A LEAKY COAXIAL CABLE AND METHOD OF MAKING SUCH A CABLE

Abstract: A leaky coaxial cable is designed to produce a defined coupling between its interior and exterior. A bonded and insulated outer conductive tape (3) of relatively large width relative to the cable diameter, is spirally wound at a low pitch angle around a dielectric cladding (2) on a central conductor (1) to provide at high frequencies a coupling level having minimal effect of the coaxial properties of the cable.

Description: FIELD OF THE INVENTION This invention relates to a leaky coaxial cable, such as is used for guided communications, obstacle detection, and perimeter security and to a method of making such a cable. Specifically, the present invention relates to a leaky coaxial cable of the type having a central conductor, a dielectric layer therearound and an outer conducting shield. BACKGROUND TO THE INVENTION Leaky coaxial cables, sometimes known as ported coaxial cables or radiating coaxial cables, are generally constructed with gaps or apertures in their outer shield which permit a portion of the internal field to couple to the external environment and external fields to couple to the cable. For example, US-A-4 300 338 discloses a design with rhombic shaped apertures in the outer conductor. Both inductive and capacitive coupling is produced have a magnitude dependent on the size, shape, orientation and density of the apertures. JP-A-5277622 describes a cable with a central conductor, a dielectric layer disposed coaxially around the central conductor and an outer conductor shield in the form of a conductive tape wound in spiral configuration to leave a helical slit between adjacent edges of the tape. The helical slit acts as energy radiating apertures. Leaky coaxial cables can also be produced with thin, solid, tubular outer shields, as described in US-A-3 681 717, in which there is diffusion coupling through the shield due to its thickness being of the same order as, or smaller than, the skin depth at the frequency of operation. Finally, it is known that by use of a spiral or solenoidal construction path along the outer conductor inductive coupling can be produced with no aperture of gap necessarily being present. US-A-3 735 293, for example, describes a cable having an outer conductor formed from closely wound metal tape with an insulating backing. In design of a leaky coaxial cable it is desired to produce a defined level of coupling with minimal effect on such coaxial cable parameters as impedance, velocity of propagation and downline attenuation. The primary components of attenuation in non-leaky cables are due to conductor and dielectric losses, but in leaky coaxial cables losses also occur due to coupling with the external environment. The presence of apertures, since they result from metal removal from the conduction path, cause an inherent increase in attenuation. Models of coupled transmission lines indicate that the capacative coupling inherent with apertures or longitudinal gaps as described in US-A-4 300 338 is generally undesirable. This coupling varies with the dielectric constant of the materials external to the cable and, thus, produces undesirable environmental sensitivity. It may also reduce the signals transferred by inductive coupling by producing components of opposite phase to them. Finally, capacitive coupling also produces a loss which contributes to attenuation. Diffusion coupling cables, such as are described in US-A-3 681 717, are limited in leaky cable applications both because the resulting coupling is weak and a substantial increase in attenuation results from the requirement that the thickness of the outer shield must be reduced. Cables relying on a solenoidal conductive path in the outer conductor, such as are described in US-A-3 735 293, called induction cables, have been restricted to use at low frequencies, because the resulting large inductive coupling increases linearly with frequency. This has been found to cause large mismatch effects and high coaxial attenuation due to a high degree of coupling when used in the frequency range of typical applications, greater than 30 MHz. Frequencies in the 30-200 MHz band are used for the detection of humans or obstacles which have a dimension of approximately 1/4 wavelength in this band. Also coaxial attenuation is inherently high for cables using high pitch angle conductors to produce the solenoidal currents since the conductor path is long. Typical application angles for spiral tapes in normal manufacturing practice is in the range 30-70 degrees (e.g. see US-A-3 735 293, 3 949 329 and 3 870 977). Coaxial attenuation increases approximately as the inverse of the cosine squared of the pitch angle for full coverage spiral tapes. For many applications it is desirable to be able to 'grade' or modulate the cable coupling, as described in US-A-4 432 193, by varying some cable parameters with length. This can, for example, be used to compensate for cable attenuation so that the external field along the cable from the signal input is maintained of uniform magnitude. SUMMARY OF THE INVENTION It is an object of the present invention to procide a leaky coaxial cable exhibiting low coaxial attenuation together with coupling levels that are sufficient for detection, without resulting in undersirable variations in the other cable parameters. As is known from JP-A-5277622, for example, a leaky coaxial cable constructed in accordance with the preamble of claim 1 comprises a central conductor extending along a longitudinal axis, a dielectric layer disposed coaxially around the central conductor and an outer coaxial conducting shield comprising a tape of selected width having a continuous conductive layer wound around the dielectric layer with the pitch angle of the tape with respect to the longitudinal axis of the cable being less than 30°. In contrast to JP-A-527762 and other cables with aperture type coupling, the tape in accordance with the invention also has an insulating layer and the tape is arranged in spiral configuration with adjacent edges of the conductive layer insulated from one another by said insulating layer and being at least closely spaced or preferably overlapping so as to attain an acceptable level of inductive coupling, low capacitive coupling and low attenuation. The invention also provides a method of making a leaky coaxial cable having an acceptable level of inductive coupling, low capacitive coupling and low attenuation comprising the steps of: (a) providing a core consisting of a central conductor and a dielectric layer disposed coaxially thereon; (b) providing a laminated tape of selected width and having a conductive layer and an insulating layer the ratio of the tape width to the circumference of the coaxial cable being selected to produce a low level of capacitive coupling in the assembled cable; (c) winding the tape coaxially around said core at a pre-determined pitch angle less than 30° with adjacent edges of the conductive layer overlapping or closely spaced to an extent to substantially eliminate radiation gaps therebetween, said edges being insulated from one another to provide an acceptable level of inductive coupling, said winding of the tape being such as to provide a sufficient surface coverage to provide a low level of capacitive coupling. The cable of the invention is of the inductive coupling type. Although it is preferred to have the edges of the conductive tape layer overlapping it is possible to have a small spacing therebetween. The use of such low pitch angles has the following advantages. Coupling levels, which increase approximately in a linear manner with frequency and as the square of the tangent of pitch angle, are sufficient for detection, yet do not detrimentally affect the coaxial cable properties. Conductor losses, which vary approximately inversely as the cosine squared of the pitch angle, are not excessive at this low angle, and hence coaxial attentuation, which has components due to both this and to coupling losses, is low. Because of the difficulty of applying and retaining wide tapes at such low angles the conductor is typically bonded both to the dielectric layer, and to itself, providing mechanical stability during production and flexing in use. The bonding also serves to provide protection of the underlying dielectric from moisture ingress from the environment. The full surface coverage of the dielectric by the outer conductor results in almost no capacitive coupling, and hence negligible losses and adverse interaction effects due to this factor. In referring to conductive tape it is intended to include also served or braided wires which function in the same manner. Brief Description of the Drawings Particular embodiments of the invention will be described in conjunction with the accompanying drawings, in which: Figure 1 shows the construction of a leaky coaxial cable in accordance with the present invention; Figure 2 is a graph showing inductive coupling at one frequency as a function of the tape width and pitch angle; Figure 3 is a graph showing capacitive coupling as a function of the same cable parameters; Figure 4 shows an alternative construction of a leaky coaxial cable including a drain wire and retaining tape; and Figure 5 shows the manner of grading a leaky coaxial cable in accordance with this invention. Figure 1 shows the construction of a leaky coaxial cable in accordance with the invention. A centre conductor 1 has a concentric dielectric layer 2 formed thereabout. The centre conductor is typically but not necessarily copper, copper-clad aluminum, copper-clad steel, or aluminum. The insulating dielectric layer is typically a solid, foamed or air-spaced plastic compound such as polyethylene, polypropylene, or teflon. A laminated tape 3 is spirally wound about the dielectric layer. The tape 3 has layers, from the inside to the outside of adhesive 5, a non-conductive plastic such as mylar, polyester or polypropylene 6, bonded to a conductor 7 such as copper or aluminum. The insulating plastic is not a necessary element if the adhesive itself provides an insulating layer and the conductor is of adequate thickness for mechanical strength. When the tape is wound with a width W and a pitch angle ϑ the relationship between these parameters and C the cable circumference at the dielectric layer is maintained so that: This allows edges of adjacent turns to be in close proximity to one another, located between the limits of being slightly gapped and have a slight overlap. In any case there is no conducting path short circuiting the turn. The conductive tape thickness can be selected to be several multiples of the skin depth at the frequency of operation to minimize attenuation. The tape layer 3 may be covered with an insulating dielectric jacket 4 to provide mechanical protection. It will be clear that the relative location of the adhesive is not critical to the invention. It could be applied to the dielectric layer or on the outside of the tape at least on the portions which overlap. An additional dielectric flooding compound can be introduced between the tape layer and jacket to provide moisture protection and, again as an option, the adhesive layer or additional adhesive layers can be formed between the tape and the jacket. The tape pitch and width are selected with regard to the data shown in Figures 2 and 3. Figure 2 shows the inductive coupling as a function of the outer conductor tape width and pitch angle. High coupling is produced with a narrow (W/C <<1) tape or wire wound at high pitch angle. From experience with leaky cables it has been found that cables constructed with parameters in the upper region of the plot exhibit extremely high coupling, producing strong interaction with the environment and unacceptable changes in coaxial properties such as impedance and attenuation. Cables that are constructed in accordance with the present invention require very wide tapes and very low pitch angles as indicated by the operating region of the plot. Figure 3 shows the related capacitive coupling as a function of tape width and pitch angle. High capacitive coupling is also produced with a narrow (W/C <<1) tape or served wires. At a constant tape width, capacitive coupling decreases as the pitch angle, and hence physical coverage of the tape, increases. For the desired minimum capacitive coupling at a particular tape width the curve indicates that the maximum available full coverage tape pitch angle be used, as the curve asymptotically approaches zero at this angle. The results of Figure 2 and 3 taken together require the leaky cable to be such that the tape pitch angle is typically in the range of 5 to 30 degrees, parameter W/C typically in the range of .5 to 1.1 and almost full coverage or a slight overlap maintained on the dielectric surface. In Figure 1 the adhesive layer 5 is used primarily to ensure tapes of such extremely high width and low pitch angle can be retained in the prescribed position. It also serves as a protective barrier to prevent moisture ingress to the dielectric. An alternative construction of the leaky cable is shown in Figure 4. In this construction the outer conductor, from the inside out, consists of a metallic drain wire conductor 10 in contact with a laminated tape consisting of a metallic conductive layer 11 in contact with the drain wire, and an insulating layer 6 providing insulation between turns. The drain wire and laminated tape are wound at pitch angles selected in accordance with the above range. To affix the laminate in the desired position relative to the dielectric an insulating tape 9 is wound at a relatively higher pitch than the laminated tape. This tape 9 can be wound either with the same or opposite lay as the laminated tape. The drain wire performs its conventional function of ensuring that the surface formed by the tape is at a uniform electrostatic potential. It will be clear that the order of the conducting layer and insulating layer can be reversed and the cable will function in the same manner. Other methods of mechanical restraint for the spiral tape are possible. For example, it is possible to interlock the adjacent insulated edges of the conductor as in armouring or folding, or to extrude a dielectric sleeve or jacket directly over the conductor immediately after it has been applied. Similar constructions using the present invention include the use of commercially available laminate tapes having several conductive and insulating layers of same or different widths or the use of more than one parallel spiral conductive tape or served wires. The latter could be used, for example, to improve mechanical characteristics such as flexibility. The same low pitch angle and coverage are required. Grading or modulation of the leaky cable can also be achieved by ensuring that the inductive coupling is modified with distance along the cable relative to the incremental coaxial attenuation at the frequency of operation. Referring to Figure 2 it is evident that coupling can be increased by moving up the full coverage line from a low to higher pitch angle and decreasing tape width. Figure 5 shows the outer conductive tape at two different sections along a radiating cable constructed to provide for constant sensitivity along the cable length. The information of Figures 2 and 3, as well as information relating to attenuation at the frequency of operation is used to derive the precise variation of tape width and pitch angle with distance along the cable.

Claims: A leaky coaxial cable having a central conductor (1) extending along a longitudinal axis, a dielectric layer (2) disposed coaxially around the central conductor and an outer coaxial conducting shield comprising a tape (3) of selected width (W) having a continuous conductive layer (7) wound around the dielectric layer with the pitch angle (ϑ) of the tape with respect to the longitudinal axis of the cable being less than 30°; characterised in that the tape also has an insulating layer (6,5), and the tape is in spiral configuration with adjacent edges of the conductive layer insulated from one another by said insulating layer and being at least closely spaced or preferably overlapping so as to attain an acceptable level of inductive coupling, low capacitive coupling and low attenuation. A leaky coaxial cable according to claim 1, wherein the tape (3) is laminated and also comprises an adhesive layer (5), the adhesive (5), the insulating (6) and the conductive layers (7) being arranged in that order from the dielectric layer to the outside of the cable whereby the adhesive layer (5) bonds to the dielectric layer (2) to hold the laminated tape (3) in place. A leaky coaxial cable according to claim 1 wherein the tape (3) further comprises a conductive drain wire (10) in electrical contact with the conductive layer (11). A leaky coaxial cable according to claim 3, wherein the drain wire (10) the conductive layer (11) and the insulating layer (6) are arranged in that order from the dielectric layer to the outside of the cable. A leaky coaxial cable according to claim 3, wherein the insulating layer (6), the conductive layer (11) and the drain wire (10) are arranged in that order from the dielectric layer to the outside of the cable. A leaky coaxial cable according to any one of claims 1 to 5 and further comprising external means for holding the tape (3) in place. A leaky coaxial cable according to claim 6 wherein said external means is an insulating tape (9) wound over the tape or a dielectric sleeve or jacket (4) surrounding the tape. A leaky coaxial cable according to any one of claims 1 to 7, wherein either the tape width or the pitch angle is varied along the length of the cable to provide a cable with coupling characteristics varying with distance along the cable. A leaky coaxial cable according to any one of claims 1 to 7, wherein both the tape width and the pitch angle are varied along the length of the cable to provide a cable with coupling characteristics varying with distance along the cable. A method of making a leaky coaxial cable comprising the steps of: a) providing a core consisting of a central conductor (1) and a dielectric layer (2) disposed coaxially thereon; b) providing a laminated tape (3) of selected width and having a conductive layer (7) and an insulating layer (6), the ratio of the tape width (W) to the circumference (C) of the coaxial cable being selected to produce a low level of capacitive coupling in the assembled cable; c) winding the tape coaxially around said core at a pre-determined pitch angle less than 30° with adjacent edges of the conductive layer overlapping or closely spaced to an extent to substantially eliminate radiation gaps therebetween, said edges being insulated from one another to provide an acceptable level of inductive coupling, said winding of the tape being such as to provide a sufficient surface coverage to provide a low level of capactive coupling.

Inventors: MAKI, MELVIN C.
Applicants: SENSTAR CORPORATION
Application year: 1986
Main classification: H01Q13
IPC codes: H01Q13, H01P11