Abstract:
The radiating cable comprises at least one cable segment ( 1 ) comprising a pair of insulated conductor wires ( 2 ) having first ends ( 3 ) connected to a load ( 4 ) equal to an impedance characteristic of the insulated conductor wires, and second ends ( 5 ) connected to a connector ( 6 ).

Description:
The present invention relates to a radiating cable for use in particular in the field of cellular telephony or in local area networks for transmitting data by wireless at up to about 2.4 gigahertz (GHz). 
     BACKGROUND OF THE INVENTION 
     The provision of radio coverage in large buildings often requires dedicated equipment to be installed. This coverage is obtained by means of antennas placed inside such buildings. 
     Technically, it would be advantageous to use radiating cables based in passages, however that gives rise to costs that are often unacceptable. Present-day radiating cables are coaxial cables with slots in periodic patterns and they are expensive, bulky, rigid, and difficult to lay. 
     Furthermore, when cabling buildings, the high levels of performance provided by present-day radiating cables are unnecessary. The object of the invention is to propose a radiating cable of low cost that is easy to lay, while presenting performance that is sufficient to ensure satisfactory transmission of signals within a building or a vehicle. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     The present invention provides a radiating cable comprising a pair of insulated conductor wires, at least one cable segment having first ends connected to a load equal to a characteristic impedance of the cable segment, and second ends connected to a connector. This provides a cable of very great flexibility and compactness which can easily be fixed in the passages of a building by means of the usual techniques for fixing an ordinary telephone cable and which also presents impedance that is independent of length. 
     In an advantageous version of the invention, the cable has at least two cable segments whose second ends are connected in parallel to the connector. Given the equivalent impedance obtained by connecting the cable segments in parallel, this makes it possible to provide a cable that presents impedance matched to the transceiver to which the radiating cable is connected while making the radiating cable out of cable segments each presenting an impedance that is higher, i.e. generally having better transmission performance than a single cable matching the nominal impedance of the transceiver. 
     In yet another advantageous aspect of the invention, the two cable segments are identical. This minimizes constraints on storage, and the cable can be installed without any need to identify the cable segments. 
     Other characteristics and advantages of the invention will appear on reading the following description of a particular non-limiting embodiment of the radiating cable of the invention, given with reference to the accompanying figures, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a radiating cable of the invention comprising two cable segments connected in parallel; and 
     FIG. 2 is a perspective view of a portion of a cable of the invention. 
    
    
     With reference to the figures, the radiating cable constituting the particular embodiment shown comprises two cable segments given overall references  1 , each segment comprising a twisted pair of insulated conductor wires  2  having first ends  3  connected to a load  4  and second ends  5  connected in a parallel configuration to a connector  6 . 
     DETAILED DESCRIPTION OF THE INVENTION 
     In this preferred embodiment, both cable segments  1  are identical and each is made from a pair of solid copper conductors having a diameter of 1.38 millimeters (mm) and covered in insulation having a thickness of 2.2 mm of cellular polystyrene expanded by 41% and covered in a polyethylene skin having a thickness of 0.08 mm. The capacitance of the wire made in this way is 210 picofarads per meter (pF/m) and the insulation has a dielectric constant of 1.463. A cable segment comprising a twisted pair of insulated conductors as described above then has a characteristic impedance of 100 ohms (Ω) so that when the wires are connected to 100 Ω load, the impedance of the cable segments is maintained at 100 Ω, regardless of its length. Two cable segments connected in parallel then have an equivalent impedance of 50 Ω corresponding to the nominal impedance normally required at the input/output (I/O) of a transceiver. The resulting cable is well-balanced, both for transmission and for reception, and when account is taken of its linear attenuation, each cable segment can be up to about 100 meters (m) long for transmission at 450 megahertz (MHz), about 75 m long for 900 MHz, about 45 m long for 1800 MHz, and about 35 m long for 2.4 GHz. 
     As shown in FIG. 2, the insulated conductors are held together by a dielectric tape  7  made of polyester, polypropylene, or more simply of paper, but preferably made of a material that enables the cable to withstand fire, such as a mineral tape of mica or of glass silk. In this embodiment, the dielectric tape  7  is covered in a series of helically-wound metal tapes  8 , having edges that are spaced apart by gaps that are preferably about one or two times the width of the metal tapes so that at high frequency the metal tape contributes to maintaining the characteristic impedance of the radiating cable at a value that is constant, while allowing radiated energy to escape through the gaps between the metal tapes  8 . It is also possible to replace the metal tapes  8  by a plurality of metal wires wound around each of the insulated conductor wires. 
     The cable segment preferably also includes a thin outer sheet  9  of thermoplastic material or of elastomer. 
     Naturally, the invention is not limited to the particular embodiment described and can be modified without going beyond the ambit of the invention as defined by the claims. 
     In particular, although the cable of the invention is described in an embodiment comprising identical cable segments connected in parallel, it is possible to provide cable segments that differ either in length or in impedance. Depending on the structure of the zone to be covered, it can be advantageous to use cable segments presenting differing performance, the attenuation in each cable segment being related to the average impedance thereof. For cable segments of different lengths, the cable having the higher impedance preferably covers the longer zone while the cable having the lower impedance covers the shorter zone. 
     If the geometrical configuration of the premises to be covered is complex, it is also possible to envisage connecting more than two cable segments in parallel, with the characteristic impedance of each cable segment being selected so that the equivalent impedance of the radiating cable corresponds to the nominal impedance of the transceiver used. 
     In order to increase radiation from the cable, it is possible to provide unbalance between the various elements of the cable, either by using different dimensions or different capacitances per unit length between the various conductor wires by varying the thickness or the nature of the insulating material, or by varying the pitch at which the insulating conductor wires are twisted together, with it being possible for pitch variation to go all the way to reversing the twist direction and/or to keeping the insulated conductor wires parallel to each other over a portion of the cable, with the helical pitch in twisted portions preferably being about 15 to 30 times the diameter of the insulated conductors and with the length of each portion at constant twist being about ten times the helical pitch in question or ten times the adjacent helical pitch for a portion in which the wires are parallel. 
     When the zone to be covered is very small, e.g. in a building of small dimensions or in a vehicle, it is possible to privilege radiation at the expense of linear attenuation and to provide a cable comprising a parallel pair of wires connected to the load. 
     The flexibility of the cable can be improved by replacing solid conductors with wires made up of multiple twisted strands. 
     It is also possible to make the cable of the invention without using metal tapes and/or a dielectric tape.