Patent Publication Number: US-10784598-B2

Title: Coaxial cable connector

Description:
PRIORITY 
     This application is a U.S national application of the international application number PCT/FI2016/050858 filed on 8 Dec. 2016, which claims priority of European patent applications EP 15397542.0 filed on Dec. 9, 2015 and EP 16397507.1 filed on Mar. 11, 2016, the contents of all of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to television network installations, and more particularly to a coaxial cable connector. 
     BACKGROUND OF THE INVENTION 
     F type connectors, specified in the standard IEC 60169-24, have been used for decades for terrestrial, cable, and satellite TV installations. The F connector has become a popular coaxial cable connector due to its inexpensiveness, good impedance matching to 75Ω, and wide bandwidth usability. The male F connector body is typically crimped or compressed onto the exposed outer braid of the coaxial cable. Female F Type connectors have an external thread to which male connectors having a matching internally threaded connecting ring are connected by screwing. 
     In various TV installations, it is vital that the metal-to-metal contact resistance between the connector and the cable braiding is optimised and maintained over time for good contact resistance. Any degradation in overall contact resistance will result in increasing the transfer impedance and will degrade the screening effectiveness. 
     In light of the new 4G LTE wireless services, which operate within the CATV frequency spectrum, it has become imperative that cable interconnect assemblies, i.e. the coaxial cable with a connector attached, meet a very high screening effectiveness as a market requirement based on a CENELEC standard. 
     However, practically none of the current coaxial cable assemblies can maintain Class A++ shielding efficiency over time. It has turned out that while a cable interconnect assembly may meet the Class A++ requirements when manufactured, the coupling transfer function of the same assembly has degraded significantly after having been installed in a CATV network some time. 
     Consequently, there is a need for an improved arrangement for connecting a coaxial cable to a connector. 
     SUMMARY OF THE INVENTION 
     Now an improved arrangement has been developed to alleviate the above-mentioned problems. As an aspect of the invention, there is provided a coaxial cable connector, which is characterized in what will be presented in the independent claim. The dependent claims disclose advantageous embodiments of the invention. 
     According to a first aspect, there is provided a coaxial cable connector for connecting a coaxial cable, wherein the connector comprises a ferrule arranged to be configured in electrical contact with at least one metal braid layer of the coaxial cable, the ferrule comprising an elongated body for the electrical contact with said at least one metal braid layer of the coaxial cable, wherein at least the body is plated with tin or made of zinc or zinc alloy; and a base comprising an opening for insertion of a centre connector of the coaxial cable through the elongated body, wherein at least surfaces of the opening of the base are covered with a material preventing short-circuit between the centre connector of the coaxial cable and the base. 
     According to an embodiment, the base is plated with nickel or nickel-tin. 
     According to an embodiment, the body and the base of the ferrule are arranged to be plated separately and connected together after plating. 
     According to an embodiment, at least the surfaces of the opening of the base are covered with a plastic or silicone. 
     According to an embodiment, the body and the base form a one-piece ferrule plated with tin or made of zinc or zinc alloy, wherein at least the surfaces of the opening of the base are covered with an insert adaptable to the base, said insert being plated with nickel or nickel-tin or made of a plastic or silicone. 
     According to an embodiment, the connector further comprises a compression fitting arranged around said ferrule; and means for applying a pressure force to the compression fitting such that a surface of the compression fitting applies a force either to an outer insulating layer of the coaxial cable surrounding said at least one metal braid layer or directly to said at least one metal braid layer substantially over the whole length of said surface of the compression fitting. 
     According to an embodiment, a cross-section of the compression fitting comprises a first surface arranged substantially co-axially with the base of the ferrule such that, in an uncompressed state, there is a space between said first surface and the ferrule, and at least one slanted second surface to which said pressure force is configured to be applied. 
     According to an embodiment, the compression fitting is made of silicone. 
     According to an embodiment, said means for applying the pressure force comprise a taper arranged to slide against said at least one slanted second surface upon pushing the coaxial cable into the connector. 
     According to an embodiment, said taper is arranged to an outer body of the connector, which is arranged to move towards the ferrule upon pushing the coaxial cable into the connector. 
     According to an embodiment, the compression fitting is made of silicone having Shore A hardness value of 20 to 70. 
     According to an embodiment, said means for applying the pressure force comprise a framing of the outer body of the connector, which is arranged to slide against said at least one slanted second surface upon screwing the outer body to the connector. 
     According to an embodiment, the compression fitting is made of silicone having Shore A hardness value of 60 to 100. 
     According to an embodiment, the connector is a F type male compression connector or a F type male crimp connector. 
     As a second aspect, there is provided use of tin for plating a body of a ferrule of a coaxial cable connector, wherein said body is arranged to be configured in electrical contact with at least one metal braid layer of the coaxial cable, and use of nickel or nickel-tin for plating a base of the ferrule of the coaxial cable connector. 
     These and other aspects of the invention and the embodiments related thereto will become apparent in view of the detailed disclosure of the embodiments further below. 
    
    
     
       LIST OF DRAWINGS 
       In the following, various embodiments of the invention will be described in more detail with reference to the appended drawings, in which 
         FIG. 1  an example of the structure of a coaxial cable; 
         FIGS. 2 a , 2 b    illustrate the effect of ageing to the coupling transfer function of a coaxial cable interconnect assembly; 
         FIGS. 3 a , 3 b    illustrate the effect of galvanic reaction to the coupling transfer function of a coaxial cable interconnect assembly having a NiSn plated F connector and aluminium cable braiding; 
         FIG. 4  shows a schematic cross-sectional view of a prior art F male compression connector with a coaxial cable connected to a F female connector; 
         FIG. 5 a    show a connector ferrule according to prior art; 
         FIG. 5 b    shows a two-piece dual plated connector ferrule according to an embodiment of the invention; 
         FIGS. 5 c -5 f    show examples where the body and the base form a one-piece ferrule plated with tin according to some embodiments of the invention; 
         FIG. 5 g    shows a dual plated one-piece ferrule comprising a body plated with tin and a base plated with nickel or nickel-tin according to an embodiment of the invention; 
         FIG. 6  shows an example of the mechanism for connecting the F male compression connector to the coaxial cable; 
         FIGS. 7 a , 7 b    show a connector design according to an embodiment of the invention; 
         FIG. 8  shows a connector design according to another embodiment of the invention; 
         FIG. 9  shows a connector design according to another embodiment of the invention combining the two-piece dual plated connector ferrule and a compression fitting; and 
         FIGS. 10 a -10 e    show a one-piece ferrule part plated with tin and an insert plated with nickel or nickel-tin adaptable into the opening of the base of the ferrule part according to an embodiment of the invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following, the problems relating to prior art are first described more in detail. Subsequently, the actual technical reasons underlying the problems, only revealed in the recent studies by the applicant, are elucidated. 
       FIG. 1  shows an example of the structure of a coaxial cable. The cable  100  comprises an inner (or centre) conductor  102  for conducting electrical signals. The inner conductor  102  is typically made of copper or copper plated steel. The inner conductor  102  is surrounded by an insulating layer  104  forming a dielectric insulator around the conductor  102 . The insulator surrounding the inner conductor may be solid plastic, such as polyethylene (PE) or Teflon (PTFE), a foam plastic, or air with spacers supporting the inner conductor. 
     The insulating layer  104  is surrounded by a thin metallic foil  106  typically made of aluminium. This is further surrounded by a woven metallic braid  108 .  FIG. 1  shows only one braid layer  108 , but there may be two (inner and outer) layers of braid, or even more braid layers. Braiding is typically made of unalloyed aluminium, copper or tinned copper, depending on the intended field of use of the coaxial cable. For example, coaxial cables used in various TV assemblies typically have the braiding made of unalloyed aluminium. The cable is protected by an outer insulating jacket  110 , typically made of polyvinylchloride (PVC). 
     The structure of the coaxial cable enables to minimize the leakage of electric and magnetic fields outside the braiding by confining the fields to the dielectric and to prevent outside electric and magnetic fields from causing interference to signals inside the cable. The shielding efficiency of each coaxial cable is characterized by its coupling transfer function, which may be defined as the transfer impedance and the screening attenuation measured together. The coupling transfer function is primarily dependent on the make-up of the coaxial cable, in part the outer and inner metal braiding and foil construction of the cable. However, for the practical use in various TV assemblies, the cable needs to be connected to the coaxial F connector. 
     There are two basic functional types of coaxial F type connectors currently available, i.e. crimp connectors and compression connectors. Both connector types include an outer body, a ferrule and a fixing nut. In order to make a ground connection between the cable braiding and connector, both of said connector types use a simple method of compressing the (outer) braid of the coaxial cable onto the connector ferrule. Both achieve the same outcome of connecting the coaxial cable to the connector by compression via the cable PVC outer jacket. 
     In order to achieve optimum transfer impedance, it is imperative that the metal-to-metal contact resistance between the connector and the cable braiding is optimised and maintained over time for good contact resistance. Any degradation in overall contact resistance will result in increasing the transfer impedance. 
     In light of the new 4G LTE wireless services, which operate within the CATV frequency spectrum, it has become imperative that cable interconnect assemblies, i.e. the coaxial cable with a connector attached, meet a very high screening effectiveness. For example, cable TV operators generally require the screening effectiveness to remain at −105 dB for the frequency range of 30-1000 MHz and the transfer impedance at 0.9 mΩ/m for 5-30 MHz, which are substantially in line with the CATV industry EN50117-2-4 Cenelec Standards as Class A++. Previous cable assemblies required only Class A+, i.e. −95 dB for 30-1000 MHz. 
     It has turned out that practically none of the current coaxial cable assemblies can maintain Class A++ shielding efficiency over time. The cable TV industry has identified the problem that while a cable interconnect assembly may meet the Class A++ requirements when manufactured, the coupling transfer function of the same assembly has degraded significantly after having been installed in a CATV network some time. 
     The phenomenon can be illustrated by the test results shown in  FIGS. 2 a  and 2 b   .  FIG. 2 a    shows the coupling transfer function of a non-used cable interconnect assembly. It can be seen that the coupling transfer function meets rather well the Class A++ requirements, especially on the low frequency 5-30 MHz transfer impedance requirements. 
       FIG. 2 b    shows the coupling transfer function of the same cable interconnect assembly after a temperate cycle test. The temperature cycle test simulates the basic ageing of the cable assembly by taking the cable to its minimum and maximum temperature limits. In this particular test, a one week temperature cycling was carried out from −20° C. to +60° C. with a dwell time of 5 minutes. As can be seen in  FIG. 2 b   , the coupling transfer function has seriously degraded. Both the low frequency transfer impedance and overall screening effectiveness have degraded. 
     Now the research has proven that the issue relates to a degradation of the metal-to-metal contact resistance between the coaxial cable braiding and the connector ferrule. This contact resistance degrades over time, and is a result of the PVC cable outer jacket being used to apply the required pressure when the connector is compressed. 
     After detailed research, it has turned out that the problem is caused by two phenomena. The first relates to the cable braiding, which in CATV coaxial cables is mainly unalloyed aluminium. The second relates to the PVC jacket of the cable. Both these materials exhibit an issue called “creep”. Material creep (a.k.a. cold flow) is defined as a solid material moving slowly or deformed permanently under the influence of mechanical stresses. It occurs as a result of long-term exposure to high levels of stress that are still below the yield strength of the actual material. 
     In the case of unalloyed aluminium, creep may exist under the slightest force and the contact force will gradually decrease over time. PVC polymers exhibit the same issue and are very unstable in joint applications. In current coaxial cable/connector scenarios, the cable jacket and braid polymers are in series with the main joint compression. Polymers have large temperature and moisture expansion rates and will creep over time until joint contact is eventually reduced to almost zero. 
     There are three key stages to creep, i.e. primary, secondary and tertiary creep. In the initial stage, i.e. primary creep, the strain rate is relatively high, but slows with increasing time. This is due to work hardening. The strain rate eventually reaches a minimum and becomes near constant. This is due to the balance between work hardening and annealing (i.e. thermal softening). The substantially constantly growing stage is known as secondary or steady creep. The characterised “creep strain rate” typically refers to the rate in this secondary stage. Stress dependence of this rate depends on the creep mechanism. Finally with tertiary creep, the strain rate exponentially increases with stress because of necking phenomena. Fracture always occurs at the tertiary stage. 
     In the case of the degradation in the metal-to-metal contact resistance of the cable/connector, it is the primary stage and the secondary stage of creep that are most applicable, although the tertiary creep may apply over a long time period and exposure to temperature extremes, which can be the case in some CATV applications. 
     In addition to creep phenomenon, a further problem was identified during the above research. This problem relates to the metal-to-metal galvanic reaction between the CATV F connector plating material and that of the coaxial cable aluminium braid. This is even more serious problem than creep, and it specifically affects the low frequency transfer impedance of the coaxial cable, as well as to some extent the screening effectiveness. 
     Any galvanic reaction between the connector and coaxial cable grounding contact points will eventually lead to one of the most serious problems in any broadband cable network, namely the generation of Common Path Distortion (CPD). CPD is a collective term, which includes all beat products which are generated within a broadband cable system, that fall within the upstream return path frequency spectrum. The beat energy generated that falls within the upstream spectrum results when the forward path signals pass over a connection point. This excludes any beat energy generated by active components. CPD is caused by a connection point that exhibits a nonlinear transfer characteristic as shown above. CPD is one of the most difficult and problematic issues within any broadband cable system, since any faults of the system generally exhibit as intermittent issues, and as a result, are very difficult to identify. For this reason, CPD can sometimes be misinterpreted as upstream ingress noise. 
     Major in-depth research over many years has shown that the F connector metal-to-metal contact between the actual device it connects to and the cable mating parts is a key issue regarding CPD. Research has shown that NiSn (nickel tin) against NiSn plating produces the best option for metal-to-metal contact with minimum effect on CPD. As a result, most connectors are plated with either NiSn or nickel. Nickel does not perform as well, as it is harder plating than NiSn, but nevertheless is still deployed in large volumes. 
     Consequently, the NiSn or nickel plating of the coaxial F connector is connected to the coaxial cable braid of unalloyed aluminium. However, aluminium is one of the worst possible materials when it comes to avoiding any form of galvanic corrosion effect with other metals. It is generally known that NiSn and nickel are a major problem when in contact with aluminium producing a galvanic voltage differential of 290 and 660 mV, respectively. 
     Moreover, the fact that the contact force reduces due to creep means that aluminium will start to further oxidise as it becomes exposed to air and possible moisture. Aluminium oxidisation is in two parts, and has two key issues with pressure type contacts. The first relates to poor surface conductivity due to insulating A1203 layer (known as sapphire) forming and constantly growing on the surface area, when the aluminium is exposed to air. The A1203 layer is a diamond-like layer and it is an excellent insulator. Any presence of water/moisture would also form an additional insulating material of aluminium hydroxide in the joint. 
     The galvanic reaction between a NiSn plated F connector and aluminium cable braiding can be illustrated by the test results shown in  FIGS. 3 a  and 3 b   .  FIG. 3 a    shows the coupling transfer function of a non-used cable interconnect assembly with a NiSn plated F connector and aluminium cable braiding. It can be seen that the coupling transfer function meets the Class A++ requirements practically throughout the required frequency range. 
       FIG. 3 b    shows the coupling transfer function of the same cable interconnect assembly after the same temperate cycle test as above in connection with  FIG. 2 b   , but with the cable assembly then left in open air for 4 weeks. As can be seen in  FIG. 3 b   , both the low frequency transfer impedance and overall screening effectiveness are very far from meeting the Class A++ requirements. The low frequency transfer impedance from 5 MHz to the cut-off frequency is in effect showing the degradation in the contact resistance between the cable braid and the connector body. The transfer impedance is shown in mΩ/metre and is a clear indication of potential CPD problem. The transfer impedance shows a serious increase in the metal-to-metal contact resistance between the cable braiding and the connector. This is clearly caused by galvanic reaction, which was further proven by cutting off the connectors and fitting the cable with fresh connectors whereby the cable reverted back to its original performance before temperature cycling. 
       FIG. 4  shows a schematic cross-sectional view of a prior art F male compression connector with a coaxial cable connected to a F female connector. The dimensions of various parts in  FIG. 4  are not in scale. It is noted that the structure of the F female connector is not relevant for illustrating the underlying problems. The F male compression connector comprises the fixing nut  400 , the ferrule  402  and the body  404 . The F male compression connector is connected to the coaxial cable  406  such that the stripped dielectric insulator  408  and the inner conductor  410  of the coaxial cable are inserted in the ferrule  402  and the PVC jacket  412  of the cable is tightly compressed. The aluminium braiding  414  of the coaxial cable is in contact with the outer surface of the ferrule, thus providing ground connection. The body  404  of F male compression connector is connected to the F female connector  416  by screwing the fixing nut  400  to a corresponding thread in the body of the F female connector  416 . 
     The problems arise from the fact that the ferrule  402  is typically NiSn plated and the braiding  414  of the coaxial cable is aluminium. The metal-to-metal contact points between the coaxial cable aluminium braid  414  and the NiSn plated connector ferrule  402  are the points at which said two parts mate to form the overall grounding point, but also the points which are subjected to galvanic corrosion due to above-described phenomena. Since the coaxial cable aluminium braid  414  and the NiSn plated connector ferrule  402  are not making an intimate metal-to-metal contact, an oxidising layer is developed, in this case due to dissimilar metals, as well as lack of contact pressure. It is this energy that generates what is called the diode effect that in effect causes the nonlinear energy transfer (i.e. CPD) to occur. 
     Consequently, there is a need for an improved arrangement for connecting a coaxial cable to a connector so as to reduce the galvanic reaction between the cable braid and the connector ferrule. 
     Now there has been invented a new connector design for compensating the galvanic reaction, which is applicable to both F type compression connectors and F type crimp connectors. 
     Accordingly, there is provided a coaxial cable connector for connecting a coaxial cable, wherein the connector comprises a ferrule arranged to be configured in electrical contact with at least one metal braid layer of the coaxial cable, the ferrule comprising an elongated body for the electrical contact with said at least one metal braid layer of the coaxial cable, wherein at least the body is plated with tin and a base comprising an opening for insertion of a centre connector of the coaxial cable through the elongated body, wherein at least surfaces of the opening of the base are covered with a material preventing short-circuit between the centre connector of the coaxial cable and the base. 
     There are various embodiments for covering at least surfaces of the opening of the base with a material preventing short-circuit between the centre connector of the coaxial cable and the base. According to an embodiment, the base is plated with nickel or nickel-tin. 
     Thus, by using tin plating on the elongated body, a nearly optimal metal-to-metal contact to unalloyed aluminium may be obtained in terms of minimum galvanic reaction of dissimilar metals. As mentioned above, aluminium is one of the worst possible materials when it comes to avoiding any form of galvanic corrosion effect with other metals. According to galvanic charts of metal-to-metal contacts, pure gold or cadmium are primarily recommended for contact with aluminium. However, as gold and cadmium are rare and expensive metals, a more preferred metal for contact with aluminium in industrial applications is tin, which is, according to the galvanic charts, the third preferred metal for contact with aluminium. 
     According to an embodiment, instead of simply plating the entire ferrule with tin plating, the ferrule may be divided in the body, which is tin plated, and the base, which is nickel or nickel-tin plated. The reason underlying the division stems from a phenomenon called “tin whiskers”. 
     Tin whiskers are electrically conductive, crystalline structures of tin that sometimes grow from surfaces where tin (especially electroplated tin) is used as a final finish. Tin whiskers have been observed to grow to lengths of several millimetres (mm) and in rare instances to lengths in excess of 10 mm. Numerous electronic system failures have been attributed to short circuits caused by tin whiskers that bridge closely-spaced circuit elements maintained at different electrical potentials. Tin is only one of several metals that are known to be capable of growing whiskers. 
     It is noted that the term “whiskers” is different than a more familiar phenomenon known as “dendrites” commonly formed by electrochemical migration processes. A whisker generally has the shape of a very thin, single filament or hair-like protrusion that emerges outward (z-axis) from a surface. Dendrites, on the other hand, form in fern-like or snowflake-like patterns growing along a surface (x-y plane) rather than outward from it. The growth mechanism for dendrites is well-understood and requires some type of moisture capable of dissolving the metal (e.g. tin) into a solution of metal ions which are then redistributed by electro migration in the presence of an electromagnetic field. While the precise mechanism for whisker formation remains unknown, it is known that whisker formation does not require either dissolution of the metal or the presence of electromagnetic field. Consequently, the growth of tin whisker may become a significant problem causing electrical short circuit issues. 
     If tin plating were applied over the whole ferrule, the whiskers could short circuit the outer body of the connector to the coaxial cable centre conductor. Therefore, by dividing the ferrule in a tin plated body and a nickel or nickel-tin plated base, the existence of short circuit problems mainly in the area of ferrule base are prevented. 
       FIGS. 5 a  and 5 b    illustrate the difference between the prior art ferrule ( FIG. 5 a   ) and the ferrule according to the above embodiments ( FIG. 5 b   ).  FIG. 5 a    shows a one-piece ferrule  500  plated with nickel (Ni) or nickel-tin (NiSn) as currently on market. The coaxial cable  502  has a braid layer of unalloyed aluminium (Al), as the coaxial cables in typical network installations currently have. When the coaxial cable  502  is connected to a connector, such as a F connector, comprising a one-piece ferrule  500  plated with nickel or nickel-tin, the Ni/NiSn-to-Al contact eventually causes a serious galvanic reaction. 
       FIG. 5 b    shows a dual plated two-piece ferrule  510  comprising a body  510   b  plated with tin (Sn) and a base  510   a  plated with nickel (Ni) or nickel-tin (NiSn). The coaxial cable  512  again has a braid layer of unalloyed aluminium (Al). Now when the coaxial cable  512  is connected to a connector, such as a F connector, comprising a dual plated two-piece ferrule  510 , the aluminium braining is arranged in electrical contact only with the tin plated body  510   b  of the ferrule, i.e. the aluminium braining does not get in electrical contact with the Ni/NiSn plated base  510   a  of the ferrule. The Sn-to-Al contact then minimises the galvanic reaction. 
     According to an embodiment, the body and the base of the ferrule are arranged to be plated separately and connected together after plating. Due to the different plating materials, the ferrule according to the embodiments is easier to manufacture, if the base and the body are separate parts, which are plated separately. After plating, the two ferrule parts are connected together, e.g. by pressing, to form a complete dual plated ferrule assembly, which achieves a nearly minimum galvanic potential between the tin plated ferrule body and the unalloyed aluminium coaxial cable braid. The assembly is then inserted into a connector, such as a body of the F connector, as normal. 
     According to another embodiment, the advantageous galvanic properties of tin may be utilised such that the base of the ferrule, especially the surfaces of the opening of the base through which the coaxial cable centre conductor extends, is covered with plastic or silicone. The base may be at least partially plated with a plastic or silicone, or there may be a separate plastic or silicone ferrule insert covering the outer part of the base of the ferrule. Thus, the actual metal ferrule may be implemented as a one-piece ferrule plated with tin, while the plastic cover at the outer part of the base of the metal ferrule prevents the growth of tin whiskers in said area, and thereby no short circuit problems are caused in the area of ferrule base. 
     Zinc as a stand-alone material has similar galvanic properties as tin. Therefore, the ferrule can be made as one-piece component, which is either plated with tin or made of zinc or zinc alloy. 
     According to an embodiment, the body and the base form a one-piece ferrule either plated with tin or made of zinc or zinc alloy, wherein at least the surfaces of the opening of the base are covered with an insert adaptable to the base, said insert being plated with nickel or nickel-tin or made of a plastic or silicone.  FIGS. 5 c -5 f    show some examples according to this embodiment. 
       FIG. 5 c    shows an example of a one-piece ferrule  520  plated with tin or made of zinc or zinc alloy and an insert  530  plated with nickel or nickel-tin adaptable into the opening of the base of the ferrule  520 . The left-hand side figure shows the ferrule  520  and the insert  530  as separated and right-hand side figure shows the ferrule  520  and the insert  530  as fully connected. In this example, the insert covers the complete existing front surface of the base, including the outer circular surface of the base and the inner surfaces of the opening of the base where the coaxial cable dielectric and the centre connector of the coaxial cable are fitted. 
     Compared to the embodiment with the dual plated two-piece ferrule, the end result here is the same in that the ferrule body is tin plated and the front part of the ferrule is nickel or nickel-tin plated. Keeping the ferrule as a standard one part ferrule and then plating the complete ferrule with tin or manufacturing the ferrule of zinc, the manufacturing process is made much easier. When a simple press-in front Ni/NiSn-plated insert is included in the ferrule, the technical effect is that the tin whiskers issues are avoided at the front end of the connector. 
       FIG. 5 d    shows another example of a one-piece ferrule  520  plated with tin or made of zinc or zinc alloy and an insert  530  plated with nickel or nickel-tin adaptable into the opening of the base of the ferrule  520 . Again, the left-hand side figure shows the ferrule  520  and the insert  530  as separated and right-hand side figure shows the ferrule  520  and the insert  530  as fully connected. In this example, the insert covers substantially the existing front surface of the base and the inner surfaces of the opening of the base where the coaxial cable dielectric and the centre connector of the coaxial cable are fitted, but not the outer circular surface of the base. 
       FIG. 5 e    shows yet another example of a one-piece ferrule  520  plated with tin or made of zinc or zinc alloy and an insert  530  plated with nickel or nickel-tin adaptable into the opening of the base of the ferrule  520  as fully connected. In this example, the insert covers substantially the existing front surface of the base as a simple press-in ring. Herein, the front surface of the base may comprise a cavity into which the insert can be fitted. 
       FIG. 5 f    shows an example of a one-piece ferrule  520  plated with tin or made of zinc or zinc alloy and an insert  530  made of plastic or silicone adaptable into the opening of the base of the ferrule  520  as fully connected. In this example, the insert covers only a part of the existing front surface of the base, as well as partly the inner surface of the opening of the base where the coaxial cable dielectric and the centre connector of the coaxial cable are fitted, as a simple press-in ring. 
       FIG. 5 g    shows an example according to an embodiment, where the ferrule is implemented as a dual plated one-piece ferrule comprising a body  540  plated with tin (Sn) and a base  550  plated with nickel (Ni) or nickel-tin (NiSn). This may complicate the plating process, but nevertheless, manufacturing the ferrule as a standard one part ferrule may provide compensating advantages. 
       FIGS. 10 a -10 e    show yet another embodiment of a one-piece ferrule part plated with tin or made of zinc or zinc alloy and an insert plated with nickel or nickel-tin adaptable into the opening of the base of the ferrule part. The main difference to the above embodiments is that the insert is first inserted into the fixing nut (e.g.  400  in  FIG. 4 ) of the F male connector and then through a central hole of the fixing nut further into the opening of the base of the ferrule part. 
       FIG. 10 a    shows the zinc(-alloy) made or tin-plated part  1010  of the ferrule and the insert  1020  plated with nickel or nickel-tin as adapted into the opening of the base of the ferrule part. The ferrule part  1010  is placed inside the body  1030  of the connector. It is noted that in  FIG. 10 a    the fixing nut is not shown.  FIG. 10 b    shows more in the detail an exemplified structure of the opening of the base of the ferrule part. 
       FIG. 10 c    shows an exemplified structure of zinc(-alloy) made or the tin-plated part  1010  of the ferrule. The tin-plated part extends inside the body as an elongated part, similar to the embodiments shown in  FIGS. 5 a -5 g   , wherein there is a longitudinal through-hole in the middle of the tin-plated part for the insertion of the coaxial cable dielectric and the centre connector of the coaxial cable. 
       FIG. 10 d    shows a side view of the zinc(-alloy) made or tin-plated part  1010  of the ferrule and the insert  1020  plated with nickel or nickel-tin as adapted into the opening of the base of the ferrule part. It can be seen that the insert  1020  comprises a stepwise narrowing such that there is a gap  1040  between the insert  1020  and the tin-plated part  1010  of the ferrule to accommodate the thickness of the base of the fixing nut material. 
       FIG. 10 e    shows the fixing nut  1050  as connected to the zinc(-alloy) made or tin-plated part  1010  of the ferrule. The insert  1020  is not shown since it has been inserted into the fixing nut  1050  and further through a central hole of the fixing nut into the opening of the base of the tin-plated part  1010  of the ferrule. It can be seen that the base of the fixing nut is fitted in the gap  1040  shown in  FIG. 10 d   . The arrangement of  FIGS. 10 a -10 e    may provide the advantage of an improved mating of the nickel part of the ferrule to the fixing nut when the F male connectors are installed and tightened onto the corresponding F female connector. 
     The various embodiments of ferrules as described above address well the problem of the galvanic reaction between the cable braid and the connector ferrule. However, there still remains the problem caused by material creep of the PVC jacket of coaxial cable when connected to a typical F connector. 
     The mechanism for connecting the F male compression connector to the coaxial cable is further illustrated in  FIG. 6 . The coaxial cable  600  is shown on the right side before the cable insertion. The coaxial cable  600  comprises the centre conductor  602  and the dielectric insulator  604 . The coaxial cable  600  further comprises the braiding  606  and the PVC jacket  608 , which have been stripped away around the dielectric insulator  604  for the installation. A stand-alone F male compression connector  610  is shown on the left side as before the cable insertion. The connector comprises the ferrule  612 , the outer body  614  of the fixing nut, and the inner body  616  of the fixing nut. The inner body  616  is typically made of plastic. The side of the outer body  614  facing the inner body is slanted such that when pushed against the inner body  616  upon the insertion of the coaxial cable  600 , the inner body bends inside and compresses the PVC jacket  608  of the coaxial cable. 
     The mechanism is typical for most F type compression connectors. When coaxial cable  600  has been properly inserted in the connector  610 , the bended inner body  616  applies pressure between the cable braid  606  and the connector ferrule  612 , which is the key metal-to-metal electrical contact between the cable and connector that will maintain optimum RFI shielding and transfer impedance. Whilst the connector compression is carried out, primarily to secure the cable and to prevent it from pulling out of the connector, the process adds some pressure force between the ferrule  612  and the braid  606 . 
     However, as described above, the pressure between the cable braid  606  and the connector ferrule  612  will degrade over time due to the inherent material creep of the PVC jacket  608 . As the PVC jacket creeps, it becomes thinner and thinner at the pressure point, and consequently the pressure will slowly degrade to a point whereby there is practically no pressure. Moreover, the pressure point between the cable braid  606  and the connector ferrule  612  is rather narrow and situated close to the end of the ferrule. In addition to F type compression connectors, the problem applies to F type crimp connectors currently on market. 
     According to an embodiment, to at least alleviate the above problem, the coaxial cable connector may further comprise a compression fitting arranged around said ferrule; and means for applying a pressure force to the compression fitting such that a surface of the compression fitting applies a force to an outer insulating layer of the coaxial cable surrounding said at least one metal braid layer substantially over the whole length of said surface of the compression fitting. 
     Thus, in comparison to a standard coaxial connector, there is provided a compression fitting around the ferrule. When the coaxial cable is inserted in the connector, a pressure force is applied on the compression fitting, which is compressed in a direction perpendicular to the elongated ferrule. Hence, a surface of the compression fitting applies a force to an outer insulating layer of the coaxial cable, i.e. the PVC jacket, and further to the area of the electric contact surface between the metal braid layer and the ferrule. The force applied by the surface of the compression fitting to the PVC jacket is advantageously distributed substantially over the whole length of said surface of the compression fitting. 
     Hence, the amount of surface area of the pressure point at the metal-to-metal contact is significantly increased, and the pressure force is distributed to a much wider area. As a result, the PVC cable jacket and aluminium cable creep is prevented, which would otherwise reduce the contact force over time. Consequently, the eventual total signal failure and major RF screening leakage is prevented. 
     According to an embodiment, a cross-section of the compression fitting comprises a first surface arranged substantially co-axially with the ferrule such that, in an uncompressed state, there is a space between said first surface and the ferrule, and at least one slanted second surface to which said pressure force is configured to be applied. 
     As mentioned above, the compression fitting is arranged around the elongated ferrule in ring-like manner. When the compression fitting is in uncompressed state, the cross-section of the compression fitting comprises a first surface arranged substantially co-axially with the ferrule such that there is a space between said first surface and the ferrule. When the coaxial cable is inserted in the connector, the metal braid layer and the PVC jacket are guided in the space between the surface of the compression fitting and the ferrule. 
     The cross-section of the compression fitting may further comprise at least one slanted second surface to which said pressure force is configured to be applied. When the coaxial cable is inserted in the connector, the pressure force is applied to the slanted surface, which pressure force, in turn, compresses the first surface tightly against the PVC jacket, whereby the space no longer exists. 
     Thus, the pressure effect achieved by the compression fitting resembles that of a plumbing olive; i.e. a compression ring or ferrule used in joining two tubes or pipes together, wherein a compressed olive seals a space between the pipe, a compression nut and a receiving fitting, thereby forming a tight joint. 
     According to an embodiment, the compression fitting is made of silicone. Silicone, being a rubber-like elastic polymer, has turned out to be a suitable material for the compression fitting such that a constant, sufficiently high pressure force can be applied substantially over the whole area of the metal-to-metal contact between the metal braid layer and the ferrule. 
     A connector design according to an embodiment is shown in  FIGS. 7 a  and 7 b   .  FIG. 7 a    shows the connector design and the compression fitting in an uncompressed state. The coaxial cable  700  is shown on the right side before the cable insertion. The coaxial cable  700  comprises the centre conductor  702  and the dielectric insulator  704 . The coaxial cable  700  further comprises the braiding  706  and the PVC jacket  708 , which have been stripped away around the dielectric insulator  704  for the installation. 
     A stand-alone connector  710  is shown on the left side as before the cable insertion. The connector comprises the dual plated two-piece ferrule  712  and a compression fitting  714  arranged around said ferrule. The cross-section of the compression fitting  714  comprises a first surface  714   a  arranged substantially co-axially with the body of the ferrule. In the uncompressed state, there is a space  716  between said first surface  714   a  and the ferrule  712 , and at least one slanted second surface  714   b  to which said pressure force is configured to be applied. 
     According to an embodiment, said means for applying the pressure force may comprise a taper  718  arranged to slide against said at least one slanted second surface  714   b  upon pushing the coaxial cable into the connector. Thus, when the taper slides against the slanted second surface, there is a pressing force on the compression fitting. When the compression fitting  714  is compressed towards the ferrule  712 , the first surface  714   a  of the compression fitting eventually applies a force to the PVC jacket of the coaxial cable surrounding the metal braid layer substantially over the whole length of the first surface  714   a  of the compression fitting. 
     According to an embodiment, said taper  718  is arranged to an outer body  720  of the connector  710 , which outer body  720  is arranged to move towards the ferrule  712  upon pushing the coaxial cable into the connector. As a result, when the coaxial cable is inserted into the connector, the taper  718  automatically slides against the slanted second surface  714   b  and applies a pressing force on the compression fitting. 
       FIG. 7 b    shows the connector design and the compression fitting in a compressed state when the coaxial cable has been inserted into the connector. In  FIG. 7 a   , the centre conductor  702  and the dielectric insulator  704  of the coaxial cable have been inserted in a cavity of the ferrule (not shown) such that the centre conductor  702  extends to the other side of connector so as to be connected to a female connector. Upon the insertion of the coaxial cable  700 , the braiding  706  and the PVC jacket  708  have been guided to the outer surface of the ferrule such that the cable braid  706  forms a metal-to-metal electrical contact (not shown) with the connector ferrule. 
     Now, upon the insertion of the coaxial cable  700 , the taper  718  attached to the outer body  720  of the connector has slid against the slanted second surface  714   b  of the compression fitting  714 , thereby applying a pressing force on the compression fitting. The outer surface of the compression fitting may be coated with silicone grease to reduce friction from taper  718  when the connector is compressed. As a result, the first surface  714   a  of the compression fitting has moved towards the ferrule and finally applied a force to the PVC jacket of the coaxial cable surrounding the metal braid layer. The pressure points of the force, indicated by arrows  722 , distribute evenly substantially over the whole length of the first surface  714   a  of the compression fitting. When the PVC cable jacket creeps and becomes thinner at the pressure point, the silicone surface  714   a  of the compression fitting compensates for the deformation by expanding against the PVC jacket such that the pressure force at the electrical contact remains substantially constant. This is particularly important when using unalloyed aluminium cable braiding, as the contact resistance may degrade significantly due to the aluminium braiding oxidising and galvanic reaction between dissimilar metals. 
     According to an embodiment, before the insertion of the coaxial cable  700 , the PVC jacket  708  of the coaxial cable is stripped away at least for such length that, when inserted, the first surface  714   a  of the compression fitting applies a force to the metal braid layer. Due to the evenly distributed force of the compression fitting, the metal braid layer is not damaged and no aluminium braiding oxidising occurs. 
     According to an embodiment, the compression fitting is made of silicone having Shore A hardness value of 20 to 70. The hardness of materials may be measured according to Shore scales. There are at least 12 different Shore scales, and the hardness of various elastic materials, such as polymers, elastomers, and rubbers, are typically measured in Shore scales OO, A and D. Herein, the material hardness needs to be considered carefully, as it needs to be able to maintain a constant, high pressure force distributed over the length of the compression fitting on to the cable PVC jacket at the pressure point. Silicone can be manufactured at various hardness levels. The experiments have shown that best results for the compression fitting shown in  FIGS. 7 a  and 7 b    are achieved by a soft to medium hard silicone having Shore A scale hardness value of about 20-70. 
     A connector design according to another embodiment is shown in  FIG. 8 , which shows the connector design and the compression fitting in a compressed state when the coaxial cable has been inserted into the connector. When compared to the connector design shown in  FIGS. 7 a  and 7 b   , the structure is otherwise similar, but according to an embodiment, said means for applying the pressure force comprise a framing  800  of the outer body  802  of the connector, which is arranged to slide against said at least one slanted second surface  804  upon screwing the outer body  802  to a thread  806  of the connector. 
     The same advantages as in the embodiment disclosed in  FIGS. 7 a  and 7 b    are obtained herein, as well. The pressure points of the applied force distribute evenly substantially over the whole length of the compression fitting against the ferrule. The silicone surface of the compression fitting compensates for the deformation of the PVC jacket due to creep by expanding against the PVC jacket such that the pressure force at the electrical contact remains substantially constant. 
     According to an embodiment, the compression fitting is made of silicone having Shore A hardness value of 60 to 100. Herein, the forces applied by the framing, when the outer body is screwed to the thread of the connector, may be greater than in the embodiment disclosed in  FIGS. 7 a  and 7 b   . Therefore, it may be preferable to have a stronger structure of the compression fitting. The experiments have shown that best results for the compression fitting shown in  FIG. 8  are achieved by a medium to hard silicone having Shore A scale hardness value of about 60-100. 
     According to an embodiment, the connector is a F type male compression connector or a F type male crimp connector. However, it is noted that the idea underlying the embodiments is not limited to F type connectors only. The compression fitting according to the embodiments may be applied to any other type of connector having an elongated ferrule. Moreover, while the means for applying a pressure force to the compression fitting in these examples refer to the pressure force applied by a slanted surface arranged to the outer body of the connector, said means may be implemented in various ways, depending on the structure of the connector in question. 
       FIG. 9  shows the dual plated two-piece ferrule and the compression fitting combined in the same connector. Thus, the ferrule of the connector  910  is implemented as the dual plated two-piece ferrule comprising a body  914  plated with tin (Sn) and a base  912  plated with nickel (Ni) or nickel-tin (NiSn) such that the aluminium braiding of the coaxial cable  900 , when inserted in the connector  910 , is arranged in electrical contact only with the tin plated body  914  of the ferrule, but not with the Ni/NiSn plated base  912 . 
     The body  914  of the ferrule is surrounded by the compression fitting  916 , which, when the coaxial cable  900  is inserted in the connector  910 , applies a pressure force to an outer insulating layer of the coaxial cable, i.e. the PVC jacket, and further to the area of the electric contact surface between the metal braid layer and the ferrule, wherein the pressure force is distributed substantially over the whole length of said surface of the compression fitting. 
     Consequently, by combining the dual plated two-piece ferrule and the compression fitting in the same connector, the dual plated two-piece ferrule addresses effectively the galvanic reaction at the metal-to-metal contact and the compression fitting advantageously compensates for the creep phenomenon of the outer insulating layer of the coaxial cable. 
     According to an embodiment, the cable interconnect assembly, i.e. the coaxial cable connected with the connector according to the embodiments, is sealed once connected to the end device to further ensure that no moisture can enter the connector. This is especially advantageous if cables with aluminium braiding are used with the connector. 
     The sealing may be carried out, for example, using a so-called air shrink rubber. That is a sleeve around the cable interconnect assembly, which is chemically swellable and which is initially in dilated configuration, and which subsequently shrinks into place by evaporation of the volatile dilation composition. The air shrink rubber provides a protective cover for a cable connection or splice which can be easily installed, quickly shrunk into tight vapor resistant protective covering within a matter of a few minutes, and can be installed without the need for any application of heat or use of special tools, equipment or materials. For further details of the usage of the air shrink rubber, a reference is made to U.S. Pat. Nos. 5,801,333 and 5,977,484 
     A skilled man appreciates that any of the embodiments described above may be implemented as a combination with one or more of the other embodiments, unless there is explicitly or implicitly stated that certain embodiments are only alternatives to each other. 
     It is obvious that the present invention is not limited solely to the above-presented embodiments, but it can be modified within the scope of the appended claims.