FR2690280A1 - Coaxial cable antenna for tunnel telecommunications or confined spaces - has coaxial cable with outer slots and second single wire, both enclosed separately by outer figure-eight shaped sleeve - Google Patents

Abstract

The coaxial cable comprises inner (21) and outer (23) conductors, the latter having regular spaced slots in it (24). The two are separated by a dielectric (22). A second single wire (3) runs parallel to the coaxial line and spaced from it. The wire is made of steel with outer copper plating. The cable and wire are encased in an outer sheath (4) in the form of an eight, holding the wire separate from the cable. RF is applied direct to the cable and via a transformer to the single wire and outer casing of the cable. ADVANTAGE - Wide pass band produced in small space with less complexity, and cost effectively.

Description

RADIATION DEVICE WITH BROADBAND BANDWIDTH
The present invention relates to a radiating device with wide bandwidth.

 A radiating device is a device comprising at least two radiating lines. One of these radiating lines is made up of either a radiating coaxial cable or a radiating waveguide, and the second of these lines is made up of either a monofilament line or a two-wire line.

 In the following, the case of radiating devices comprising a radiating coaxial cable will be treated, but the comments made also relate to devices comprising a radiating waveguide.

 Coaxial radiating cables are generally intended to provide telecommunications in confined environments (galleries or tunnels for example). They generally consist of a central conductor surrounded by an envelope made of a dielectric material, itself surrounded by a tubular external conductor provided with openings distributed over its length. The assembly can be sheathed with an insulating material. Such cables operate in a frequency band, called bandwidth, determined by a number of parameters.

 It is also well known to suspend a radiating cable from a wall by means of a carrier arranged parallel to the external conductor of the cable and integral with the latter by means of a sheath made of an insulating material surrounding both the conductor outside and the carrier, so that this sheath has a well-known eight-shaped section.

 The upper limit of the bandwidth is a function of the acceptable linear losses along the cable and the arrangement of the openings made in the external conductor. It can be fixed for example by the level of line losses specified by the user of the cable for example. In practice, the upper limit of the bandwidth is of the order of a few gigahertz.

 The lower limit of the bandwidth depends mainly on the desired level of radiated field. At frequencies below this limit, the field radiated by the cable is considered too weak to be usable.

 Indeed, at low frequencies, the dimensions of the openings of the external conductor are small compared to the wavelength. The gain of the elementary antenna equivalent to an opening being proportional to the ratio between the surface of the opening considered and the wavelength, it is therefore very low at low frequencies. In practice, the lower limit of the bandwidth is to twist a few megahertz.

 To reduce this lower limit, it is known to use a radiating device comprising on the one hand the radiating coaxial cable, and on the other hand a two-wire line installed in the same medium as the radiating cable, and taking over from the transmission. at low frequencies, in practice up to around 100 to 150 kHz. The two-wire line consists of two conductors isolated from each other and forming a closed circuit between a load and a transmitter (or a receiver). The two conductors are relatively close to each other, so that the two-wire line is not very sensitive to disturbances from the surrounding environment.

 However, such a solution is not satisfactory, since it requires the installation of three elements: the radiating cable and the two conductors of the two-wire line. This is complex and costly, especially in a confined environment where the space required for the devices used to carry out the installation is often reduced.

 The object of the present invention is therefore to produce a radiating device with a large passband, the implementation of which in a confined environment is less complex than that of conventionally used devices.

 The present invention provides for this purpose a radiating device comprising - a tubular conductor elongated along a longitudinal axis and provided with openings, a carrier consisting of a metallic element arranged parallel to said tubular conductor and integral with said tubular conductor by intermediate a sheath made of an insulating material arranged around said carrier and said tubular conductor so as to electrically isolate said carrier from said tubular conductor, - a two-wire line consisting of two insulated conductors arranged parallel to each other, characterized in that that said conductors of said two-wire line consist respectively of said metal carrier and said tubular conductor.

 A cylindrical element of arbitrary section constant over its entire length is called tubular.

 By using the carrier of a radiating cable or waveguide as an element of the two-wire line, one avoids having to use an independent two-wire line whose installation is expensive and complex to carry out. In addition, such a device can be easily produced from existing products or technologies, cables or radiating waveguides with carriers having already been manufactured for a long time.

 According to a characteristic of the preceding device, a system for transmitting or receiving energy is electrically connected to one of the ends of the two-wire line between the carrier and the tubular conductor. It is possibly possible to electrically connect a transformer between this system and the two-wire line. This is particularly interesting in the case where the system used is asymmetrical, the transformer then being an asymmetrical-symmetrical transformer. One can also use in this case instead of a transformer any equivalent circuit making it possible to fulfill the same function, in particular a mounting of transistors.

 Furthermore, when a transformer is used, it is advantageous to choose the respective dimensions of the sections of the carrier and the tubular conductor and the distance between these two elements so that the characteristic impedance of the two-wire line formed is in accordance with transformer input or output impedance standards available on the market.

 Of course, it is preferable to connect to the end of the two-wire line opposite to that connected to the transmission or reception system, between the tubular conductor and the carrier, a load adapted to this system.

 The metal carrier will preferably be made of a metal having good mechanical properties, in particular of tensile strength, and electrical, in particular of low ohmic resistance, such as copper-plated steel or an alloy of aluminum and magnesium such as " 'ALMELEC "(registered trademark). In addition, this carrier may consist of a strand of metal son or a metal monofilament.

 According to a possible embodiment, the device according to the invention ~ may further comprise a conductor said inner conductor disposed internally and coaxially with the tubular conductor, and a casing made of a dielectric material interposed between the tubular conductor and the inner conductor, to constitute a radiating coaxial cable.

 In this case, a second energy transmission or reception system is electrically connected to one of the ends of the radiating coaxial cable between the inner conductor and the tubular conductor to allow the operation of the radiating cable. It is then preferable to connect electrically to the end of the radiating coaxial cable opposite to that connected to the transmission or reception system, between the internal conductor and the tubular conductor, a load adapted to this system.

 According to another possible embodiment, the tubular conductor has a circular, elliptical or rectangular section, and constitutes a radiating waveguide.

 Note that, in the field of underground radiating cables, US Pat. No. 4,605,914 describes a radiating device comprising a radiating coaxial cable and a two-wire line constituted by the tubular external conductor of the radiating cable and by an external conductor parallel to the radiating cable. and made integral with the latter by means of an insulating outer sheath molded both around the cable and around the external conductor. Such a device is intended to be buried, and can in no case be suspended in a tunnel or in a gallery for example.

 Other characteristics and advantages of the present invention will appear in the following description of a radiating device according to the invention, this description being given by way of illustration and in no way limitative.

 In the following figures - Figure 1 is a perspective view of a radiating device according to the invention, - Figure 2 is an elevational view of the device of Figure 1, with the electrical diagram corresponding to its operation as a transmitter, - Figure 3 is a perspective view of another radiating device according to the invention, with the electrical diagram corresponding to its operation as a transmitter.

 In these figures, the common elements have the same reference numbers.

 In the following, we will take the example of a radiating emitting device, but it is obvious that the device according to the invention can also be used as a receiver.

 FIG. 1 shows a radiating device 1 with radiating cable 2 according to the invention. The radiating cable 2 more precisely comprises an inner conductor 21 surrounded by an envelope 22 made of a dielectric material and then an outer tubular conductor 23. The conductor 23 is provided with openings 24 arranged over its entire length.

Parallel to the cable 2 is a carrier 3 consisting of a strand of metallic wires made of copper-plated steel for example.

The carrier 3 is made integral with the cable 2 by means of an insulating sheath 4 surrounding both the carrier 3 and the cable 2 and insulating these two elements from each other, the sheath 4 thus having a shaped section of eight. The external conductor 23 and the carrier 3 constitute a two-wire line which takes over from the radiating cable 2 for transmission at low frequencies.

 FIG. 2 shows a possible electrical installation diagram for the device of FIG. 1.

In this diagram, a first transmitter El is connected between the conductor 23 and the carrier 3 to excite the two-wire line formed by these two elements. The transmitter El is also connected to ground, so that it constitutes an asymmetrical transmitter. So, we have arranged between the transmitter
El and the line which it feeds an asymmetric asymmetric transformer T.

 On the other hand, the radiating cable 2 is excited by a second emitter E2 connected between the interior conductor 21 and the tubular exterior conductor 23.

 In order to avoid mismatches, a load Z1 adapted to the secondary output impedance of the transformer T and to that of the two-wire line constituted by the carrier 3 and the conductor 23 is connected to the end of the two-wire line opposite to that connected to the transformer T, and similarly, a load Z2 adapted to the output impedance of E2 and of the radiating coaxial cable 2 is connected to the end of the radiating coaxial cable 2 opposite to that connected to E2.

 Furthermore, in order to be able to use a standard transformer available on the market, that is to say compatible both with the conventional output impedances of the transmitters used, which are 50D, and with the characteristic impedances of two-wire lines, which are generally between 300 and 600n, the respective diameters dl and d2 of the conductor 23 and of the carrier 3 will be chosen, as well as the distance D between these two elements so that the characteristic impedance of the two-wire line of the device 1 is actually between 300 and 600Q.

An example of embodiment fulfilling the constraints mentioned above is given below.

 In the device 1 of FIG. 1, the diameter d1 is 24.6 mm and the diameter d2 is 10 mm. The characteristic impedance of the two-wire line is fixed at 300Q.

In this case, if it is assumed that the quantity of dielectric material constituting the sheath 4 is low and therefore does not generate a significant increase in the permittivity of the medium, that is to say of the surrounding air, the distance D is equal to 70 mm.

 Tests carried out using the device 1 of FIG. 1 at low frequencies, that is to say at the frequencies where the two-wire line formed by the external conductor 23 and the carrier 3 takes over from the transmission, show that the performance of the device 1 at these frequencies is of the same order of magnitude as that obtained using an independent two-wire line according to the prior art.

 On the other hand, there is shown in Figure 3 an electrical diagram of a device 30 according to the invention in which a radiating waveguide is used instead of a radiating cable. The device 30 comprises a metal waveguide 31 of rectangular section, provided with slots 32 arranged over its entire length. As in FIG. 1, a metal carrier 33 is made integral with the waveguide 31 by means of an insulating sheath 34 surrounding both the carrier 33 and the waveguide 31. Thus, the waveguide 31 and the carrier 33 constitutes a two-wire line which takes over from the radiating waveguide 31 for transmission at low frequencies.

A transmitter E'1 is connected via a transformer T 'between the waveguide 31 and the carrier 33. As in the case of the device 1 of FIGS. 1 and 2, an adaptation load Z '1 is connected to the end of the two-wire line opposite to that connected to the transmitter
E'1. Furthermore, the adaptation of the waveguide 31 is carried out by a fictitious load Z'2 formed by the end of this waveguide opposite to that connected to the transmitter E'1.

 The main mode of operation of the rectangular section waveguide 31 is mode TE10.

 The device 3 has the same advantages as the device 1.

 Obviously, the invention is not limited to the embodiment which has just been described.

 In particular, the insulating sheath does not necessarily have an eight-shaped section over the entire length of the carrier; such a section can be repeated periodically for example, and interspersed with empty spaces.

 On the other hand, the metallic material constituting the carrier can be any. However, it is preferable that it has both good mechanical properties, and in particular good tensile strength, since the device is suspended through it, and good electrical properties, and in particular low ohmic losses, in order to provide the best performance in terms of radiated power. Copper-plated steel and "ALMELEC", in particular, have these properties.

 The carrier may consist of a strand of metal son or a metal monofilament.

 As already indicated, the device according to the invention can also be used as a receiver. In this case, the transmitters will be replaced by receivers.

 When the device according to the invention uses a waveguide, the latter can have a rectangular, circular or elliptical section. When the waveguide section is circular, the operating mode is TE11 mode, and when it is elliptical, the operating mode is TEC11 mode. Whatever the section of the waveguide used in a device according to the invention, the electrical diagram can be that shown in FIG. 3.

 Finally, any means can be replaced by equivalent means without departing from the scope of the invention.

Claims (13)

1 / Radiating device comprising - a tubular conductor (23) elongated along a longitudinal axis and provided with openings (24), a carrier consisting of a metal element (3) arranged parallel to said tubular conductor (23) and integral said tubular conductor (23) by means of a sheath (4) made of an insulating material arranged around said carrier (3) and said tubular conductor (23) so as to electrically isolate said carrier (3) from said tubular conductor (23 ), - a two-wire line consisting of two insulated conductors arranged parallel to each other, characterized in that said conductors of said two-wire line consist respectively of said metal carrier (3) and said tubular conductor (23). 2 / Device according to claim 1 characterized in that a system for transmitting (El) or receiving energy is electrically connected to one of the ends of said two-wire line between said carrier (3) and said tubular conductor (23). 3 / Device according to claim 2 characterized in that a transformer (T) is electrically connected between said system and said two-wire line. 4 / Device according to claim 3 characterized in that, when said system is asymmetrical, said transformer is an asymmetrical-symmetrical transformer. 5 / Device according to one of claims 3 or 4 characterized in that the respective dimensions of the sections of said carrier (3) and said tubular conductor (23) and the distance between the latter are chosen so that the characteristic impedance of said two-wire line corresponds to the output impedance of said transformer (T). 6 / Device according to one of claims 2 to 5 characterized in that a load (Z1) adapted to said system is connected to the end of said two-wire line opposite to that connected to said system. 7 / Device according to one of claims 1 to 6 characterized in that said carrier (3) is made of a material chosen from copper-plated steel and "ALMELEC". 8 / Device according to one of claims 1 to 7 characterized in that said carrier (3) consists of a strand of metal son. 9 / Device according to one of claims 1 to 7 characterized in that said carrier (3) consists of a single metal wire. 10 / Device according to one of claims 1 to 9 characterized in that it comprises a conductor said inner conductor (21) disposed internally and coaxially with said tubular conductor (23), and a casing (22) made of a dielectric material interposed between said tubular conductor (23) and said inner conductor (21), so as to constitute a radiating coaxial cable (2). 11 / Device according to claim 10 characterized in that a second system for transmitting (E2) or receiving energy is electrically connected to one of the ends of said radiating coaxial cable (2) between said inner conductor (21 ) and said tubular conductor (23). 12 / Device according to claim 11 characterized in that a suitable load (Z2) to said second system is electrically connected to the end of said cable (2) opposite to that connected to said system. 13 / Device according to one of claims 1 to 9 characterized in that said tubular conductor (31) has an elliptical, circular or rectangular section, and in that it constitutes a radiating waveguide.