Patent Publication Number: US-7214883-B2

Title: Electrical signal cable

Description:
PRIORITY CLAIM 
     This patent application is a non-provisional application of U.S. provisional patent application No. 60/674,514 filed on Apr. 25, 2005 and entitled “Conductors Insulated by Layers of Dielectric Material,” which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the field of electrical cables and, more particularly, to electrical cables that transmit analog and/or digital signals. 
     BACKGROUND OF THE INVENTION 
     Electrical cables used to transfer audio, data and radio frequency signals are characterized by capacitive, inductive and resistive properties. Collectively, these properties establish the impedance of the electrical cable. One or more of these three properties can cause degradation of the original signal as it is transferred through the cable. It remains a great challenge in the electrical cable industry to minimize and/or carefully control each of these properties over a broad range of frequencies. 
     A well known technique to limit the inductive properties of an electrical cable involves the use of twisted pair construction. A conventional twisted pair cable design incorporates a pair of insulated conductors twisted around each other in a helical fashion. One advantage of twisted pair cable construction its reduced sensitivity to common mode magnetic interference. This reduced sensitivity occurs when one conductor is designated to carry a signal source current and the other conductor is designated to carry the signal return current. As the opposing currents travel through their respective twists, the opposing magnetic fields cancel each other at the interior of the cable. This cancellation, in effect, reduces the total loop area available within the circuit created by the electrical cable. With a smaller effective loop area achieved by adding additional twists, not only is the amount of magnetic energy stored and radiated reduced when compared to non-twisted electrical cable, but a smaller effective loop area also means the electrical cable is less sensitive to interfering magnetic fields. It is in this fashion that the effective inductance of a twisted pair cable is lower than the inductance of a non-twisted cable having the same overall length. 
     While twisted pair cabling has the inherent advantage of reduced magnetic fields, there are still improvements necessary to maintain complete minimization and/or control of the reactive portions of the cable impedance. Capacitance in a twisted pair design will depend on conductor length, cross sectional area, distance between conductors and the dielectric constant of the material used to insulate the conductors. As twists are added to a cable of fixed length, more conductor and insulating material will be required and capacitive coupling between conductors will increase as a result. So, while the total magnetic field will be reduced in a twisted pair design, a natural consequence is an increased amount of electric field energy stored in the dielectric medium, a phenomenon known as capacitive coupling, within the proximity of the two conductors. 
     For any required value of capacitance, an insulating material with a low dielectric constant separating source and return conductors allows these conductors to be arranged in closer spatial proximity to each other than the same conductors separated by a material with higher dielectric constant. Therefore, while the two dielectric materials can produce two different cables having the same measured value of capacitive coupling, the design utilizing the low dielectric constant insulator can result in a cable that has a lower measured inductance. 
     Various means of reducing capacitive coupling between the conductors of twisted pair cables are known to the art. Air is considered to be the best dielectric element for the purposes of reducing capacitive coupling. Air is also one of the most difficult elements to incorporate in an electrical cable design. Several techniques for incorporating air as a dielectric element have been disclosed. U.S. Pat. No. 1,305,247 discloses a strip of flexible insulating material separating conductors and maintained in an elastic and compressible condition. The insulating material can be formed with a continuous central hollow or air cavity. U.S. Pat. No. 2,804,494 discloses a twin lead twisted pair RF cable utilizing an elongated hollow tube with grooves formed in diametrically opposite sides of the tube and extending the length of the cable. Conductors are positioned in open spaces formed by the groove and rely on spacers to maintain the conductors in a constant spatial relationship. 
     When designing a cable to be used for high fidelity signaling an additional concern is the phenomenon of eddy currents known to cause a conductor to increase its resistance in proportion to the frequency being transmitted through the cable. Higher frequency signals will result in a current density that is concentrated at the surface of a conductor where these is less conductor area available. This effect is known in the art as skin effect. Skin effect contributes an additional measurable component of AC resistance to the over all impedance properties of the cable. 
     Litz wire techniques are well known in the electrical industry for reducing power losses due to eddy currents and the resulting skin effect. Power conversion circuits that utilize transformer or inductor windings make use of Litz wire properties to improve power efficiencies at high frequencies, typically less than 1 MHz. Among other techniques exploiting skin effect, U.S. Pat. No. 4,538,023 describes using conductors of various diameters with smaller diameter conductors surrounding larger diameter conductors as a means for adjusting the relative speed of high and low frequency components. 
     The twisted pair configurations which incorporate a continuous air channel suffer from several major limitations. In the previously mentioned patents, a continuous air cavity of significant cross sectional area with respect to the conductor diameter must be used to prevent deformation or collapse of the air cavity under the compression forces achieved during the twisting process and the compression forces exerted during normal bending necessary to conform the cable to environmental and installation conditions. An air gap that is larger than the minimum necessary to achieve a desired impedance results in a larger loop area with a consequentially larger inductance. In addition, these techniques require relatively complex and expensive manufacturing methods. Other known methods of providing substantially solid insulating strips between conductors do not incorporate any method for providing air as a substantial dielectric element. Moreover, these strip and air cavity dielectric techniques do not incorporate a specific means for reducing skin effect. Previously mentioned Litz wire designs intended to minimize skin effect do not address a specific means for incorporating air to reduce the capacitive effects between the signal source and return conductors of an electrical cable. 
     Accordingly it is desirable to have an electrical cable that overcomes these limitations and concerns. 
     SUMMARY OF THE INVENTION 
     The present invention provides an electrical cable with minimal values of resistance, capacitance and inductance. In addition, the present invention provides an economical means for incorporating air as a dielectric element in electrical cables. The present invention also provides an electrical cable with minimal susceptibility to skin effect. Furthermore, the present invention allows the transmission of low and high power electrical signals with minimal signal degradation. 
     More specifically, the present invention provides an electrical cable having two or more conductors, one or more multistrip insulators separating the two or more conductors from one another, and a protective cover formed around the two or more conductors and one or more multistrip insulators. The multistrip insulator may include one or more dielectric strips, one or more protective strips or a combination thereof. 
     In addition, the present invention provides an electrical cable having two or more conductors, one or more multistrip insulators separating the two or more conductors from one another and a protective cover formed around the two or more conductors and one or more multistrip insulators. Each multistrip insulator includes at least a dielectric strip disposed between a first protective strip and a second protective strip. The dielectric strip contains a set of air spaces distributed throughout the entirety of the dielectric strip. 
     The present invention also provides a method for manufacturing an electrical cable by providing two or more conductors, separating the two or more conductors from one another using one or more multistrip insulators, and forming a protective cover around the two or more conductors and one or more multistrip insulators. Each multistrip insulator includes at least a dielectric strip disposed between a first protective strip and a second protective strip. The dielectric strip has a set of air spaces distributed throughout the entirety of the dielectric strip. 
     The present invention is described in detail below with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which: 
         FIG. 1  is an elevation view of an electrical cable constructed in accordance with one embodiment of the present invention; 
         FIG. 2  is a greatly enlarged view taken along the lines  2 — 2  of  FIG. 1 ; 
         FIG. 3  is an elevation view of  FIG. 2  with the protective dielectric layers broken away to disclose the dielectric strip having spaces for air in accordance with one embodiment of the present invention; 
         FIG. 4  is a cross sectional view of another embodiment of the present invention having a multistrip insulator having alternating layers of dielectric strips and protective strips; 
         FIG. 5  is a cross sectional view of another embodiment of the present invention having several layers of dielectric strips forming a multistrip insulator; 
         FIG. 6  is a cross-sectional view of another embodiment of the present invention having three conductors; 
         FIG. 7  is a cross-sectional view of another embodiment of the present invention having four conductors; and 
         FIG. 8  is a cross-sectional view of another embodiment of the present invention having a shielding element. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention. The discussion herein relates primarily to an electrical signal cable, but it will be understood that the concepts of the present invention are applicable to any electrical signal medium where it is desirable to reduce resistance, capacitance and inductance. 
     The present invention provides an electrical cable with minimal values of resistance, capacitance and inductance. In addition, the present invention provides an economical means for incorporating air as a dielectric element in electrical cables. The present invention also provides an electrical cable with minimal susceptibility to skin effect. Furthermore, the present invention allows the transmission of low and high power electrical signals with minimal signal degradation. Note that the term “Conductor” is used herein to describe the part of the cable used to carry the electrical signal. The conductors described in the present invention may be constructed of multiple strands of insulated or uninsulated conducting material. The conductors may also be constructed of a single conducting strand. In any embodiment of the present invention the conductors and multistrip insulator may be either twisted together or remain untwisted. 
     Now referring to  FIG. 1 , an elevation view of an electrical cable  10  constructed in accordance with one embodiment of the present invention is shown. The electrical cable  10  includes two or more electrical conductors  11 ,  12 . The two or more conductors  11 ,  12  can be an insulated conductor, an uninsulated conductor, a stranded conductor, a solid conductor or a combination thereof. Conductors  11  and  12  are separated by one or more multistrip insulators  17 . The two or more electrical conductors  11 ,  12  and the one or more multistrip insulators  17  are encased in a protective cover  16 . The multistrip insulator  17  may include one or more dielectric strips, one or more protective strips or a combination thereof (e.g., a first dielectric strip  14  and a first protective strip  13 , etc.). As shown, the multistrip insulator  17  includes a first dielectric strip  14  disposed between a first protective strip  13  and a second protective strip  15 . Protective strips  13 ,  15  are constructed from a material with low surface friction, such as a strip of polytetrafluoroethylene (PTFE). The dielectric strip  14  is formed from a material fabricated with a plurality of spaces for air  30  such as polypropylene or PTFE mesh. The multistrip insulator  17  maintains the structural integrity and dimensional stability of the spaces for air  30  enclosed within the dielectric strip  14  when the conductors  11 ,  12  and multistrip insulator  17  are flexed and/or twisted together.  FIG. 2  further illustrates the interaction between the protective strips  13 ,  15 , dielectric strip  14  and the conductors  11 ,  12 . Likewise,  FIG. 3  further clarifies the arrangement of protective strips  13 ,  15  and dielectric strip  14  in the formation of a multistrip insulator  17 . Spaces for air  30  within dielectric strip  14  are enclosed by the strands  31  of the mesh material and the protective strips  13 ,  15 . 
     The spaces for air  30  may be formed from apertures or indentations of varying dimensions and geometries. The use of a mesh material formed by orthogonally placed plastic strands may result in a plurality of air spaces uniformly distributed along the length of the cable with each space for air  30  forming an aperture having a rectangular geometry. The use of a perforated plastic strip may result in a plurality of air spaces  30  uniformly distributed along the length of the cable with each space for air forming an aperture with round geometry. Other possible aperture geometries for  30  include rhomboid, elliptical, triangular, square and random. Any combination of the various geometries may be incorporated in the dielectric strip  14 . 
     Protective strips  13 ,  15  can be made from any suitable material. A partial list of example materials suitable for  13 ,  15  include polyvinylchloride (PVC), PTFE, polyethylene, FEP, polyethylene, nylon, Kapton to which may be added flexible ceramic fibers and/or flexible fiberglass strands. Dielectric strip  14  can be made from any suitable material constructed with a plurality of spaces for air such as braided, woven, meshed, screened or foamed strips. A partial list of example materials suitable for dielectric  14  include polyvinylchloride (PVC), PTFE, FEP, polyethylene, polypropylene, nylon, Kapton to which may be added flexible ceramic fibers and/or flexible fiberglass strands. Strips  13 ,  14 ,  15  may be made with any of these materials or any combination of them. As result, each dielectric strip or each protective strip may include a material (e.g., polyvinylchloride (PVC), polytetrafluoroethylene (PTFE), polyethylene, FEP, polyethylene, nylon, Kapton, set of flexible ceramic fibers, set of flexible fiberglass strands, glass tape, glass fabric, plastic tape, plastic fabric, plastic braid or a combination thereof) that is braided, woven, meshed, screened, perforated, foamed or a combination thereof. In certain cases it may be desirable to apply an adhesive to either the protective strips or dielectric strips to help secure the various layers of the multistrip insulator  17 . 
       FIGS. 1 ,  2  and  3  are provided for illustration purposes only and are not meant to be limitations of the present invention. For example, the multistrip insulator  17  can include any combination and number of protective layers and dielectric layers.  FIGS. 4 and 5  illustrate alternate embodiments of the multistrip insulator  17 . In  FIG. 4 , a first dielectric strip  44  is disposed between a first protective strip  41  and a second protective strip  42 , and a second dielectric strip  45  is disposed between the second protective strip  42  and a third protective strip  43 . It should be obvious to those skilled in the art that many modifications and adaptations to the multistrip insulator  17  will occur by varying the order and number of layers of dielectric material and/or protective strips and these modifications and adaptations are within the scope of the present invention. The use of a protective strip is not required for all possible embodiments claimed by this invention. For example, as shown in  FIG. 5 , alternate embodiments may be created by constructing a multistrip insulator  17  by using one or more layers comprised of dielectric strip material  51 ,  52 ,  53 . 
     In keeping with one of the principal objects of the present invention, conductors  11 ,  12  are comprised of separately insulated conducting strands woven together such that each of said conducting strands tends to take all possible positions within the cross-section of the entire conductor. To those skilled in the art, this construction is known as Litz wire construction. Litz wire construction has the advantage of reducing signal loss at higher frequencies due to the phenomenon of skin effect. The conductors  11 ,  12  described in the present invention may be constructed of multiple strands of insulated or uninsulated conducting material. In the preferred embodiment shown in  FIG. 1 , the conductors  11 ,  12  and multistrip insulator  17  are twisted together. 
       FIGS. 6 and 7  also illustrate additional alternate embodiments of the present invention. In these figures, it can be seen that cables comprising more than two conductors can incorporated within the scope of the present invention.  FIG. 6  illustrates how the present invention can be applied to the construction of an electrical cable with three conductors  61 ,  62 ,  63 .  FIG. 7  illustrates how the present invention can be applied to the construction of an electrical cable with four conductors  71 ,  72 ,  73 ,  74 . 
     In yet another embodiment,  FIG. 8  illustrates a shielding element  80  for the electrical cable  10  surrounding conductors  11 ,  12  and multistrip insulator  17  and disposed inside the protective jacket  16 . The shielding element  80  may be manufactured of any material suitable for attenuating magnetic and/or electric fields such as metal foil, metal braid, metalized plastic foil or a combination thereof. 
     Protective cover  16  in the preferred embodiment is fabricated from heat shrinkable, clear tubing. Heat shrinkable tubing has the advantage of providing a constant compressive force securing the conductors  11 ,  12  firmly to the multistrip insulator  17 . The compressive force provided by the heat shrink tubing maintains the overall dimensional stability of the electrical cable and as a result the cable impedance is uniform along the length of the electrical cable and the impedance specification will remain consistent over independent manufacturing runs. For ease of fabrication, the tubing may be applied as a contiguous series of short strips. Alternate embodiments of the present invention may utilize suitable extruded or pressure jacket methods for fabricating the protective cover  16 . 
     The present invention also provides a method for manufacturing an electrical cable ( 10 ) by providing two or more conductors ( 11 ,  12 ), separating the two or more conductors ( 11 ,  12 ) from one another using one or more multistrip insulators ( 17 ), and forming a protective cover ( 16 ) around the two or more conductors ( 11 ,  12 ) and one or more multistrip insulators ( 17 ). Each multistrip insulator ( 17 ) includes at least a dielectric strip ( 14 ) disposed between a first protective strip ( 13 ) and a second protective strip ( 15 ). The dielectric strip ( 14 ) includes a set of air spaces ( 30 ) distributed throughout the entirety of the dielectric strip ( 14 ). The method can also include the steps of twisting the two or more conductors ( 11 ,  12 ) and the one or more multistrip insulator ( 17 ), and/or forming one or more shielding elements ( 80 ) around the two or more conductors ( 11 ,  12 ) and the one or more multistrip insulators ( 17 ). 
     While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.