FR2599560A1 - Coaxial cable forming an induction antenna for moving appliance radio communication installation, laid for example along a road or in a tunnel - Google Patents

Abstract

This cable comprises a core 11, an inner conductor 12 wound helically around the core, an insulator 13 and an outer conductor 14 wound helically around the insulator. The two conductors 12, 14 have similarly directed helices and the winding pitch B of the outer conductor 14 is two to three times greater than the winding pitch A of the inner conductor 12.

Description

The present invention relates to an induction antenna cable for

  radiocommunications on mobile devices, for example on a road, a railway line, in a tunnel or the like.

  Figure 1 shows a known induction antenna cable for radiocommunications. This coaxial cable comprises a core 1 made of magnetic or insulating material, which is successively covered by an inner conductor 2, an insulator 3, an outer conductor 4 and a sheath 5. The inner 2 and outer 4 conductors are formed by helical winding of conductors on the core 1 respectively 10 on the insulator 3, the inner conductor 2 having a helical direction

  contrary to that of the external conductor 4.

  A current flowing between the inner and outer conductors produces ur. inductive radiation in the space outside

  of the antenna cable and this radiation can be picked up by an antenna 15 on a moving vehicle.

  However, the transmission loss with this known induction cable-antenna is very significant, which poses the problem that the radiocommunication installation requires a large number of

  repeaters when the antenna cable extends over a large distance, which makes the installation expensive.

  The invention therefore aims to provide an antenna cable

  induction with low transmission loss.

  According to the invention, an induction antenna cable for radiocommunications, comprising a core, an inner conductor wound in a helix around the core, an insulator surrounding the inner conductor and an outer conductor wound in a helix around the insulator, is characterized in that the two conductors have the same direction of helix and that the winding pitch of the external conductor is 2 to 3 times greater than the winding pitch of the internal conductor. Other features and advantages of the invention

  will emerge more clearly from the description which follows of a nonlimiting exemplary embodiment, as well as from the appended drawing,

  in which: FIG. 1 shows an induction antenna cable for radio35 communications of known type, already described; Figure 2 is a similar view of a cable according to an embodiment of the invention; and FIGS. 3 and 4 are graphs representing the inductance values as a function of the ratio of the turns of the interior and exterior conductors in an antenna cable according to the invention, FIG. 3 indicating the value of inductance of the exterior conductor

  and Figure 4 showing the total inductance value.

  The antenna cable according to the invention is suitable for long distance communications, its transmission loss is reduced and it allows a high electric field intensity to be obtained.

  Unlike the known antenna cable described in the above, the two conductors of the antenna cable of the invention have the same winding direction, with the result that a high frequency power source signal excites a ground return circuit, 15 so as to allow reception by an antenna of any

  which mobile radio device is in the vicinity.

  Following further research to increase the intensity of the electromagnetic field produced outside the induction antenna cable according to the invention, the Applicant has discovered that this result can be obtained by maintaining the ratio of the steps d winding of the inner and outer conductors in

a defined range.

  FIG. 2 represents an embodiment of a cable antenna according to the invention. The central core 11 of this cable is made of magnetic or insulating material, in particular of magnetic material having a magnetic permeability greater than unity. The inner conductor 12 is formed by helically winding a copper wire around the core II, with a winding pitch A. The inner conductor is covered by an insulator 13. The outer conductor 14 14 is formed by helical winding a copper wire around the insulation 13, with a winding pitch B. The outer conductor

  is covered by a plastic sheath 15.

  The direction of winding of the copper wire forming the inner conductor 12 is the same as that of the outer conductor 14 and the pitch B of the latter is not less than twice and not. more

  three times the winding pitch A of the inner conductor 12.

  As can be seen in Figure 2, it is not necessary that the inner 12 and outer 14 conductors are each formed by a single conductor, but can each be composed of a number of parallel strands or filaments. In this case, the steps A and B are the winding steps or distances between the successive turns of the same conductive strand and not the distances

  between the neighboring parallel strands.

  The reason why step B must be between two 10 and three times step A will be explained below.

  With the cable according to the invention, a circuit between the earth and the external conductor is excited by the inductance of the external conductor, causing a current of earth to flow through this circuit, so that a strong electromagnetic field is induced 15 in the outdoor space. The intensity of this electromagnetic field is proportional to ZT / o ZT represents the coupling impedance of the external conductor and 4 represents the phase shift constant

coaxial cable.

  In other words, to increase the intensity of the external electro-magnetic field, the coupling impedance ZT of the external conductor must be increased; therefore, the inductance L of the external conductor in the earth return circuit must be large because ZT = juL. It is also necessary to ensure that the phase shift constant 4 of the coaxial cable is not high, therefore to make the total inductance L0 of the coaxial cable low. The inductance L and the mutual inductance M12 of the interior and exterior conductors can be expressed as follows: Interior conductor: L = ira Ji N1 1 1 0lp 1 M12 1 sa P1 l2 Exterior conductor: L2 = N22Oo {a2Ji + (b2 - a2)}

2 2 2

  M12 = a PoN1N2 where A represents the radius of the nucleus; m1 the coefficient of permeability of the core 1; p0 the permeability constant of the free space; N1 the number of turns of the inner helix over a unit length; N2 the number of turns of the outer helix over the same unit length; and b the radius of the outer helix. The winding steps being inversely proportional to the number of turns, A / B = N2 / N1. Consequently, the conditions under which the inductance of the external conductor in the earth return circuit, L = M12 - L2, is made maximum and the total inductance LO = L1 - 2M12 + L2 is made minimum, can be calculated as illustrated in figures 3 and 4 (calculations for b = 2a). As shown in the graphs of Figures 3 and 4, the inductance L of the external conductor can be made almost maximum and the total inductance LO can be made as low as possible if the

  N2 / N1 turn ratio is limited to the range from 1/3 to 1/2.

  As has been described in the foregoing, in an induction cable for radiocommunications according to the invention, the interior and exterior conductors are wound in the same direction and their winding pitches are defined. It thus becomes possible to obtain a cable with a low loss of transmission and capable of creating a strong electromagnetic field, which was impracticable in the prior art. Therefore, the cable according to the invention is very advantageous when it is used in an induction radio installation requiring communications to

great distance.

R E V E N D I CATIONS

  1. Induction cable antenna for radiocommunications, comprising a core (11), an inner conductor (12) wound helically around the core, an insulator (13) surrounding the inner conductor and an outer conductor (14) wound helically around of the insulator, characterized in that the two conductors (12,14) have the same helical direction and that the winding pitch (B) of the external conductor (14) is two to three times greater than the pitch

  winding (A) of the inner conductor (12).

  2. Antenna cable according to claim 1, the core of which

  (11) is made of a magnetic material whose magnetic permeability is greater than unity.