Abstract:
There is provided a cable which retards lightning. The cable includes at least one internal conductor which may be a power conductor or a signal conductor. A choke conductor is wound about the internal conductor in the shape of a spiral. If lightning strikes near the cable or a device which is attached to the cable, such as an antenna, the choke conductor presents a high impedance to the current caused by lightning and will prevent the lightning current from flowing down the choke conductor, thus entering the internal conductor, thereby preventing damage to the internal conductor and any associated electronic equipment. Preferably, a shield is also wound about the internal conductor adjacent to the choke conductor in a direction opposite to the choke conductor, whereby the angle formed by the crossing of the choke conductor and the shield is approximately 90° to block the magnetic field component of the lightning discharge. The choke conductor and the shield may be wound about a conduit which houses the cable.

Description:
RELATED APPLICATION 
     This is a continuation-in-part of U.S. application Ser. No. 09/066,237, filed on Apr. 24, 1998, now U.S. Pat. No. 5,930,100 which is a continuation-in-part of U.S. patent application Ser. No. 08/741,536, filed Oct. 31, 1996, which issued as U.S. Pat. No. 5,744,755 on Apr. 28, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to electrical cable. More particularly, it relates to electrical cable which retards lightning so that the cable is not substantially affected by the lightning and, in the case of communication cable, the communication signal on a signal conductor within the cable is not substantially affected, as well as its associated equipment. 
     While this invention is applicable to both power and communication cable, most of the detailed discussion herein will focus on communication cable used in conjunction with an antenna. 
     As used herein, the term antenna includes television and radio antenna, satellite dishes and other devices which receive electromagnetic signals. A major problem associated with an antenna is caused by lightning striking the antenna. Often the high current associated with the lightning will travel through the communication cable which is attached between the antenna and electronic equipment. This current will damage the electronic equipment. 
     According to  The Lightning Book , by Peter E. Viemeister, self-induction in a conductor may occur during a lightning strike. This occurs because lightning currents may rise at a rate of about 15,000 amperes in a millionth of a second. For a straight conductor with the usual cross section, this surging current can produce nearly 6,000 volts per foot of wire, which is enough to jump an insulated gap to a nearby conductor, such as the center conductor, in a coaxial cable. 
     Currently lightning protection of cable is more focused on the installation of cable within a system. The National Electric Code attempts to insure a proper path for lightning to discharge, thus reducing the damage of equipment connected to the end of the cable. The cable in and of itself offers little or no protection from electric fields or magnetic fields associated with the lightning strike. Even though electrical codes provide suggestions on installing and grounding equipment, their primary focus is providing a straight path to ground for lightning to discharge and eliminating the differences of potential between the two items. 
     FIG. 1 is an example of a home TV antenna installation according to the National Electric Code. If lightning were to strike antenna  10 , half of the charge would be on ground wire  12  which is attached to the mast  14  of the antenna, and the other half would be on the coaxial cable&#39;s outer shield  16  which is connected to the antenna terminals  18 . Theoretically, the current on coaxial cable  16  would travel to antenna discharging unit  20  and then through grounding conductor  22 . The center conductor or signal conductor of the coaxial cable, however, is unprotected, which means that damage to the electronics in the receiver and other components within the home is likely. Furthermore, the longer the lead-in wire, the greater the problem. As lightning strikes this antenna  10  and discharges to ground, a large electric field is set up along the coaxial lead-in wire  16  and ground wire  12 . At right angles to this electric field is an exceptionally strong magnetic field which surrounds all of the cable. 
     In addition, lightning follows the straightest, closest and best path to ground. Any sharp bends, twists or turns of the ground wire sets up resistance to the quick discharge. See Page 201 of  The Lightning Book , referred to above. This resistance usually causes the discharge to jump off the ground wire with the bend and into a path of least resistance. 
     OBJECTS OF THE INVENTION 
     It is one object of this invention to provide an improved lightning retardant cable which may or may not be received in a conduit. 
     It is another object to provide a lightning retardant cable which deals with both electric and magnetic fields caused by lightning. 
     SUMMARY OF THE INVENTION 
     In accordance with one form of this invention there is provided a lightning retardant cable which includes at least one internal conductor. The internal conductor may be a signal conductor or a power conductor. A signal conductor conducts a signal containing information. A power conductor conducts current for operating devices and equipment. 
     A choke conductor is provided. The choke conductor is wound about the internal conductor in the shape of a spiral. The choke conductor is not in contact with the internal conductor. The choke conductor presents a high impedance to the electrical current caused by lightning when the lightning strikes near the cable. 
     Preferably, the internal conductor is made of metal for conducting electrical signals or current, although the internal conductor may be an optical fiber. 
     It is also preferred that a spiraled shield be placed underneath the choke conductor. The spiraled shield is also wound about the internal conductor, but in an opposite direction to the choke conductor. The adjacent windings of the shield are not in electrical contact with one another and act as another choke. Preferably, 90° angles are formed at the crossing points between the choke conductor and the shield. 
     The choke conductor dissipates the electric field caused by the lightning strike. The shield performs two functions. It acts as a choke in the opposite direction of the choke conductor and thus enhancing the cancellation process and it acts as a Faraday Cage to greatly reduce the associated magnetic field. 
     It is also preferred that one side of the shield be insulated so that when the shield is wound about the cable a winding is not in electrical contact with the previous or next winding. The insulation over the shield may extend over one of the edges of the shield to reduce the likelihood of arcing. 
     The choke conductor may also be insulated. The choke conductor may be substantially rectangular in shape with, preferably, round edges. In addition, each end of the insulated choke conductor may be electrically connected to a corresponding end of the shield. This connection may be made by winding an insulated part of the choke conductor about an uninsulated part of the shield at each end of the cable. 
     It is also preferred that an overall outer jacket be provided for the cable and that a ground conductor be attached to the outer jacket. 
     Also, the choke conductor and shield may be wound about the cable as described above, or they may be wound about a conduit which receives the cable. It is preferred that the induction of the choke and the shield be substantially equal. The number of turns in which the choke is wound may be adjusted to equalize their inductance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter which is regarded as the invention is set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof may be better understood in reference to the accompanying drawings in which: 
     FIG. 1 is a simplified electrical diagram showing a prior art antenna signal transmission and grounding system; 
     FIG. 2 is a simplified electrical diagram showing the antenna signal transmission and grounding system of the subject invention; 
     FIG. 3 is also a simplified electrical diagram showing the antenna signal transmission and grounding system of the subject invention; 
     FIG. 4 is a side elevational view of the lightning retardant cable of the subject invention; 
     FIG. 5 is a side elevational view of an alternative embodiment of the lightning retardant cable of the subject invention; 
     FIG. 6 is a side elevation view of another alternative embodiment of the lightning retardant cable of the subject invention; 
     FIG. 7 is a side elevational view of yet another alternative embodiment of the lightning retardant cable of the subject invention; 
     FIG. 8 is a cross sectional view of the spiraled shield of FIGS. 5,  6  and  7 ; 
     FIG. 9 is a side elevational view of another alternative embodiment of the lightning retardant cable of the subject invention for a power application; 
     FIG. 10 shows a cross section of an insulated choke conductor which may be used with another embodiment of the invention; 
     FIG. 11 shows an inductive meter measuring the inductance of a straight wire; 
     FIG. 12 shows a pair of oppositely wound inductors; 
     FIG. 12A shows the inductors of FIG. 12 being closely spaced and connected together at their opposing ends; 
     FIG. 12B shows the inductors of FIG. 12A having an inductive meter connected there across; 
     FIG. 13 shows the cable which utilizes the choke conductor construction of FIG. 10, wherein only one end of the choke conductor is connected to one end of the shield; 
     FIG. 14 is a more detailed view of the cable of FIG.  13 . 
     FIG. 15 is a perspective view showing a cable received within a conduit with the choke and shield conductors being spiraled about the conduit. 
     FIG. 16 is a sectional view showing an insulated shield with the insulation extending past one of the edges of the shield. 
     FIG. 17 is a side elevational view of the shield of FIG. 16 applied to a cable with one of the side edges of the shield shown in phantom. 
     FIG. 18 is a sectional view showing a substantially rectangular shaped choke conductor which is insulated. 
     FIG. 19 is a section view showing an uninsulated substantially rectangular choke conductor with round edges. 
     FIG. 20 is a section view showing the choke conductor of FIG. 19 being insulated. 
     FIG. 21 is a sectional view of a cable showing the choke conductor of FIG. 19 forming a part thereof. 
     FIG. 22 is a side elevation view of a cable having portions of the jacket removed for clarity showing one end of a choke conductor terminated to one end of a shield conductor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now more particularly to FIG. 3 which relates to an embodiment of the invention where the lightning retardant cable is a communication cable, there is provided antenna signal transmission and grounding system  24  for grounding antenna  10 . As previously indicated, antenna  10  may also be a satellite dish or another device for receiving signals from the air. System  24  includes lightning retardant cable  26 , which is the cable of the subject invention and will be described in more detail below. Lightning retardant cable  26  is attached to antenna  10  at connector lead box  28 . Cable  26  is also connected to standard antenna discharge unit  30 . A typical antenna discharge unit  30  is a Tru Spec commercially available from C Z Labs. A coaxial cable  32  is connected to the discharge unit  30  and to electronic equipment (not shown). 
     A ground wire  34  connects the antenna discharge unit  30  to ground clamps  36  and  38 . Ground clamp  38  is, in turn, connected to ground rod  39 . In addition, the antenna mast  40  is connected to ground clamp  38  through ground wire  42 . 
     FIG. 2 is similar to FIG. 3, but illustrates some of the details of cable  26 . In the communication cable embodiment of this invention, cable  26  is preferably a coaxial cable, although, cable  26  could be a fiber optic cable or twin lead cable. A communication cable must include at least one signal conductor. In the preferred communication cable embodiment of this invention, however, cable  26  is a coaxial cable. FIG. 2 illustrates the center conductor  44 . Center conductor  44  is the signal conductor and is connected to terminal box  46  attached to the mast of the antenna  10 . Signal conductor  44  is connected through antenna discharge unit  30  to coaxial cable  32 . Spiraled choke conductor  56  surrounds signal conductor  44  and is connected to antenna discharge unit  30  which, in turn, is connected to ground conductor  34 . Cable  26  will be discussed in more detail below. 
     FIG. 4 shows lightning retardant cable  26  having signal center conductor  44  which is surrounded by foam dielectric  50 . A standard coaxial cable shield  52  surrounds the dielectric  50 . Insulated jacket  54  surrounds shield  52 . A choke conductor  56  is wound about outer jacket  54  in a spiraled fashion. An overall outer insulated jacket may be placed over the cable to provide protection for the cable. The choke conductor  56  should be large enough to handle the high currents caused by lightning without melting. Choke conductor  56  should be at least  17  gauge and preferably is  10  gauge. Preferably the choke conductor is made of copper. If the choke conductor is made of a bundle of round copper wires, the bundle should be equivalent to at least  17  gauge wire or larger. 
     Referring now to FIG. 2, if lightning strikes antenna  10 , the energy of that strike would normally be split, that is, one-half would follow ground wire  42  and the other half would follow cable  26  to ground rod  39 . However, since cable  26  forms an electrical choke due to spiraled choke conductor  56 , that is, conductor  56  actually chokes out the flow of current due to its high impedance to lightning current which has a very fast rise time, the majority of the surge follows ground wire  42  to ground and does not follow cable  26  to ground. One-half of the energy from the strike that would start down cable  26  after a lightning strike would quickly be cancelled out by the action of the choke. Each time the choke conductor  56  is twisted around the cable, it causes the electric field generated by the lightning to interact upon itself, thus blocking the flow of current. 
     As with any electrical discharge, there is an electric field, as well as a magnetic field at right angles to the electric field. Lightning causes a tremendously large magnetic field due to the huge discharge of electric current. FIG. 5 shows an alternative embodiment of the lightning retardant cable of the subject invention which includes a special shield to block the magnetic component of the lightning discharge, thus acting as a Faraday Cage. 
     In FIG. 5 there is provided a center signal conductor  44 , dielectric  50 , standard coaxial cable shield  52  and coaxial cable jacket  54 . A substantially flat spiraled wrapped shield  58  is wound over the top of coaxial cable jacket  54 . 
     As shown by a cross section of the spiraled shield  58  in FIG. 8, the shield includes a conductive top metal portion  60  which is insulated by plastic insulation  62  on the bottom. Thus the shield may be spiraled upon itself without causing an electrical short. Metal portion  60  of shield  58  is preferably made of aluminum or copper. Shield  58  is commercially available. 
     Choke conductor  56  is spiraled over the top of shield  58  in the opposite direction to the spiral of shield  58 . Preferably, both shield  58  and choke conductor  56  are spiraled at 45° angles with respect to signal conductor  44 . Thus the shield and the choke conductor cross at 90° angles. Alternatively, the spirals for both the choke conductor and the shield could be adjusted to various angles to maximize inductance depending on the desired effect. 
     In the embodiment of FIG. 5, choke conductor  56  is in electrical contact with the metallic portion  60  of shield  58 . However, in the embodiment of FIG. 6, an insulated jacket  64  is provided between spiraled shield  58  and choke conductor  56  and a small drain wire  61  is placed in contact with shield  58  between shield  58  and jacket  64 . The drain wire  61  enables one to conveniently terminate the shield. In the design shown in FIGS. 5 through 8, both electric and magnetic fields are addressed. The electric field is addressed by the spiraled choke conductor  56  which, as indicated above, functions as an electrical choke. The magnetic field is addressed by the spiraled shield  58 , which acts as a Faraday cage. Also, the spiraled shield acts as a flat choke in the opposite direction of the spiraled electrical choke  56 , thus enhancing the cancellation effect. Therefore, shield  58  has two functions. 
     As indicated above, preferably, the shield  58  is preferably at a 45° angle with respect to center transmission signal conductor  44  and is spiraled in counterclockwise wrap. The choke conductor  56  is preferably also at a 45° angle with respect to center conductor  44 , but is spiraled in the opposite direction around the shield  58 , i.e., clockwise. The directions in which the choke conductor and signal conductor are wound could be reversed. The result is a 90° angle between the magnetic shield and the electric choke. The choke conductor  56  could be in the form of a second shield. 
     Referring now more particularly to FIG. 7, for ease of installation, a ground wire  66  may be made as a component of the cable  26 . Ground wire  66  is attached to the outer jacket  65  of the cable and is embedded in plastic which forms part of the extruded jacket  65 . The ground wire  66  runs the length of the cable. The ground wire is set apart from the main cable so that it may easily be detached and attached to a grounding rod. 
     The cable shown in FIG. 5 has been tested in the laboratory and in the field. The results show a substantial improvement over the prior art. 
     The detailed description above primarily discusses communication cable applications of the invention. FIG. 9 shows a lightning retardant cable  69  of the subject invention for power applications. Internal conductor  70  and  72  are power conducts which are normally heavier gauge than communication conductions. Often a gravel conductor (not shown) is placed adjacent to the power conductors. Conductors  70  and  72  are covered by insulated jacket  74 . Choke conductor  56  is spiraled about jacket  74  in the same fashion as shown and described in reference to FIG.  4 . In addition, the shield arrangement shown in FIGS. 5,  6  and  7  may also be used in power cable applications. 
     The choke conductor  56  can be insulated with insulation so that it is not in electrical contact with shield  58 . This insulation will electrically isolate the choke conductor  56  from shield  58  so that one may separate the electrical and magnetic fields. This will allow one to adjust the two windings, i.e., the shield and the choke, separately for maximum inductance. FIG. 10 shows a cross view of an insulated choke conductor. Item  56  is the choke conductor and item  76  is an insulative jacket. 
     It may become necessary, depending upon the application, that the choke conductor&#39;s insulative jacket  76  be slightly conductive. A compound, such as carbon, can be added to the insulation to increase this conductivity, i.e., to make the insulation semi-conductive. 
     Lightning will usually follow the path of least resistance or least inductance to ground. Every straight wire has an inductance. To minimize the inductance, you can actually use two coils wound opposite of each other. The fields of these two coils will cancel out each other and result in “0” induction. In FIG. 11, item  78  illustrates an inductive meter measuring the inductance of a straight wire  77 . In FIG. 12, items  79  and  80  illustrate inductors. If the second inductor  80  is wound opposite inductor  79 , as shown by  81  in FIG. 12A, and the two are electrically connected at both ends  82 , then the inductance should read “0”, as illustrated by meter  78  in FIG.  12 B. 
     Certain applications of lightning retardant cable may be enhanced if only one end of the cable has the choke  56  connected or grounded to shield  58 . This allows the shield to function as a Faraday cage shielding the inner coax or wires from the magnetic fields of any induced energy. FIG. 13 illustrates this construction. In this illustration, choke  56  and shield  58  are in electrical contact at one end of the cable only. This can be accomplished by winding the choke  56  around shield  58  so that they are in mechanical and electrical contact, as illustrated in FIG.  14 . 
     FIG. 14 shows a cross view of cable  65 . Item  58  is the spiral shield wrapped so that there is 100% full overlapping coverage. Choke  56  is stripped of insulation and wrapped around shield  58  so that it is in mechanical and electrical contact. 
     Referring now more particularly to FIG. 15, there is provided insulated cable  84  including conductor  86  and an insulation layer  88 . Cable  84  is received within conduit  90  which may be a typical plastic extruded conduit. Insulated shield  92  is wound about the outside of conduit  90  and the uninsulated side of shield  92  makes contact with the outer surface of plastic conduit  90 . Choke conductor  94  is wound about conduit  90  in the opposite direction to shield  92 . Preferably, the choke conductor and shield  92  cross one another in an angle of 90°. The choke conductor may or may not be insulated. If the choke conductor is uninsulated, it should make contact with the insulated side of shield  92  for the entire length of the cable, except at the far ends. The far end of the choke conductor is electrically connected to the shield conductor by a connection device, such as bolt  93 , at one end and by a connection device, such as bolt  95 , at the other end. 
     Referring now more particularly to FIG. 16, a specially designed insulated shield may be used to reduce arcing problems, namely, insulated shield  96 . Insulated shield  96  includes flat conductor  98  which is insulated by insulation  100 . Insulation  100  extends beyond edge  102  of the shield so as to form an expanded section  104  of the insulation. This enables overlap  106  between adjacent turns of the shield, as indicated in FIG. 17, so as to reduce the probability of arcing between adjacent turns of the shield. This expanded insulation could be placed on top of the shield, depending on how it is wrapped. In addition, the insulation material  100  may not be attached at all to the metal portion of the shield  96 , but it may be a separate piece of insulation applied to the cable during the manufacturing process between the windings of the shield. 
     In many situations, the choke conductor is simply a # 10  or # 12  round wire. The size of the wire was chosen since it meets usual National Electric Code requirements for grounding and has been shown to be large enough to handle direct lightning hits without burning through. A large wire wrapped around a cable alters the normally smooth round appearance, resulting in a so-called spiral hump on the cable due to the outer choke conductor&#39;s size. In practices, spiral hump could be a problem if the cable is pulled through a conduit with other cables since it would tend to cause binding on the spiral hump as it slides over the cables or joints in the conduit. This can be solved or improved upon by using a different shape of choke conductor wire, such as a so-called flat wire which, in reality, is substantially in a rectangular shape, as illustrated in FIGS. 18-21. FIG. 18 shows substantially rectangular shaped choke conductor  108  which has been insulated by insulation  110 . However, because a rectangular shaped conductor includes sharp edges  112 , it is preferred that edges  112  be rounded, as shown in FIG.  19 . FIG. 20 shows the substantially rectangular shaped rounded edge conductor being insulated by insulation  114 . As shown in FIG. 21, the substantially rectangular shaped conductor  111  or an insulated flat conductor, shown in FIG. 18, will not cause this spiraled hump, but will present a smooth surface outer jacket cable. 
     The lightning retardant cable discussed above preferably includes two chokes, one in the form of a so-called choke conductor, and the other in the form of a spiraled shield magnetically opposite, but having substantially identical inductances. The shield and the choke conductor are normally terminated at each end, as referred to above. Various techniques may be used to terminate the shield to the choke conductor. One technique is illustrated in FIG. 21 which shows the uninsulated portions  116  and  118  of choke  120  being placed in contact with uninsulated portion  122  of shield  124 . The insulation of the shield is stripped and if the choke conductor is insulated, its insulation is also stripped, so as to make electrical contact with one another. 
     When energize the two opposing coils&#39; magnetic fields cancel because they are oppositely wound, therefore the current does not flow down the coils outside the cable. When manufacturing the cable, the shield is normally wound first. The flat shield is usually, but not always, one inch in width. The electrical induction of the flat choke can be measured with an induction meter or an impedance bridge. The choke conductor or drain wire, which is usually a round configuration, but, as stated above, could be a substantially rectangular configuration, is a solid wire due to its physical characteristics. If it is wrapped at a 45° angle opposite to the shield, its electrical characteristics, i.e., its inductance will be slightly different. In order for the lightning retardant cable to achieve maximum performance, the two coiled inductors should have substantially the same inductance, as measured by an impedance bridge. This can be accomplished by adjusting the number of turns of the drain wire if the shield turns are fixed. Once the choke conductor is applied, it can be tuned to the shield&#39;s inductance by wrapping extra turns at one end of the cable or both ends until the inductance is the same. 
     From the foregoing description of the preferred embodiments of the invention, it will be apparent that many modifications may be made therein. It will be understood, however, that the embodiments of the invention are exemplifications of the invention only and that the invention is not limited thereto. It is to be understood therefore that it is intended in the appended claims to cover all modifications as fall within the true spirit and scope of the invention.