Patent Publication Number: US-6664474-B1

Title: Shield slot tap

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The invention relates to electrical cable taps. More specifically, the invention relates to a cable tap 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, taps, are made. “Piggy back” cable taps permit the diversion of a signal from a trunk cable to a second cable, without requiring attachment of connectors to the trunk cable. 
     Conventional co-axial cable taps 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 tap creates a significant impedance discontinuity with mismatch loss. The mismatch loss, 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 tap, trunk cable type and the operating frequency. Where energy conservation is desired, or multiple taps are required, the insertion loss from a conventional center conductor contacting co-axial cable tap may be significant. 
     Taps 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 reactive tap, in-line. The separate structure of the tap, required connectors and time-consuming installation procedure may cause this type of tap to be undesirably expensive. 
     Competition within the tap 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 tap which overcomes 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. 2A shows an external top view of an alternative embodiment of the invention. 
     FIG. 2B shows an external top view of an alternative embodiment of the invention. 
     FIG. 3 shows an external side view of one embodiment of a cover usable with the invention. 
     FIG. 3A shows an external side view of another 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 a  shows test data representing the coupling loss of a center conductor tap (10 dB nominal) with respect to operating frequency. 
     FIG. 8 b  shows test data representing the insertion loss of a center conductor tap (10 dB nominal) with respect to operating frequency. 
     FIG. 9 a  shows test data representing the coupling loss with respect to operating frequency of a first embodiment of the invention. 
     FIG. 9 b  shows test data representing the insertion loss with respect to operating frequency of the first embodiment of the invention. 
     FIG. 10 a  shows test data representing the coupling loss with respect to operating frequency of a second embodiment of the invention. 
     FIG. 10 b  shows test data representing the insertion loss with respect to operating frequency of the second embodiment of the invention. 
     FIG. 11 shows a chart comparing the insertion loss with respect to operating frequency test data from FIGS. 8 b ,  9   b  and  10   b . 
     FIG. 12 is an isometric view of a fixture usable for guiding a cutting tool for forming a channel having specific dimensions. 
    
    
     DETAILED DESCRIPTION 
     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, interupts an RF current path between either shield area. The interupted 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 tap 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 tap 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, tap cable  20  has a tap center conductor  22  spaced away from a tap outer conductor  24  by a tap dielectric  26 . The trunk dielectric  15  and or tap dielectric  26  may be any material with suitable dielectric properties, including air. Further, the tap 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 . 
     The tap 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 tap 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 tap center conductor  22  and tap 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  is 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 tap center conductor  22  and tap 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 tap from environmental contaminants and/or maintain the electrical coupling of the tap 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 tap 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  15  or trunk center conductor  11 . 
     The tap cable  20  is connected similarly to the first embodiment, with connections, for example, of the tap 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 tap 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, 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 . 
     The second embodiment may also use 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 tap. The coupling factor is a measure of how much of the total RF energy present in the trunk cable  10  is coupled to the tap cable  20 , the remainder continuing along the trunk cable  10 . Generally, increases in length L and width W increases coupling. 
     FIGS. 8 a  and  8   b  show test data for coupling factor and insertion losses of a Andrew Corporation Radiax™ Cable Tap (center conductor tap connector) attached to LDF4, ½ inch diameter, co-axial cable manufactured by Andrew Corporation. Note the scale of the insertion loss plot is 0.2 dB/division. 
     FIGS. 9 a  and  9   b  show test data for coupling factor and insertion losses of a tap connector according to the first embodiment of the invention with an open ended rectangular channel  40  having a width W of 0.63 inches (arc length) and a length L of 1.5 inches formed in LDF4 cable manufactured by Andrew Corporation. Connections are soldered and the cover  30  is removed. Note the scale of the insertion loss plot is 0.1 dB/division. 
     FIGS. 10 a  and  10   b  show test data for coupling factor and insertion losses of a tap connector according to the second embodiment of the invention with a rectangular trough  60  having a width W of 0.63 inches (arc length along the trunk outer conductor  13 ) and a length L of 1.62 inches formed in LDF4 cable manufactured by Andrew Corporation. Connections are soldered and the cover  30  is removed. Note the scale of the insertion loss plot is 0.1 dB/division. 
     Frequency test points in the FIG. 8 a - 10   b  are identified at 0.8, 1.0, 2.0 and 2.2 Gigahertz. For comparison, the coupling factors of each device are intended to be in the area of 10 dB but have not been tuned to be exactly the same and therefore the different insertion losses should be viewed accordingly. As shown by the comparison chart in FIG. 11, the test data demonstrates that the invention provides a significant broadband decrease in insertion loss, when compared to the performance of prior center conductor contacting cable taps. 
     As shown in FIG. 12, a fixture  80 , jig or cutting guide may be supplied to guide a cutting tool during formation of the channel or trough. A plurality of fixtures in a tap kit may define a range of different channel/trough dimensions thereby enabling a user to form a channel/trough dimensioned for a specific coupling factor. Alternatively, a single fixture may be configured with graduations identifying widths and or lengths associated with specific coupling factors. 
     As described, the shield slot tap provides the following advantages. The shield slot tap has a reduced insertion loss and may be formed without any external elements beyond the trunk cable  10  and the tap cable  20 , thereby decreasing component costs. 
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                 Table of Parts 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 10 
                 trunk cable 
               
               
                 11 
                 trunk center conductor 
               
               
                 13 
                 trunk outer conductor 
               
               
                 15 
                 dielectric 
               
               
                 20 
                 tap cable 
               
               
                 22 
                 tap center conductor 
               
               
                 24 
                 tap outer conductor 
               
               
                 26 
                 tap 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 
               
               
                 80 
                 fixture 
               
               
                   
               
            
           
         
       
     
     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 if 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 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.