position by screwing up a rear bush (21).

3. A muff which may be employed for putting into effect the method as in either of the Claims 1 and 2, characterized by the fact that it includes an inner hollow (10) which has a cylindrical rear portion (11) for introduction, a central frustoconical portion (12) which narrows towards the front of the muff a cylindrical front portion (13).

4. A muff as in Claim 3, characterized by the fact that the frustoconical portion (12) exhibits an angle of the cone (a) lying between about 10° and about 45° and preferably of the order of 15°.

5. A muff as in either of the Claims 3 and 4, characterized by the fact that it includes at the front end of it a cheek in the form of a small collar (14) the front face (15) of which ensures the electrical connection between the end of the outer conductor or conductors of the cable and the start of the coaxial line of the connector.

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