GB2094065A

GB2094065A - Improvements relating to radio frequency radiating communication cables

Info

Publication number: GB2094065A
Application number: GB8106242A
Authority: GB
Original assignee: Coal Industry Patents Ltd
Current assignee: Coal Industry Patents Ltd
Priority date: 1981-02-27
Filing date: 1981-02-27
Publication date: 1982-09-08

Abstract

A cable used in a radiating transmission system operating at radio frequencies to communicate between stations situated along the cable is made up of an inner conductor (1) surrounded by a dielectric (2) and a plurality of circumferentially spaced conductor elements (3) arranged to lay parallel to the conductor (1). An insulating sheath (4) envelopes the elements (3). <IMAGE>

Description

SPECIFICATION Improvements relating to radio frequency radiating communication cables This invention relates to radio frequency radiating communication cables and is particlarly concerned with cables forming part of systems operating over a long radiating or leaky transmission line.

Radiating or leaky transmission line systems basically comprise a base transmitter which has a radio frequency transmission line extending from it along which radio frequency signals are sent. The transmission line is deliberately constructed as an imperfect one such as a co-axial cable with a perforate outer conductor, so that signals in the inner conductor can set up electromagnetic fields outwardly of the line and can be picked up by mobile receivers in the vicinity of the line.

Many forms of transmission line have been proposed. A particularly popular form is that of a coaxial line having a braided outer conductor in which the braid is loosely woven so as to give poor screening effect.

Atypical coaxial cable comprises a central inner conductor of either solid or stranded conductive material such as copper surrounded by dielectric material around which is woven the braided outer conductor. An outer plastic sheath is provided for protecting the coaxial line. The braided outer conductor comprises a number of wires wound together in spindles which are woven in an interlaced manner around the dielectric material, the spindles being arranged around the dielectric at a preselected angle of lay with respect to the longitudinal axis of the coaxial line. Typical, the angle of lay is approximate lay 30".

Recent experimental work has established that best results are obtained when the angle of lay is small. However, constructional problems have been encountered in producing a woven braid comprising spindles arranged around the dielectric at small angles of lay.

An object of the present invention is to provide a radiating or leaky transmission line which tends to overcome or reduce the above mentioned constructional problems.

According to the present invention, a radio frequency radiating communication cable comprises an inner conductor surrounded by a sleeve of dielectric material, an outer conductor comprising a plurality of conductor elements arranged substantially parallel to the inner conductor and spaced circumferentially from each other, an outer insulating sheath retaining the conductor elements in position substantially parallel to the inner conductor.

Preferably, each outer conductor element comprises a single strand although a plurality of strands can be used.

The elements may be equally spaced around the dielectric, but if a concentration of a leakage field is required in one area a larger space may be created.

By way of example only, one embodiment of the present invention will be described with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a partly exposed end portion of a radiating or leaky transmission line constructed in accordance with the present invention; and Figure 2 is a cross sectional view of the line of Figure 1 The drawings to which reference is now made show the radiating or leaky transmission line to comprise a single strand central inner conductor 1 surrounded by a sleeve 2 of dielectric material, and on an outer conductor comprising a plurality of spaced, single strand conductor elements 3 arranged substantially parallel to the inner conduc tor The outer conductor elements are retained in position by an outer sheath 4 of insulating material which in Figure 1 is partly cut away to expose the outer conductor elements 3. The sheath 4 is provided with recesses for accomodating and positioning the outer conductor elements. In other embodiments the sleeve 2 has recesses for at least partly accommodating the outer conductor elements which then are retained in position by the sheath 4.

Typically, the line comprises between 2 and 100 outer conductor elements.

In different embodiments of the invention each outer conductor element comprises a plurality of strands. The conductor 1 and the elements 3 are made of copper in this example although aluminium on a suitable electrically conductive alloy could be used.

If the cable is to be placed alongside a wall of a tunnel or along a linear route and is to be thus used in a situation where the radiation from the cable is only required in one direction, i.e. into the tunnel or into the path of the linear route the spacing of the conductor elements 3 may be altered so as to be greater on the side of the cable adjacent the tunnel or linear path and to be closer elsewhere. This effectively gives a "ballooning" effect of signal propagation into the tunnel or linear path but a shielding of the signal in the area of the tunnel wall or in the area where no signal propagation is required. This enables a more efficient use of the radiated signal to be made.

From the above description it will be seen that the present invention provides a radiating or leaky transmission line which enables the outer conductor elements to be arranged substantially parallel to the axis of the line, i.e. the angle of lay substantially is zero, which tends to overcome or reduce constructional problems, and which helps to improve the radio frequency properties of the cable.

1. A radio frequency radiating communication cable comprising an inner conductor surrounded by a sleeve of dielectric material, an outer conductor comprising a plurality of conductor elements arranged substantially parallel to the inner conductor and spaced circumferentially from each other, and an outer sheath of insulating material retaining the conductor elements in position substantially parallel to the inner conductor.

2. A cable as claimed in Claim 1 in which each

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvements relating to radio frequency radiating communication cables This invention relates to radio frequency radiating communication cables and is particlarly concerned with cables forming part of systems operating over a long radiating or leaky transmission line. Radiating or leaky transmission line systems basically comprise a base transmitter which has a radio frequency transmission line extending from it along which radio frequency signals are sent. The transmission line is deliberately constructed as an imperfect one such as a co-axial cable with a perforate outer conductor, so that signals in the inner conductor can set up electromagnetic fields outwardly of the line and can be picked up by mobile receivers in the vicinity of the line. Many forms of transmission line have been proposed. A particularly popular form is that of a coaxial line having a braided outer conductor in which the braid is loosely woven so as to give poor screening effect. Atypical coaxial cable comprises a central inner conductor of either solid or stranded conductive material such as copper surrounded by dielectric material around which is woven the braided outer conductor. An outer plastic sheath is provided for protecting the coaxial line. The braided outer conductor comprises a number of wires wound together in spindles which are woven in an interlaced manner around the dielectric material, the spindles being arranged around the dielectric at a preselected angle of lay with respect to the longitudinal axis of the coaxial line. Typical, the angle of lay is approximate lay 30". Recent experimental work has established that best results are obtained when the angle of lay is small. However, constructional problems have been encountered in producing a woven braid comprising spindles arranged around the dielectric at small angles of lay. An object of the present invention is to provide a radiating or leaky transmission line which tends to overcome or reduce the above mentioned constructional problems. According to the present invention, a radio frequency radiating communication cable comprises an inner conductor surrounded by a sleeve of dielectric material, an outer conductor comprising a plurality of conductor elements arranged substantially parallel to the inner conductor and spaced circumferentially from each other, an outer insulating sheath retaining the conductor elements in position substantially parallel to the inner conductor. Preferably, each outer conductor element comprises a single strand although a plurality of strands can be used. The elements may be equally spaced around the dielectric, but if a concentration of a leakage field is required in one area a larger space may be created. By way of example only, one embodiment of the present invention will be described with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a partly exposed end portion of a radiating or leaky transmission line constructed in accordance with the present invention; and Figure 2 is a cross sectional view of the line of Figure 1 The drawings to which reference is now made show the radiating or leaky transmission line to comprise a single strand central inner conductor 1 surrounded by a sleeve 2 of dielectric material, and on an outer conductor comprising a plurality of spaced, single strand conductor elements 3 arranged substantially parallel to the inner conduc tor The outer conductor elements are retained in position by an outer sheath 4 of insulating material which in Figure 1 is partly cut away to expose the outer conductor elements 3. The sheath 4 is provided with recesses for accomodating and positioning the outer conductor elements. In other embodiments the sleeve 2 has recesses for at least partly accommodating the outer conductor elements which then are retained in position by the sheath 4. Typically, the line comprises between 2 and 100 outer conductor elements. In different embodiments of the invention each outer conductor element comprises a plurality of strands. The conductor 1 and the elements 3 are made of copper in this example although aluminium on a suitable electrically conductive alloy could be used. If the cable is to be placed alongside a wall of a tunnel or along a linear route and is to be thus used in a situation where the radiation from the cable is only required in one direction, i.e. into the tunnel or into the path of the linear route the spacing of the conductor elements 3 may be altered so as to be greater on the side of the cable adjacent the tunnel or linear path and to be closer elsewhere. This effectively gives a "ballooning" effect of signal propagation into the tunnel or linear path but a shielding of the signal in the area of the tunnel wall or in the area where no signal propagation is required. This enables a more efficient use of the radiated signal to be made. From the above description it will be seen that the present invention provides a radiating or leaky transmission line which enables the outer conductor elements to be arranged substantially parallel to the axis of the line, i.e. the angle of lay substantially is zero, which tends to overcome or reduce constructional problems, and which helps to improve the radio frequency properties of the cable. CLAIMS

2. A cable as claimed in Claim 1 in which each outer conductor element comprises a single strand.

3. A cable as claimed in Claim 1 in which each outer conductor element comprises a plurality of strands.

4. A cable as claimed in Claims 1,2, or 3 in which the elements are equally spaced around the dielectric.

5. A cable as claimed in Claims 1,2 or 3 in which the elements are unequally spaced on at least one area of the circumference of the dielectric.

6. A cable as claimed in any preceding claim in which recesses are provided in the sheath to receive and position the elements.

7. A radio frequency radiating communication cable substantially as hereinbefore described and with reference to the accompanying drawings.