Abstract:
A radiating coaxial cable having a longitudinal axis comprises an inner conductor having a longitudinal axis wherein the axis of the inner conductor defines the axis of the cable. A dielectric material surrounds the inner conductor. A continuous outer conductor surrounds the dielectric in direct contact therewith and is spaced from the inner conductor. The outer conductor has a plurality of slots disposed therein. Adjacent slots are spaced in the axial direction a distance S. One or more adjacent slots are grouped together in a cell. The cable has a plurality of cells. Adjacent cells are angularly disposed from each other by an angle α.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to radiating transmission lines, particularly coaxial cables having helically disposed slots, and to radio communication systems that use such radiating transmission lines. 
     BACKGROUND OF THE INVENTION 
     Radiating coaxial cable has been used for many years in various types of radio communication systems. An improved radiating cable is disclosed in commonly-owned U.S. Pat. No. 5,809,429, which is incorporated herein by reference in its entirety. An embodiment of this improved cable contains one row of slots in the cable&#39;s outer conductor which are configured to produce a radiated field polarized perpendicularly to the axis of the cable to avoid the radiation of a field polarized parallel to the cable axis and to provide coupling energy between the interior of the cable and the slots. Another embodiment of this improved cable contains two parallel rows of slots disposed in the outer conductor diametrically opposite each other so that the cable performance would be independent of the wall-mounting position. 
     In practice, when using the cable with a single row of slots, attention must be given to the slot position during the mounting of this cable on a wall. Preferably, for best performance, all the slots should be facing outward away from the wall. A cable mounted with all of the slots facing outward away from the wall performs superior (see FIG. 1 a ) to a cable which has slots over a substantial length of cable facing inward towards the wall (see FIG. 1 b ). FIG. 1 c  illustrates a cable  10  containing a row of axially aligned slots  11  according to one embodiment of the cable disclosed in commonly-owned U.S. Pat. No. 5,809,429, incorporated by reference above. 
     Cable machines used in industry today tend to twist the cable as the cable is formed during manufacturing and/or reeled for shipping. The effect of the cable twist is the random rotation of slots over unpredictable lengths of cable. It has been observed that during cable manufacturing the slots of the cable can be rotated 360° over 180 feet of the cable. For example, this rotation can occur abruptly for a substantial length of cable so that the slots switch from being rotated 0° in the circumferential direction to being rotated 180° over a length of cable, and then again being rotated another 180° back to the first position for the next length of cable, where the rotations between 0° and 180° are random. 
     Another problem associated with the manufacture of radiating coaxial cable having all slots aligned in a row along the axis of the cable is mechanical slot compression. Such a cable is manufactured by wrapping the outer conductor, already having the slots formed therein, around the cable. During wrapping, the slots are compressed in the circumferential direction with respect to the cable causing the slots to become narrower. This mechanical slot compression results in less slot area through which the cable can emit or receive a signal. To remedy mechanical slot compression, tape is often affixed to the outer conductor before wrapping. The tape reinforces the outer conductor to help maintain the shape of the slots during wrapping. However, taping does not prevent slot compression; rather, it lessens its effect. Further, taping increases manufacturing time and expense. 
     FIGS. 1 a  and  1   b  provide an example which illustrates the effect that facing slots towards the wall has on the received signal level. A 180 foot length of cable that experienced the aforementioned twisting during manufacturing and reeling contained a 90 foot mid-portion having slots rotated so as to face inward towards the wall. The remaining portion of the cable was situated so that the slots faced outward away form the wall. This degree of slot rotation is not uncommon for a cable that has experienced twisting during manufacturing and reeling. The coupling amplitude of a 900 MHz signal was measured along the length of this cable. The type of signal obtained is shown in FIG. 1 b  and is undesirable because the drop in signal strength can result in degraded information received over such a long interval or a complete loss of communication over the interval. The magnitude of this null can be appreciated by comparing FIG. 1 b  with FIG. 1 a.  Thus, there is a need to remedy this effect in order to use a radiating cable in a radio communication system that is to provide a steady signal. Furthermore, there is a need for a radiating cable whose performance is independent of the wall mounting position of the cable. 
     SUMMARY OF THE INVENTION 
     An object of some embodiments of the present invention is to provide an improved radiating coaxial cable which can be mounted close to, or even on, a wall (even a metallic wall) or other surface independent of the cable orientation without significantly degrading the operation of the radio communication system in which the radiating cable is used. 
     Another object of some embodiments of the present invention is to provide an improved radiating coaxial cable which can be manufactured without experiencing mechanical slot compression. 
     In accordance with one embodiment of the present invention, the foregoing objectives are realized by providing a radiating coaxial cable having a longitudinal axis comprising an inner conductor having a longitudinal axis wherein the axis of the inner conductor defines the axis of the cable. The cable also comprises a dielectric material surrounding the inner conductor. A continuous outer conductor surrounds the dielectric and is in direct contact therewith and is spaced from the inner conductor. The outer conductor has a plurality of slots disposed therein with adjacent slots being spaced in the axial direction. According to some embodiments of the present invention, the slots are helically disposed in the circumferential direction. 
     According to some embodiments of the present invention, the radiating coaxial cable having helically disposed slots in the cable&#39;s outer conductor can be mounted without regard to the direction which the slots are facing in relation to the signal transmitter or receiver. 
     Also according to some embodiments of the present invention, an improved radio communication system is provided which includes the above radiating cable located within or adjacent to a prescribed area containing a multiplicity of radio transmitters, receivers or transceivers (“radio units”), which may be either mobile or fixed. Signals are transmitted to and received from the various radio units via the radiating cable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a plot of the indoor measurements of the continuous wave signal level (in dB) taken along a 180 foot length of a radiating coaxial cable having linearly aligned slots facing outward, measured at a perpendicular distance of six feet away from the coaxial cable and at the same height as the coaxial cable, while operating at a fixed frequency of 900 MHz. 
     FIG. 1 b  is a plot of the indoor measurements of the continuous wave signal level (in dB) taken along a 180 foot length of a radiating coaxial cable having slots experiencing a 360° rotation over 180 feet, measured at a perpendicular distance of six feet away from the coaxial cable and at the same height as the coaxial cable, while operating at a fixed frequency of 900 MHz. 
     FIG. 1 c  is a perspective view of a cable with a linear arrangement of slots according to an embodiment of the cable disclosed in commonly-owned U.S. Pat. No. 5,808,429. 
     FIG. 1 d  is a plot of the indoor measurements of the continuous wave signal level (in dB) taken along a 180 foot length of a radiating coaxial cable having helically disposed slots, wherein adjacent slots are angularly disposed at 72° from each other, according to one embodiment of the present invention, measured at a perpendicular distance of six feet away from the coaxial cable and at the same height as the coaxial cable, while operating at a fixed frequency of 900 MHz. 
     FIG. 1 e  is a plot of the indoor measurements of the continuous wave signal level (in dB) taken along a 180 foot length of a radiating coaxial cable having helically disposed slots, wherein adjacent slots are angularly disposed at 120° from each other, according to one embodiment of the present invention, measured at a perpendicular distance of six feet away from the coaxial cable and at the same height as the coaxial cable, while operating at a fixed frequency of 900 MHz. 
     FIG. 2 a  is a perspective view of a radiating coaxial cable having slots helically disposed at 72° according to one embodiment of the present invention, and associated radio units (“R.U.”); 
     FIG. 2 b  is a cross-sectional view of a radiating coaxial cable having slots helically disposed at an angle α according to one embodiment of the present invention; 
     FIG. 3 is another perspective view of a radiating coaxial cable having slots helically disposed at 72° according to one embodiment of the present invention; 
     FIG. 4 is a perspective view of a radiating coaxial cable having two slots per cell according to an alternative embodiment of the present invention; 
     FIG. 5 is a perspective view of a radiating coaxial cable having tilted slots helically disposed according to an alternative embodiment of the present invention; 
     FIG. 6 is a perspective view of a radiating coaxial cable having slots in tilted alternating directions helically disposed according to an alternative embodiment of the present invention; 
     FIG. 7 is a perspective view of a radiating coaxial cable having helically disposed slots in an alternative embodiment of the present invention. 
     FIG. 8 is a perspective view of a radiating coaxial cable having many slots per wavelength helically disposed according to an alternative embodiment of the present invention. 
     FIG. 9 is a perspective view of a radiating coaxial cable having zigzagged slots helically disposed according to an alternative embodiment of the present invention. 
     FIG. 10 a  is an indoor measurement of the coupling loss (in dB) taken over a frequency range of 200 to 1000 MHz for a radiating coaxial cable such as shown in FIG. 1 c  having lineally aligned slots, wherein each plot represents a 90° rotation of the cable. 
     FIG. 10 b  is an indoor measurement of the coupling loss (in dB) taken over a frequency range of 200 to 1000 MHz for a radiating coaxial cable such as shown in FIG. 3 having slots helically disposed at 72°, wherein each plot represents a 90° rotation of the cable. 
     FIG. 10 c  is a comparison of the indoor measurement of the coupling loss (in dB) taken over a frequency range of 200 to 1000 MHz for a radiating coaxial cable such as shown in FIG. 3 having slots helically disposed at 72° and a cable such as shown in FIG. 1 c  having axially aligned slots facing away from the wall. 
     FIG. 10 d  is a comparison of the indoor measurements of the coupling loss (in dB) taken over a frequency range of 200 to 1000 MHz for a radiating coaxial cable such as shown in FIG. 3 having slots helically disposed at 72° and a cable such as shown in FIG. 1 c  having axially aligned slots facing inward towards the wall. 
     FIG. 11 a  is an indoor measurement of the coupling loss (in dB) taken over a frequency range of 200 to 1000 MHz for a radiating coaxial cable such as shown in FIG. 3 but having slots helically disposed at 120°, wherein each plot represents a 90° rotation of the cable. 
     FIG. 11 b  is a comparison of the indoor measurements of the coupling loss (in dB) taken over a frequency range of 200 to 1000 MHz for a radiating coaxial cable such as shown in FIG. 3 but having slots helically disposed at 120° and a cable such as shown in FIG. 1 c  having axially aligned slots facing outward away from the wall. 
     FIG. 11 c  is a comparison of the indoor measurements of the coupling loss (in dB) taken over a frequency range of 200 to 1000 MHz for a radiating coaxial cable having slots such as shown in FIG. 3 but helically disposed at 120° and a cable having axially aligned slots facing inward towards the wall, such as shown in FIG. 1 c.    
     FIG. 12 a  is a plot of the indoor insertion loss (in dB/100 m) of a radiating coaxial cable such as shown in FIG. 3 having slots helically disposed at 72° according to one embodiment of the present invention, measured over a frequency range of 50 to 1000 MHz. 
     FIG. 12 b  is a plot of the indoor insertion loss in (dB/100 m) of a radiating coaxial cable , such as shown in FIG. 1 c  having axially aligned slots facing outward away from the wall, measured over a frequency range of 50 to 1000 MHz. 
     FIG. 12 c  is a plot of the indoor insertion loss (in dB/100 m) of a radiating coaxial cable such as shown in FIG. 1 c  but having slots experiencing a 360° rotation over 180 feet, measured over a frequency range of 50 to 1000 MHz. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but, to the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     One embodiment of the radiating coaxial cable  20  according to the present invention is illustrated in FIG. 2 a.  The radiating cable  20  may be used in a wide variety of different applications where multiple radio units, often mobile units, must communicate with one or more base stations within a defined area. One example of such a system is a highway or railroad communication system in which the radiating cable extends along an open highway or railroad (or, also, in a tunnel) for constant communication with mobile radio units in the various vehicles on the open highway or railroad (or in the tunnel). Another example is a wireless local area network (WLAN) of personal computers, printers, servers and the like, located in a common building or on a common floor. This invention is particularly useful in applications where the communication area is sufficiently large that the radiating cable  20  must be at least 60 feet in length. 
     Referring now to FIGS. 2 a,    2   b,  and  3 , a length of a radiating coaxial cable  20  having a series off-resonant slots  21  formed in the cable is shown. The slots  21  are helically disposed in the circumferential direction so adjacent slots  21  are angularly disposed at an angle α from each other. In the illustrated embodiments, the slots  21  are angularly disposed approximately 72° from each other so that the circumferential position of the slots  21  repeats every sixth slot. In an alternative embodiment of the present invention cells of slots are helically disposed in the circumferential direction along the length of the cable  20 . In the embodiment illustrated in FIG. 4, each cell comprises two slots axially aligned in the same angular position along the cable. In other alternative embodiments, the cells of slots may comprise more than two slots. 
     Referring again to FIGS. 2 a,    2   b,  and  3 , the cable  20  is a typical coaxial cable having an inner conductor  25  insulated from an outer conductor  27  by a dielectric material  26 . The inner conductor  25  defines the longitudinal axis of the cable. The slots  21  are spaced by a center-to-center distance, S, from each other in the axial direction. When a signal is fed into one end  22  of the cable  20  and propagated through the cable  20  to a matched load at the opposite end  23 , a portion of the signal is radiated from the slots  21  along the entire length of the cable. The radiated field is polarized perpendicularly to the axis of the cable  20  and can be detected by radio units (“R.U.”) anywhere along the length of the cable  20 . The cable  20  can also receive radiated signals from the radio units anywhere along the length of the cable  20 . These received signals are propagated through the cable to a receiver (not shown) at the end  22  of the cable  20 . To cause each slot  21  to radiate energy from the interior of the coaxial cable  20 , a coupling device such as tab  24  is provided at each slot  21 . The tabs  24  may lie in the cylinder of the outer conductor  27  of the cable  20 , or the tabs  24  may be bent into the interior of the cable  20  for increased coupling. The phase of the slot&#39;s  21  electric field are reversed for successive slots  21  by forming the tabs  24  on alternating edges of successive slots  21 , so that the tabs  24  are on opposite edges of each pair of adjacent slots  21 . 
     The slots  21  are axially spaced from each other by a distance, S. The dimensions of both the slots  21  and the tabs  24  are chosen to avoid any significant radiation attenuation of the signals that are propagated longitudinally through the cable  20 , thereby ensuring that the signal is radiated with adequate strength along the entire length of the cable  20 . Thus, the radiated energy per unit length of cable, as well as the radiated-attenuation per unit length of the cable, are relatively low. 
     While FIGS. 2 a  and  3  illustrate slots  21  that are substantially rectangular in shape, the helical disposition of the slots according to an embodiment of the present invention is applicable to a radiating coaxial cable having slots of any shape. For example, FIG. 5 illustrates an alternative embodiment of the present invention wherein a radiating coaxial cable  30  contains slots  31  that are elliptical in shape and have a longitudinal axis  33  which is tilted at an angle β with respect to the axis  32  of the cable  30 . In the illustrated embodiment, the longitudinal axis  33  of the slots  31  are tilted at an angle β of approximately30° with respect to the axis  32  of the cable  30 . In other alternative embodiments, the slots  31  may be tilted with respect to the axis  32  of the cable  30  at an angle β ranging from approximately 0° to 90°. 
     FIG. 6 illustrates another alternative embodiment of the present invention wherein a radiating coaxial cable  34  contains elliptical-shaped slots  3   1 . The longitudinal axis  33  of adjacent slots  31  are tilted in alternating directions with respect to the axis  32  of the cable  34  at an angle at an angle β. Viewing the cable  34  shown in FIG. 6 from left to right, the slot  31  in the first position  35  is tilted at an angle β of approximately positive 30° with respect to the axis  32  of the cable  34 . The adjacent slot  31  (in the second position  36 ) is tilted at an angle β of approximately negative 30° with respect to the axis  32  of the cable  34 . The tilting of the slots repeats in a similar manner along the length of the cable: the slot in the third position  37  is tilted at an angle at an angle β of approximately positive 30° with respect to the axis  32  of the cable  34 ; the slot in the fourth position  38  is tilted at an angle at an angle β of approximately negative 30° angle with respect to the axis  32  of the cable  34 ; and so on. In alternative embodiments, adjacent slots may be tilted in alternating positive and negative directions with respect to the axis  32  of the cable  34  at an angle at an angle β ranging between approximately negative 90° and positive 90°. 
     FIG. 7 illustrates a radiating coaxial cable  40  containing elliptical-shaped slots  31  having the longitudinal axis  33  of the slots  31  substantially parallel to the axis  32  of the cable  40  according to another embodiment of the present invention. 
     In other alternative embodiments, the center-to-center axial spacing, S, of adjacent slots is determined by the specified frequency range of the particular application in which the cable is used. Usually, the wavelength of the signal inside the cable varies from application to application. For example, in the embodiment illustrated in FIG. 3, the center-to-center spacing, S, is usually such that only a few slots  11  are provided in each wavelength (of the signal inside the cable) so that S is much larger than one-fourth of the wavelength. In other alternative embodiments, S is very much smaller then one-fourth the wavelength as shown in FIG.  8 . FIG. 8 illustrates a radiating coaxial cable  42  according to an alternative embodiment of the present invention which has many slots  44  per wavelength. The slots  44  of cable  42 , as shown in FIG. 8, have the longitudinal axis  33  of the slot  44  substantially perpendicular to the axis  32  of the cable  42 . In other alternative embodiments of the cable  42 , the longitudinal axis  33  of the slots  44  may be tilted with respect to the axis  32  of the cable. 
     In still another alternative embodiment, a radiating coaxial cable  46  contains zigzagged shaped slots  48  as illustrated in FIG.  9 . The zigzagged shaped slots  48  have three sections: a first section  50 ; a second section  51 ; and a third section  52 . The first and third sections  50 ,  52  are disposed substantially parallel to the axis  32  of the cable  46  and are connected via the second section  51  which is disposed substantially perpendicular to the axis of the cable  46 . In the embodiment illustrated in FIG. 9, adjacent slots are flipped so that adjacent slots face alternating directions. Viewing FIG. 9 from left to right, the slot  48  in the second position  56  is the mirror image of the slot  48  in the first position  55 . The slots  48  are flipped in this manner along the length of the cable. The slot  48  in the fourth position  58  is the mirror image of the slot  48  in the third position  57 , and so on. 
     Slot compression is often a problem with cables having a row of axially aligned slots because of the limited amount outer conductor surface area between adjacent slots. A cable having helically disposed slots according to the present invention mitigates against the aforementioned problems associated with mechanical slot compression. The cable having helically disposed slots provides increased area between adjacent slots resulting in an increased ability to maintain the slot edge position and avoids slot compression during the wrapping of the outer conductor on to the cable. Hence, the outer conductor having helically disposed slots does not need to be tapped before wrapping. Therefore, a cable having helically disposed slots according to one embodiment of the present invention can be manufactured without devoting time and money to guard against slot compression. 
     In alternative embodiments, a cable  20  having helically disposed slots  21  can have slots  21  disposed from each other at angles, ranging approximately from 36° to 120°. In the case of slots  21  disposed from each other at 120°, the circumferential slot position repeats every third slot  21 . In the case of slots  21  disposed from each other at 36°. the circumferential or angular slot position repeats every tenth slot  21 . However, it has been found that decreasing the angular distance between slots  11  beyond this range may be undesirable because positioning the adjacent slots  11  closer to each other by decreasing the angular position between the slots  11  decreases the outer conductor surface area between the slots  11  which can lead to mechanical slot compression. As a slot is compressed the effective signal radiation from that slot is reduced. Severe slot compression or slot compression along a significant length of cable  10  can greatly effect the performance of the cable  10 . According to some embodiments, adjacent slots are disposed at either 60° or 90° from one another. Disposing adjacent slots at angles of 60° and 90° causes the slots to repeat their angular position every sixth slot or fourth slot, respectively. Having slots repeat their angular position on an even number of slots reduces the cable manufacturing costs associated with tooling. 
     Referring now to FIG. 1 d,  the signal radiating performance of a radiating coaxial cable such as shown in FIG. 3 with helically disposed slots at 72° according to one embodiment of the present invention is shown. FIG. 1 d  is a plot of the strength of a fixed frequency signal radiated from the cable over the length of the cable. The cable used in connection with FIG. 1 d  as well as the cables used in connection with FIGS. 1 a  and  1   b  have the same diameter, center-to-center slot spacing, S, and slot configuration. The slot dimensions and configuration were chosen to have the cable operate optimally from approximately 380-1140 MHz. The cable was 180 feet in length and was operated at a frequency of 900 MHz. The perpendicular distance between the cable axis and the measured field point was six feet, while the cable and measured field point were at the same height. FIG. 1 e  illustrates that similar results were obtained for a cable identical to that described but having slots helically disposed at 120° according to an alternative embodiment of the present invention. 
     Comparison of FIGS. 1 a,    1   b,  and  1   d  indicate that the ideal case, a cable having all slots facing outward (FIG. 1 a  ), produces the strongest and steadiest signal. However, greater time and effort must be expended to mount a cable to a wall in the ideal manner and in some cases it may not be possible. A cable having experienced slot rotation due to cable twisting occurring during both manufacturing and/or reeling (FIG. 1 b  ) produces the most undesirable signal due to the aforementioned deep null, occurring over the 90 foot portion of the cable (from the approximately 75 feet to 165 feet point on the cable) wherein the slots are rotated towards the wall, which can result in communication loss or information degradation. While FIG. 1 d  illustrates a decrease in signal level from the ideal case (FIG. 1 a  ), the cable with helically disposed slots still radiates a steady peak signal which is relatively flat but contains some sharp dips. However, these dips are not significant because they occur over only a few inches. If the receiver is on a moving vehicle, it will only experience a signal drop for a very short time. Likewise, a fixed receiver or its antenna only has to be moved a few inches to receive a strong signal. Therefore, the cable having helically disposed slots, according to an embodiment of the present invention, can be installed without regard to cable orientation and yet radiate nearly as well as the ideal case. 
     Because a radiating cable having helically disposed slots radiates a substantially flat near-field pattern, it provides reliable (non-fading) communications to and from radio units distributed along the length of the cable. This reliability is particularly useful in digital communications because it permits the attainment of low bit error rates (“BERs”). For example, digital data communications may require BERs as low as 10 −8  to avoid loss of significant data. These low BERs are attainable with a substantially flat near-field pattern because the fluctuations, or oscillations, in the pattern arc of such a small amplitude that losses of one or more bits of data are very small. The substantially flat near-field patterns of the present invention are also desirable for analog communication signals, to avoid spurious distortions in the analog signals. 
     Referring now to FIGS. 10 a  and  10   b  the signal receiving performance of the radiating cable  20  having helically disposed slots  21  at 72° according to one embodiment of the present invention may be compared to a cable having axially aligned slots. FIG. 10 a  shows the swept frequency measurements for the cases of a cable such as shown in FIG. 1 c  having all slots being disposed along a straight line along the axis but rotated in different angular positions. The frequency of the signal received by the cable is swept from 50 to 1000 MHz in {fraction (1/20)} of a second and is transmitted by an antenna on a cart moving parallel to the cable at a rate of four inches per second. The distance covered in one frequency sweep is ⅕ inch per sweep. This distance is so small compared to the wavelength, which is at least 11.8 inches at 1000 MHz, that the distance is practically zero inches per sweep; therefore, the sweep is virtually instantaneous. The curve identified by reference number  60  refers to the case where the cable is rotated 0° so that all slots face outward away from the wall. Reference number  62  refers to the case where the cable is rotated upward 90° so that slots are facing the ceiling. Reference number  64  refers to the case where the cable is rotated downward 90° so that the slots face the floor. 
     Reference number  66  refers to the case where cable is rotated 180 ° so that the slots face inward towards the wall. Finally, reference number  68  refers to the case of a cable having experienced slot rotation due to cable twisting wherein the slots are rotated 360° over a 180 foot cable. FIG. 10 a  illustrates that large drops, up to 12 dB, occur in the signal strength between rotated positions of the cable having all slots linearly aligned. A drop of this magnitude would result in a severely reduced signal causing degradation of information or a complete loss of communication. This result indicates that is it undesirable to use a cable having slots facing towards the wall for more than a minimal portion of the length of the cable. 
     FIG. 10 b  illustrates the coupling loss experienced by a cable such as shown in FIG. 3 having slots helically disposed from each other 72° in the circumferential direction according to one embodiment the present invention for the same swept frequencies. The cable having helically disposed slots used in connection with FIG. 10 b  contains the same type of slots and the same axial slot spacing as the cables used in connection with FIG. 10 a.  All of the lines representing measurements for each rotation of the cable of FIG. 10 b  practically fall on top of one another evincing that for any given frequency the signal level is independent of cable rotation. Similar results were obtained from a cable having slots helically disposed 120° from each other according to an alternative embodiment of the present invention (see FIG. 11 a ). Therefore, a cable of the present invention can be mounted without regard to cable position because the slots are distributed about the circumference of the cable; cable twisting does not disturb that distribution. Thus, signal degradation due to the inherent slot rotation occurring during manufacturing and/or cable reeling is reduced or eliminated with a cable having helically disposed slots according to the present invention. 
     Referring to FIG. 10 c,  the coupling loss experienced by the cable of FIG. 3 having slots helically disposed from each other 72° in the circumferential direction is compared to a cable such as shown in FIG. 1 c  having all slots facing outward away from the wall. While the coupling loss of the cable having all slots facing outward is less than the cable having helically disposed slots, examination of FIG. 10 c  indicates the difference in coupling losses is at most 5 dB occurring around 850 MHz. This small difference in coupling loss experienced by the cable having helically disposed slots while receiving a signal is acceptable because this same cable produces a steady near-field signal as seen in FIG. 1 d.  When compared to the case of a cable such as shown in FIG. 1 c  having all slots facing the wall, the cable of FIG. 3 having slots helically disposed from each other 72° in the circumferential direction produces higher coupling as illustrated in FIG  10   d.  Similar results were obtained from a cable having slots helically disposed 120° from each other according to an alternative embodiment of the present invention. FIG. 11 b  compares a cable having slots helically disposed 120° from each other to a cable have axially aligned slots facing outward away from the wall for the same swept frequencies of FIG. 10 c.  FIG. 11 c  compares a cable having slots helically disposed 120° from each other to a cable have axially aligned slots facing inward toward the wall for the same swept frequencies of FIG. 10 d.    
     Helically disposing the slots of the cable does not have a significant impact on the insertion loss of the cable. Referring to FIGS. 12 a,    12   b,  and  12   c,  it can been seen that a cable such as shown in FIG. 3 having slots helically disposed from each other (FIG. 12 a  ) has only a slightly higher insertion loss than a cable having all slots facing outward (FIG. 12 b ) and a cable experiencing twisting due to cable reeling (FIG. 12 c  ). This slightly larger cable insertion loss is attributed to the slots not being as compressed because the helically disposed slots are resistant to the aforementioned mechanical compression.