Abstract:
A high frequency coaxial cable having a foil ( 7   a ) between the cable insulator ( 5 ) and cable braid ( 7   b ), is terminated to a coaxial connector ( 40 ) in a manner that allows fast and easy cable preparation and results in a termination with minimal axial electric field lines that cause a high insertion loss and a high VSWR (voltage standing wave ratio). A bore ( 46 ) at the rear portion ( 42 ) of the connector outer conductor, receives the cable insulator with foil around the cable insulator. The bore has a front part ( 54 ) that forms an interference fit around the foil, to avoid an axially-extending gap which might contain axially-extending field lines. The front of cable insulator and foil are flush and both abut the insulation ( 25 ) of the connector.

Description:
CROSS-REFERENCE  
       [0001]     Applicant claims priority from British patent application 0419303.3 filed 31 Aug. 2004.  
       BACKGROUND OF THE INVENTION  
       [0002]     This invention relates to a coaxial connector for terminating to a high performance coaxial cable of the type that has a wrapped conductive shield. A coaxial cable includes a solid or stranded inner cable conductor surrounded by a layer of polymer dielectric material. The dielectric material is precisely centered within a woven braid outer cable conductor, and the cable has an outer jacket of polymer material. The outer cable conductor defines a ground return path which is necessary for microwave signal transmission.  
         [0003]     High performance, low loss coaxial cables have been developed to transmit higher frequencies with minimal impedance discontinuities. With low loss dielectrics, these cables may transmit higher power levels with minimal attenuation. The high performance cables generally comprise an inner cable conductor surrounded by a low loss dielectric material such as cellular polyethylene, a thin wrapped metallic outer shield such as a conductive foil, a woven plated copper braid shield, and a polymer outer jacket such as polyvinyl chloride (PVC). This type of cable is desirable for use in the transmission of high rate digital signals such as those used in the High Definition Television (HDTV) industry, of a frequency of about 1 GHz and higher.  FIG. 1  shows such a high performance coaxial cable  1  which comprises a center cable conductor  3  and an outer cable conductor  7  formed by a thin wrapped metallic foil  7   a  and a woven braid outer conductor  7   b.  A dielectric material, or insulator  5  separates the center conductor  3  and the outer conductor  7 . The entire cable  1  is enclosed in an outer jacket  9 .  
         [0004]     Cables are generally prepared for termination to a coaxial connector by stripping, or removing, from around the center cable conductor, the dielectric material, the braid and the cable jacket to strip lengths specified by the manufacture of the RF coaxial connector. In the case of the high performance coaxial cable having a wrapped metallic foil shield, the foil is generally removed and stripped back approximately evenly with the jacket, as shown in  FIG. 2   a . The removal of the metallic foil in this way is an inconvenience for cable assembly manufacturers and cable installers because it requires the foil to be stripped back behind (within) the braid that surrounds it. This operation is time consuming and requires special tools, and may lead to damage of the braid.  
         [0005]     A preferred termination technique would be to leave the metallic foil intact, i.e. flush with the dielectric material and/or braid. However, this presents a problem in terms of electrical performance. At lower frequencies, cables prepared and terminated in this way exhibit no electrical performance problems, with particular respect to return loss. However, at higher frequencies, a convoluted signal path occurs, and a higher than expected return loss or VSWR (voltage standing wave ratio) is exhibited.  
       SUMMARY OF THE INVENTION  
       [0006]     According to the invention, there is provided a radio frequency coaxial connector for terminating a coaxial cable of the type that includes a center cable conductor, a dielectric cable insulation surrounding the center conductor, and a cable outer conductor that includes a conductive foil surrounding the dielectric material. The connector includes a tubular metallic connector having a rear end for receiving the coaxial cable and having a front end for interfacing with a complimentary connector, and a tubular insulator located within the connector outer conductor. The rear end of the connector outer conductor forms an open bore for receiving the cable center conductor, cable dielectric material and the conductive foil. A part of the bore is of a reduced diameter to provide an interference fit between walls of the connector bore and the cable conductive foil. The reduced inner diameter of the bore is preferably located adjacent to the connector insulator.  
         [0007]     In use, the cable center conductor, the cable insulator surrounding the center conductor and the cable conductive foil, are received into the bore in the rear end of the coaxial connector. The conductive braid is placed around the rear end portion of the connector outer connector. The cable portion with foil on the outside is easily received into a rear part of the bore in the connector outer conductor, but the reduced diameter of a front bore part provides an interference fit between the conductive foil of the cable and the inner surface of walls of the bore in the connector outer conductor. This interference fit eliminates any clearance space between the conductive foil of the cable and the inner surface of the bore, and thereby eliminates a longitudinal electric field between the conductive foil and the connector body.  
         [0008]     It has been found that prevention of such a longitudinal electric field is an effective way of maintaining the radial orientation of the electric field, thereby ensuring good electrical performance at higher frequencies.  
         [0009]     The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a partially cut away view of a prior art high performance coaxial cable.  
         [0011]      FIGS. 2   a  and  2   b  are cross sectional view on one side of the axis, of the prior art high performance coaxial cable shown in  FIG. 1 , and shown terminated with a prior art coaxial connector.  
         [0012]      FIG. 3  is a cross sectional view showing the distortion of the electric field lines within a transmission line which is caused by a change in the conductor geometry.  
         [0013]      FIG. 4  is a cross sectional view of a coaxial connector according to the invention.  
         [0014]      FIG. 5  is a cross sectional view on one side of the axis, of the high performance coaxial cable shown in  FIG. 1  terminated with the coaxial connector shown in  FIG. 4 .  
         [0015]      FIGS. 6   a,    6   b  and  6   c  show predicted return loss for the terminated coaxial connectors shown in  FIGS. 2   a,    2   b  and  5  respectively.  
         [0016]      FIGS. 7   a,    7   b  and  7   c  show predicted voltage standing wave ratios (VSWR) for the coaxial connectors shown in  FIGS. 2   a,    2   b  and  5  respectively. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]      FIG. 1  shows a prior art high performance (low losses at frequencies of about 1 GHz and somewhat higher) coaxial cable  1 . The cable includes coaxial inner and outer cable conductors  3 ,  7 , a dielectric layer or insulator  5  between the conductors, and a protective outer jacket  9 . The cable outer conductor  7  includes a conductive foil  7   a  lying around and against the insulator  5  and a conductive braid  7   b  lying around the foil.  
         [0018]      FIG. 2   a  shows the coaxial cable  1  of  FIG. 1  terminated to a prior art coaxial connector  11 . Only the right portion of the connector  11  that receives the cable  1  is shown in the Figure, and only portions on one side of the coincident cable and connector axis  12  is shown. The cable jacket  9 , has been stripped back (cut away) from around the cable center conductor  3  and the insulator  5 . The conductive foil  7   a  also has been stripped back to a location within the cable braid  7   b  to be approximately flush with the cable jacket  9 . The center conductor  3  and the cable insulator  5  are received within a rear end portion  13  of the connector outer conductor  11 . The exposed cable center conductor  3  is received in a connector center conductor contact pin  23 , and a front end of the cable insulator  5  abuts a corresponding connector insulator element  21  in the connector  11 . The braid  7   b  of the cable outer conductor is received around the outer surface of the rear end portion  13  of the connector. A ferrule, or crimp tube  15  is crimped onto an outer surface of the connector outer conductor rear end  13 , and around the cable jacket  9 . The crimp tube urges the braid  7   b  against the connector outer conductor rear end portion  13  and prevents the connector  11  from detaching from the cable  1 .  
         [0019]      FIG. 2   a  shows electric field lines L 1  extending between the cable center conductor  3  and the cable outer conductor  7 . It can be seen from the figure that the electric field lines L 1  in the intact cable insulator are radial to the axis  12 . The electric field lines are slightly distorted at L 2  in the region adjacent to the open rear end of the connector outer conductor portion  13 , where the braid is not parallel to the center conductor. However, the slight distortion of the electric field lines L 2  in this region does not cause significant reflection of energy and consequent loss. Within the rear end portion  13  of the connector outer contact, the radial orientation of the electric field lines is restored, with the field lines running from the center conductor  3  to the rear end portion  13  of the connector outer conductor (which is electrically connected to the braid  7   b ).  
         [0020]     Electric field lines of a high performance coaxial cable in the normal transverse electromagnetic mode of transmission are purely radial, and thus terminate perpendicular to the surfaces of the center and outer conductors. However, at sudden transitions in the diameter of the conductors, such as a step change in the conductor diameter of a coaxial connector, the electric field lines distort as at L 3  in  FIG. 3 , so as to maintain their perpendicular relationship with the conductor surfaces. This distortion in the electric field lines creates higher order modes of propagation. Since the connector is not usually designed to transmit these higher order modes of propagation, they are attenuated over a very short distance, and are thus localized in the vicinity of the discontinuity. The high modes of the propagation lead to a power loss from the normal transverse electromagnetic mode, which results in a higher than expected return loss, or VSWR (voltage standing wave ratio), at high frequencies. The distortions upon analysis appear capacitive, and are a major source of reflections within an otherwise matched impedance connector.  
         [0021]     It is almost impossible to avoid discontinuities in a connector design. For example, methods of terminating a cable to a connector often result in diameter variations between the cable and the connector. These variations require changes in conductor diameters to maintain the proper impedances, thus creating discontinuities. Below about 1000 MHz (1 GHZ), these discontinuities usually have no significant effect on the resulting return loss or VSWR. However, at higher frequencies, the discontinuities have a major impact on the performance of the connector.  
         [0022]     The terminated cable shown in  FIG. 2   a  provides acceptable performance in terms of return loss, even at high frequency applications such as high definition video cabling. However, as described above, the arrangement shown in  FIG. 2   a  requires that the end of the cable  11  be prepared by cutting the conductive foil  7   a  away from underneath the braid  7   b , so that the end of the conductive foil  7   a  is approximately flush with the end of the cable jacket  9 .  
         [0023]      FIG. 2   b  shows the prior art high performance coaxial cable  1  of  FIG. 1  terminated with the same prior art coaxial connector  11  shown in  FIG. 2   a . However, in this case, only the cable jacket  9  is stripped away from around or within the braid  7   b . The front end of the conductive foil  7   a  lies flush with the front end of the insulator  5 . This is the preferred way of preparing the cable, as it does not require any special effort or special tools. Again, for clarity, only the rear part of the connector  11  that receives the cable  1  is shown in the Figure.  
         [0024]     As shown in  FIG. 2   b , the cable center conductor  3 , insulator  5  and conductive foil  7   a  are received within the rear end portion  13  of the connector. The cable center conductor  3  is received into the connector center conductor contact pin  23  and the extreme front ends of the cable insulator  5  and the conductive foil  7   a  abut the insulator element  21  in the connector  11 . The conductive braid  7   b  is received around the outer surface of the outer contact end portion  13  of the connector and the crimp tube  15  is crimped onto the braid around the outer surface of the rear end  13  of the outer conductor of the connector  11 .  
         [0025]      FIG. 2   b  shows the electric field lines L 4  between the center conductor  3  and the outer conductive foil  7   a  of the known high performance coaxial cable  1  shown in  FIG. 1  when the cable is stripped in the easy way. It can be seen that electric field lines L 4  in the cable  1  are radial to the center conductor  3  and to the conductive foil  7   a.  It can also be seen that a gap region  30  exists between the outside surface of the conductive foil  7   a  and the inside surface  32  of the bore in the outer coaxial conductor rear portion  13 . Within the outer conductor rear portion  13 , electric field lines L 5  from the exposed end  34  of the cable center conductor  3  do not terminate at the conductive foil  3   a.  Instead, these field lines at L 5  extend in a longitudinal M or axial direction (parallel to the axis  42 ) from the front ends of the insulator  5  and conductive foil  7   a  and terminate at some point within the gap  30 . These longitudinal field lines are concentrated in the gap  30  formed between the conductive foil  7   a  and the inner surface  32  of the rear end portion  13  of the connector outer conductor. The gap is a result of clearance left to allow easy cable insertion. The electric field lines are considerably distorted, resulting in a so-called cylindrical reentrant cavity which causes the connector to resonate at a specific frequency.  
         [0026]      FIG. 4  shows a connector  40  of the invention for easily terminating a high performance coaxial cable having an outer conductive foil  7   a , which does not cause a cylindrical reentrant cavity and the consequential high return loss, even at high frequencies. These advantages are achieved without the need for the end of the cable to be specially prepared (as shown in  FIG. 2 ). The coaxial connector comprises a substantially tubular metallic connector outer conductor  19 , a substantially tubular insulator  25 , a connector center conductor contact pin  27  and a crimp tube  15 .  
         [0027]     A rear end portion  42  of the outer connector conductor  19  has a rearwardly R opening bore  46  for receiving the coaxial cable  44 . The rear end portion  42  of the outer connector conductor may be a different part than the rest of the outer conductor  19 , different sized rear portions  42  being provided for different sized cables  44 . An interface  19   b  is of the prior art design and provides a BNC plug for interfacing with a complimentary jack. The connector insulator  25  is located between the ends of the body  19  so as to be coaxial therewith. The insulator  25  comprises two insulator blocks  25 A,  25 B through which are formed holes on the connector axis  50 , the insulator  25 B being of harder material to guide the cable center conductor. The center, or inner conductor pin  27  is located in an axial hole of the insulator  25 . The pin comprises a pin portion  27 A for receiving, via the bore  46 , an end of the center conductor  3  of the coaxial cable. The connector  40  may also comprise a number of other components (not shown) such as a bayonet collar, gaskets, spring washers and split washers. These components are all known from existing connectors and will not be described further.  
         [0028]     The bore  46  in the rear end  42  of the connector outer conductor leads to the insulator  25 . The inner diameter of the bore steps from a first diameter A at the open rear part  52  to a second, smaller diameter B in the bore front part  54  which lies adjacent to the insulator  25 . The outer surface of the rear portion  42  of the outer conductor preferably has a knurled surface.  
         [0029]     In use, the high performance coaxial cable  44  is prepared in the same way as the cable shown in  FIG. 2   b , by stripping back the dielectric material  5  and the conductive foil  7   a  to be flush with each other (and usually with the braid  7   b , which shortens as it is expanded). This leaves an exposed portion of center conductor  3 . The prepared cable  44  is then received into the connector  40 .  
         [0030]      FIG. 5  represents the prepared cable  44  of  FIG. 4  fully installed in the connector  40 . It can be seen that the cable center conductor  3 , the cable insulator  5  and the cable conductive foil  7   a  are received within the bore  46  in the rear end of the connector outer conductor. The exposed portion of the cable center conductor  3  is received into the connector center conductor  2 . The extreme front ends  5   f  and  7   af  of the insulator  5  and conductive foil  7   a  then abut a rear end  25   r  of the insulator  25  of the connector  40 . The relative dimensions of the bore and the cable components are such that the cable insulator  5  and conductive foil  7   a  are easily received into the bore rear part  52 , but that the smaller bore front part  54  creates an interference fit with the conductive foil  7   a.    
         [0031]     In the specific example shown in  FIG. 5 , the outer diameter of the conductive foil  5  is 3.78 mm and the rear and front part inner diameters A, B of the bore are 3.9 mm and 3.68 mm respectively. Thus, there is a slight interference of about 0.1 mm between the foil and the front bore diameter. The cable insulator  5  compresses to allow the foil to fit into the front bore part. To further the connection of cable to the connector, the braid  7   b  is expanded to lie around the outer surface of the rear end portion  42  of the outer conductor and the crimp tube  15  is crimped around the braid.  
         [0032]      FIG. 5  shows the electric field lines L 6 , L 7  between the cable center and outer conductors  3 ,  7  and the connector outer conductor  19 . The electric field lines L 6  in the intact cable  44  are radial. Within the bore, the electric field lines are radial, terminating at the center conductor  3  and the conductive foil  7   a.  However, in contrast to the arrangement shown in  FIG. 2   b , there are only insignificant longitudinal electric field lines L 7  extending parallel to the axis  50 . This is because the interference fit between the conductive foil  5  and the inner surface of the bore front part  54  ensures that there are no clearance gaps and eliminates paths for electric field distortion. Instead, almost all of the electric field lines from the center conductor terminate directly to the connector body.  
         [0033]     As noted above, the elimination of the axial electric field lines reduces return loss and VSWR at high frequencies.  FIGS. 6   a,    6   b  and  6   c  are graphs showing predicted return loss for the terminated coaxial connectors shown in  FIGS. 2   a,    2   b  and  5  respectively. The graphs are directly comparable. It can be seen from the graph that the return loss for the coaxial connector of the invention ( FIG. 6   c ) is an improvement on that shown in  FIG. 6   b , and is similar to that shown in  FIG. 6   a.  For example, at a frequency of 5 GHz, the terminated coaxial connector arrangement of the invention results in a predicted return loss ( FIG. 6   c ) of −38 dB, while for the prior connector arrangement of  FIG. 2   b , the predicted return loss ( FIG. 6   b ) is −10 dB. For a large gap  32  ( FIG. 2   b ) there may be a resonance near the desired operating frequency resulting in dropoff of the signal.  
         [0034]      FIGS. 7   a ,  7   b  and  7   c  are directly comparable graphs showing predicted voltage standing wave ratio (VSWR) for the coaxial connectors shown in  FIGS. 2   a,    2   b  and  5  respectively. Again, it can be seen from the graphs that the VSWR for the coaxial connector of the invention ( FIG. 7   c ) is a considerable improvement on that shown in  FIG. 7   b , in that there is no specific resonant frequency. The VSWR for the coaxial connector of the invention is similar to that shown in  FIG. 7   a.    
         [0035]     In the connector described above, the bore of the rear end of the connector body has two inner diameters with a step between them. However, other bore profiles are suitable. For example, the inner diameter of the bore may gradually ramp from the first diameter to the second diameter, or more than two discrete inner diameters may be provided. What is important is that an interference fit is provided between the bore and the conductive foil of the cable adjacent the insulator arrangement of the connector.  
         [0036]     Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.