GB2235336A - Leaky cable antenna - Google Patents

Abstract

A communication system comprises a leaky feeder cable 1 disposed within a housing 8 which provides a discontinuous RF shield. The housing is preferably made of stainless steel, and the discontinuities, which may be apertures 9 are preferably arranged to establish a regular pattern on signals received or transmitted. As an alternative the discontinuities can be provided by a series of separate tubes (Fig 2a). Such a tube may be made with the discontinuities of RF transparent material or may be a continuous RF transparent tube with areas of RF shielding, coating or sheathing. The tube may be notched (Fig 2b). Instead of a circular tube, square or other cross-section tube may be used. Instead of terminating in resistance 2 the ends of the cable 1 can be coupled (Fig 3) to enhance received signals. The leaky feeder may be used for voice, remote control and video communication. The leaky feeder may be used for mobile to mobile communication (Fig 4). A mesh network such as used in buildings is described (Fig 5). <IMAGE>

Description

COMMUNICATION VIA LEAKY CABLES This invention relates to communications systems and in particular but not exclusively to communication systems in which leaky cables are utilised to enhance communication with a mobile transmitter or receiver.

There are various environments such as large buildings or industrial works, docks, mines, airports, railways, tunnels and so forth where communication between a base station and mobile vehicles or personnel ('mobiles') or between two mobiles is necessary. Full radio communication using significant power broadcasting presents problems in some environments due to potential hazard and in others, in terms of channel availability and regulatory conditions. More serious difficulties are encountered when wideband signals such as video signals are to be transmitted; and are aggravated by changes in geometry. It is known to provide distributed low power transmission from or reception by a leaky feeder cable which extends from a base station. A leaky feeder cable of the known type comprises a coaxial cable which has deliberately imperfect screening.

One problem that arises in a variety of circumstances is a need to shield the feeder physically while permitting propagation in the manner of a leaky feeder and particularly to enable servicing of the feeder for very long periods of time.

The present invention provides a transmission system comprising a feeder cable disposed within a housing that provides discontinuous RF shielding in which the discontinuous RF shielding comprises a plurality of intervals in the screening arranged to define a pattern on transmitted or incident RF signals.

The invention is now described by way of example with reference to the accompanying drawings in which : Figure 1 is a schematic drawing of an installation according to an embodiment of the invention; Figure 2 shows alternative embodiments of tube housings; Figure 3 is an embodiment for enhancing received signals; Figure 4 illustrates mobile to mobile communication; Figure 5 illustrates an embodiment utilising a network of leaky cables, and Figure 6 illustrates a typical single station arrangement configured for a mobile vehicle controlled from a base station.

In buildings or sites communication between a base and a vehicle may be required not only for voice but also for remote control and in many instances communication in both directions is required although not necessarily always of the same type of signals. For example in robotic systems such as in warehouses directional instructions are transmitted to the robot and video and/or other positioning signals may be transmitted back from the robot to the remote operator. Frequently the remote operator will be located at a base station, but in some instances may also be mobile.

Thus two way communcation is desirable both between stations and mobiles and between mobiles.

Referring now to Figure 1, a schematic embodiment of the invention shows a leaky feeder cable 1 that is connected to a terminating resistance 2. Signals are launched into the feeder cable 1 from a transmitter 3, and signals in the cable are picked up by a receiver 4. The transmitter 3 and receiver 4 are connected to the cable and isolated from each other by a directional coupler, which may be a low-loss frequency sensitive coupler 5. Typically the transmitted data comprises low bandwidth control data at 25KHz on a 30MHz carrier. With a leaky feeder cable externally generated RF radiation will penetrate the imperfect screening at the cable and then propagate in the cable in both directions, and this can be utilised to receive signals from an external mobile, such as mobile 6.

However, a typical rate of propagation in cable is 0.8 times the free space velocity, thus a signal such as a video picture at normal TV rates on a substantially plane wavefront 7 that impinges on a leaky cable 1 over a distance of say 6 metres will suffer multi-path distortion, in the example of a video picture blurring it over 100ns and reducing the line resolution by a factor of 2.

In the present invention it is proposed to provide an additional component comprising a member such as a metal tube 8 having spaced apart apertures 9. Where the tube is solid it acts as an RF screen preventing both egress and entry of radiation, but at the apertures radiation leaked from cable 1 is emitted as if from a series of point sources, and likewise impinging external radiation enters the cable only in the vicinity of the apertures.

The effect of the radiation being transmitted from a series of apertures rather than along a continuous length is of negligible effect in that the apertures, over the route of the leaky feeder cable, effectively constitute a near randomly distributed pattern of secondary sources. Very close to the tube (i.e. within a few centimetres) there may be voids in the transmitted signals, but this can be overcome by providing spaced apart multiple antennae for reception at the mobile.

With respect to the externally generated radiation the effect of the apertured tube is of more significance. In this instance the image blurring is quantised into periodic time-delayed elements, instead of a continuum of blurring, and this periodic blurring can be removed by adaptive filter techniques. The particular spacing and arrangement of the apertures can be selected to suit the filtering arrangement.

The use of an apertured tube in this way is significantly less expensive and less complex than the alternative of installation of directional transmission antennae on to the mobile stations. Additionally, the metal tubing has the practical advantage of providing a rugged housing for the feeder cable.

The apertured tube illustrated in Figure 1 could be replaced by alternative structures, the basic concept being to provide an RF shield with interruptions or discontinuities at particular intervals. Figure 2 shows some alternative exemplary structures: (a) a series of discrete tubes, for example 6 metre lengths separated by a 0.02 metre interval.

Such a tube may be fabricated with the discontinuities of RF transparent material so as to maintain continuous ruggedisation, or be a continuous RF transparent tube with areas of an RF shielding, coating or sheathing. Figure 4(b) illustrates a notched tube, with 0.2m width notches at intervals of 2 to 12 metres along the tube. Square or other cross-sections of tubes may also be used.

An important advantage of the present invention is that the tube, which may be, for example, of stainless steel, provides physical protection for the cable and yet does not significantly impair propagation of signals to or from the cable. Moreover, the invention allows for long-term servicing of the cable. In particular the tube may extend around a region in which are deployed mobile transmitters and/or receivers, such as may be incorporated in remotely operated vehicles. The cable may be withdrawn from the sheath or tube for inspection or replacement. A new cable may be inserted into the sheath or tube without any need for human entry into the region around or in which the sheath or tube extends.

When an externally generated signal enters the cable 1 it divides and propagates in opposite directions. With the arrangement shown in Figure 1, the far end of the leaky feeder is terminated in a load to minimise distortion from reflected signals returning back along the cable. An alternative embodiment is shown in Figure 3, in which the end of the leaky feeder is connected to a low loss coaxial cable (which could of course be a continuation of the same cable but with full rather than leaky sheathing) and the low loss coaxial cable is then connected back to the base station, so that the signals in the low loss cable are combined with the signals that propagated along the leaky cable. In this way the signal strength can be improved at the receiver which, instead of receiving from only half of the input energy transmitted along a leaky cable, also receives signals that propagated in the opposite direction.

Such an arrangement may further minimise the risk of voids as well as signal loss in the effective field pattern.

Mobile station to mobile station communication may be achieved by the technique illustrated in Figure 4.

In Figure 4 a first mobile station 10 transmits a signal f, that is picked up by the leaky cable 1 in apertured tube 8.

This signal is passed by the leaky cable to the receiver 4 where it is amplified, passed to transmitter 5 and retransmitted as signal f2 along the leaky cable. It can then be picked up by any other appropriately tuned mobile such as mobile 11.

In Figure 5 a mesh network is illustrated. Such an arrangement, incorporating the discontinuously shielded leaky cable, may be utilised in buildings or large vehicles or vessels. The leaky cables define a network of local cells coupled together and communicating with transmitters and receivers. Such an arrangement may be utilised to provide mobile to mobile communication without the attendant propagation delay and attenuation associated with remote base stations.

Figure 6 schematically illustrates a one station arrangement configured for a mobile vehicle controlled from a base station. One or more video channels may be transmitted at UHF or higher frequencies from mobile to the cable.

Telemetry data from the vehicle may be combined with video signals or, more preferably, separately transmitted.

Command data is sent from.

A modification of the invention would be to establish discrete discontinuities (or zones of imperfect screening) in the screening of the cable with 'perfect' screening between the discontinuities so that the leak pattern from the cable itself adopted the pattern described in relation to the tube.

Claims (8)

1. A transmission system comprising a feeder cable disposed within a housing that provides discontinuous RF shielding in which the discontinuous RF shielding comprises a plurality of intervals in the screening arranged to define a pattern on transmitted or incident RF signals.

2. A transmission system according to claim 2 in which the intervals comprise a regular pattern of interruptions in the shielding.

3. A transmission system according to claim 1 or claim 2 in which the feeder cable comprises a leaky feeder cable and is disposed within a tube having a plurality of regularly spaced intervals in its screening.

4. A transmission system according to claim 3 in which the tube comprises a series of spaced apart tubes.

5. A transmission system according to claim 3 in which the tube has a series of apertures.

6. A transmission system according to any preceding claim in which the housing is metal.

7. A transmission system according to any preceding claim in which the housing comprises a screening layer disposed over a supporting member.

8. A transmission system according to any preceding claim in which the far end of the feeder cable is connected via a low loss cable back to a receiver.