Patent Publication Number: US-6660939-B1

Title: Method and apparatus for shield slot signal coupler

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. utility patent application No. 10/064,946, titled “Shield Slot Tap”, filed Aug. 30, 2002, assigned to Andrew Corporation. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The invention relates to electrical cable couplers, also known as cable taps. More specifically, the invention relates to a cable coupler with low insertion losses and a variable coupling factor. 
     2. Description of Related Art 
     Many systems, for example in-building RF distribution systems or antenna arrays, utilize a trunk cable from which multiple connections, couplers, are made. For example, a trunk cable extending between several floors of a building may be coupled to supply/receive a signal to/from each floor. Each coupler applied to the trunk cable draws off a specified portion of the signal power present at the coupler location on the trunk cable. Therefore, successive couplers with a common coupling factor attached to the same trunk cable will receive a progressively lower signal level at each coupler location. 
     “Piggy back” cable couplers permit the diversion of a signal from a trunk cable to a second cable and vice versa, without requiring the attachment of connectors to the trunk cable. Conventional “piggy back” co-axial cable couplers introduce a contact pin through an aperture created in the trunk cable&#39;s shield/outer conductor that contacts the trunk cable&#39;s center conductor. Creation of the aperture, without damaging the center conductor, is time consuming and normally requires a dedicated tool. 
     The contact pin of a conventional co-axial cable coupler creates a significant impedance discontinuity with mismatch loss. The mismatch loss, of a for example 10 dB coupler, combined with radiation and coupled energy losses, may create an insertion loss of up to approximately 1.5 dB with respect to the trunk cable, depending on the coupler, trunk cable type and the operating frequency. Where energy conservation is desired, or multiple couplers are required, the insertion loss from a conventional center conductor contacting co-axial cable coupler may be significant. 
     Couplers with low insertion losses exist. However, these devices require cutting the trunk cable at a desired insertion point, installing connectors at both sides of the break and insertion of the coupler, in-line. The separate structure of the coupler, required connectors and time-consuming installation procedure may cause this type of coupler to be undesirably expensive. 
     Competition within the coupler connector market has also focused attention on minimization of materials and manufacturing costs. 
     Therefore, it is an object of the invention to provide a method, apparatus and a kit for a coupler(s) which overcome deficiencies in the prior art. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 shows a cut-away side view of a first embodiment of the invention. 
     FIG. 2 shows an external top view of the first embodiment of the invention. 
     FIG. 3 shows an external side view of one embodiment of a cover usable with the invention. 
     FIG. 4 shows a cut-away side view of a second embodiment of the invention. 
     FIG. 5 shows an external top view of the second embodiment of the invention. 
     FIG. 6 shows a cross-section view of the second embodiment of the invention. 
     FIG. 7 shows a cross-section view of a third embodiment of the invention. 
     FIG. 8 shows an external top view of a conductor embodiment of the invention. 
     FIG. 9 shows a cut-away view of the conductor embodiment of the invention. 
     FIG. 10 a  shows a cut-away side view of a variable coupling factor embodiment of the invention. 
     FIG. 10 b  shows a cut-away side view of another variable coupling factor embodiment of the invention. 
     FIG. 11 a  shows an external and partial cut-away view of an in-line embodiment of the invention. 
     FIG. 11 b  shows an external view of an in-line variable coupling factor embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     U.S. utility patent application No. 10/064,946, filed Aug. 30, 2002 is hereby incorporated by reference in the entirety. 
     RF energy in a co-axial cable, propagates in the space between a center conductor and a surrounding outer conductor. The formation of a, for example, generally u-shaped slot/discontinuity (channel) in the trunk cable outer conductor/shield, having an open end and a closed end aligned parallel to the cable center longitudinal axis and an inside and an outside shield area with respect to the channel, interrupts an RF current path between either shield area. The interrupted RF current path around the channel induces a voltage potential between either side of the closed end of the u-shaped channel. Therefore, by connecting a second, for example, co-axial cable across the channel, a cable coupler may be formed which couples RF energy but does not require contact with the center conductor of the trunk cable. Because the creation of a significant impedance discontinuity is avoided, reflective losses and therefore overall insertion loss due to the addition of the cable coupler to the trunk cable is minimized. 
     In a first embodiment, as shown in FIGS. 1 and 2, a co-axial trunk cable  10  has a trunk center conductor  11  spaced away from a trunk outer conductor  13  by a trunk dielectric  15 . A, for example co-axial, coupler cable  20  has a coupler center conductor  22  spaced away from a coupler outer conductor  24  by a coupler dielectric  26 . The trunk dielectric  15  and or coupler dielectric  26  may be any material with suitable dielectric properties, including air. Further, the coupler cable  20  may be any form of conductor, including for example micro strip conductors or a single conductor acting as a radiating element. 
     A channel  40  is formed through the trunk outer conductor  13  having a length L and a width W. The channel  40  may have a U-shape, open ended rectangular shape, V-shape or other form having an open end defined by the overall axial length L parallel to a center longitudinal axis of trunk cable  10  and a transverse width W or arc length around the trunk outer conductor  13 . An open end of the channel  40  may be aligned facing either a RF generator or RF load end of trunk cable  10 . A gap width G of the channel  40  may vary along the channel  40  but is at least large enough to create an electrical continuity break in the trunk outer conductor  13 . Length L is greater than or equal to gap width G. 
     The coupler center conductor  22  may be coupled with the trunk outer conductor  13  at an inside coupling point  50  proximate to the closed end of the channel  40 . The coupler outer conductor  24  may be coupled with an outside coupling point  52  of the closed end of the channel  40 . Alternatively, the connection points of the coupler center conductor  22  and coupler outer conductor  24  may be reversed, i.e. coupled with coupling points  52  and  50 , respectively. 
     Locations of the inside and outside coupling points  50 , 52  are not critical other than their locations being on either side of the channel  40  at points where an RF voltage differential exists. Dimensions L, W and G of the channel  40  and the location of connection points  50 , 52 , determine a coupling level that is described herein below. 
     The coupling of the coupler center conductor  22  and coupler outer conductor  24  with the trunk outer conductor  13  may be, for example, via soldering, spring clip(s), direct mechanical connection or mechanical compression via elongated straps or mechanical clamp(s). Any manner of securing electrical connection may be used; with care taken that the manner selected does not provide a short circuit across the channel  40 . 
     As shown in FIG. 3, a cover  30  held in place, for example, by elongated straps  35 , adhesive or mechanical clamps may be used to protect the coupler from environmental contaminants and/or maintain the electrical coupling of the coupler cable  20  with the trunk cable  10 . The cover  30  may also include, insulated so as not to form a short circuit across the channel  40 , an RF shield  33  to minimize RF energy radiation losses/interference from the trunk cable  10  through the channel  40  and from the exposed portion of the coupler center conductor  22 . 
     As shown in FIGS. 4,  5  and  6 , a second embodiment may use an aperture or trough  60  formed in the trunk outer conductor  13 . The trough  60  may be of any shape, generally having an overall length L and a width W. A conductor  62  is placed in the trough  60  with one end, along the trunk cable center longitudinal axis, in electrical contact with the trunk outer conductor  13  via a conductor/shield coupling  70  but otherwise electrically isolated from the trunk outer conductor  13 , thereby creating an electrical equivalent of the channel  40  structure of the first embodiment. Conductor/shield coupling  70  may be, for example, via soldering, spring clip(s), direct mechanical connection or mechanical connection via elongated straps or mechanical clamp(s). Further, the conductor/shield coupling  70  may be omitted. The positioning of conductor  62  may, for example, be aided by the use of adhesive, elongated strap(s) or mechanical connection, for example to the trunk dielectric  1   5  or trunk center conductor  11 . Alternatively, the conductor  62  may be mounted to the cover  30 , which is then secured to the trunk cable  10  as previously described. 
     The coupler cable  20  is connected similarly to the first embodiment, with connections, for example, of the coupler center conductor  22  to the trunk outer conductor  13  at an inside coupling point  50  of the closed end of the conductor  62  and a connection of the coupler outer conductor  24  with an outside coupling point  52  of the closed end of the channel formed between the conductor  62  and the trunk outer conductor  13 . 
     To ensure that the conductor  62  is isolated from the trunk outer conductor  13 , the conductor  62  may be formed as a conductive layer spaced away from the edges of all but the back end of an insulating substrate  64 , for example a printed circuit board (PCB). The portion of conductor  62  extending to a back end of the insulating substrate  64  may then become the conductor/shield coupling  70  coupled with the trunk outer conductor  13 . If no conductor/shield coupling  70  is desired, or if a jumper or other connection for conductor/shield coupling  70  is used, the conductor  62  may be spaced away from all edges of the insulating substrate  64 . 
     In a third embodiment as shown in FIG. 7, for example, where the trunk cable  10  has an air dielectric  15 , the insulating substrate  64  may be dimensioned to fit against the edges of the trunk outer conductor  13  defining the trough  60  and or against the trunk center conductor  11 , thereby maintaining a fixed location of the conductor  62 , even where there is no dielectric material to support the insulating substrate  64 . 
     Both the second and third embodiments may have a cover  30  as shown, for example, in FIGS. 3 and 6, discussed herein above. 
     The channel  40  or trough  60  may be formed by cutting or otherwise removing at least trunk outer conductor  13  material and dielectric  15 , if applicable, with a scroll saw, band saw, router, grinder, laser or other channel/trough forming device. Specific dimensions of the channel  40  or trough  60  may be formed using guides that may be made for specific trunk cable dimensions and types. 
     To form the, for example, open ended rectangular channel  40  using a scroll saw a first cross-sectional cut may be made in the trunk outer conductor  13  to a desired depth creating the W dimension. When the bottom of the first cut is reached, the angle of cutting is changed to be parallel to a center longitudinal axis of the trunk cable  10  for a length L of a longitudinal cut. The movement may then be reversed, allowing removal of the cutting element. 
     When using a cutting method that cuts across the cable, through the dielectric  15 , the depth of the first cross-sectional cut should be shallow enough so that the trunk center conductor  11  is not contacted. Manipulation of the cutting angle across the trunk outer conductor  13  or use of a router or other controlled depth cutting method removes this requirement. 
     Alternatively, a trough  60  may be formed in the trunk cable  10  by making a single, for example with a grinding tool, or series of cross-sectional cut(s), removing trunk outer conductor  13  and if applicable trunk dielectric  15  from a desired length and width of the trunk cable  10 . 
     The selected overall length L, width W and channel width G dimensions of the channel  40  or trough  60  determine a coupling factor of the cable coupler. The coupling factor is a measure of how much of the total RF energy present in the trunk cable  10  is coupled to the coupler cable  20 , the remainder continuing along the trunk cable  10 . Generally, increases in length L and width W increases coupling. 
     The channel  40  or trough  60  may also be formed in the trunk cable  10  prior to trunk cable  10  installation and or at the time of trunk cable  10  manufacture. In a controlled environment, high precision channel(s)  40  and or trough(s)  60  may be efficiently formed using dedicated computer controlled machine tooling that may be, for example, too bulky, complex and or expensive to use onsite and or upon an installed trunk cable  10 . As shown in FIG. 8, a plurality of channels  40  or troughs  60  may be pre-formed at desired coupler connection points  90  along the trunk cable  10 . 
     Where the locations of coupler connection points  90  and or the number of couplers to be installed are unknown, a trunk cable  10  may be pre-formed with coupler connection points  90  spaced at regular intervals. To reduce trunk cable  10  conductor corrosion, electrical characteristic degradation, RF leakage and or environmental contamination due to unused coupler connection points, the coupler connection points  90  may be covered after forming the channel  40  or trough  60  with, for example, a conductive foil  100  that may be secured to the trunk outer conductor  13  by, for example, an adhesive and or sealed under a trunk cable outer coating  110  of, for example, polyethylene as shown in FIG.  9 . Markings along the outer coating may be used to aid users with location of the coupler connection points. Alternatively, the unused coupler connection points  90  may be covered with a conductive foil  100  or. other sealing/shielding after installation is complete. 
     To utilize a pre-formed coupler connection point  90 , the outer coating  10  covering the desired coupler connection point, if present, may be removed along with any conductive foil  100 . Connection of a coupler cable  20  may then be completed as described herein above. 
     Because the coupler connection points  90  may be pre-formed using precision equipment, each successive coupler connection point  90  may be specifically dimensioned for a desired coupling factor to create an array or network of. couplers on the trunk cable  10  that has a common coupler cable  20  signal strength. For example, coupler connection points  90  farther along the trunk cable  10  from a signal generator may be configured to have a higher coupling factor to make up for signal strength coupled by the previous couplers and or lost to cable/conductor attenuation. 
     Single or multiple coupler connection points  90  may also be formed configured with a larger coupling factor than desired upon installation. As shown in FIGS. 10 a  and  10   b a,  for example, conductive clip, collar, ring  120  and or conductive tuning foil  130  may then be used to electrically shorten the dimensions of the channel  40  or trough  60  thereby adjusting the individual coupler to a desired coupling factor. In this way, a desired common coupler cable  20  signal strength may be quickly obtained even if the number and or spacing of the installed couplers and any other signal loss sources are unknown prior to trunk cable  10  field installation. 
     In an in-line embodiment, as shown in FIG. 11 a,  the coupler may be formed as a separate component with trunk connectors  140  for insertion, in-line with a trunk cable  10  and a coupler connector  150  for connection to the coupler cable  20 . As shown in FIG. 11 b,  the in-line embodiment may be given high precision variable coupling factor capability through the addition of a, for example, conductive collar  160  movable to electrically shorten the channel  40  or trough  60  length and or width dimensions as applicable. The conductive collar  160  may be movable, for example, via rotation about threads or axially along notches, a ridge or groove. 
     As described, the shield slot coupler provides the following advantages. The shield slot coupler has a reduced insertion loss and may be inexpensively formed without any external elements beyond the trunk cable  10  and the coupler cable  20 , thereby decreasing component costs. The coupler may be formed with an adjustable coupling factor. Trunk cables may be manufactured with regularly spaced channels  40  and or troughs  60  that can be quickly and inexpensively utilized as couplers as desired. 
     
       
         
           
               
             
               
                   
               
               
                 Table of Parts 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 10 
                 trunk cable 
               
               
                 11 
                 trunk center conductor 
               
               
                 13 
                 trunk outer conductor 
               
               
                 15 
                 dielectric 
               
               
                 20 
                 coupler cable 
               
               
                 22 
                 coupler center conductor 
               
               
                 24 
                 coupler outer conductor 
               
               
                 26 
                 coupler dielectric 
               
               
                 30 
                 cover 
               
               
                 33 
                 RF shield 
               
               
                 35 
                 elongated strap 
               
               
                 40 
                 channel 
               
               
                 50 
                 inside coupling point 
               
               
                 52 
                 outside coupling point 
               
               
                 60 
                 trough 
               
               
                 62 
                 conductor 
               
               
                 64 
                 insulator substrate 
               
               
                 70 
                 conductor/shield coupling 
               
               
                 90 
                 coupler connection point 
               
               
                 100 
                 conductive foil 
               
               
                 110 
                 trunk cable outer coating 
               
               
                 120 
                 ring 
               
               
                 130 
                 conductive tuning foil 
               
               
                 140 
                 trunk connector 
               
               
                 150 
                 coupler connector 
               
               
                 160 
                 conductive collar 
               
               
                   
               
            
           
         
       
     
     where in the foregoing description reference has been made to ratios, integers or components having known equivalents then such equivalents are herein incorporated as if individually set forth. 
     while the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus, methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of applicant&#39;s general inventive concept. Further, it is to be appreciated that improvements and/or modifications may be made thereto without departing from the scope or spirit of the present invention as defined by the following claims.