Abstract:
An omnidirectional resonant antenna in a half-plane or in the whole plane comprises a single radiating electric conductor ( 26 ) having at least three abutted wires ( 28, 30, 32 ), the length of each wire and the orientation of the wires relative to one another determining the global orientation of the electric conductor. The wires are oriented along at least three different spatial directions and the lengths of the wires are designed to obtain an omnidirectional global radiation of the electric conductor in a half-plane or in the whole plane.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to omnidirectional resonant antennas and more particularly to omnidirectional resonant antennas operating in a half-space or all of space. 
   DESCRIPTION OF THE RELATED ART 
   It is known in the prior art to produce resonant antennas, that is to say antennas of which the dimensions have been determined in such a manner that they have a resonance phenomenon for multiples of a predetermined frequency. These antennas use the resonance phenomenon in order to increase the energy of the radiation emitted and/or received at the predetermined frequency and thus have a limited pass band. These antennas also have the advantage that they are compact by comparison with non-resonant antennas, that is to say antennas which do not have a resonance phenomenon for multiples of a predetermined frequency. 
   These antennas can be produced with the aid of a single electric conductor forming a dipole or a monopole, usually of the strand type. They are for example produced with the aid of a metal cover imprinted on a dielectric substrate, these latter antennas being known by the name of “patch antennas”. Another mode of production consists of cutting out slots in a mass plane, these antennas being known by the name of “slot antennas”. However, at best, it is known nowadays to produce omnidirectional resonant antennas operating in a spatial plane, that is to say that the electromagnetic radiation emitted or received is substantially uniform irrespective of the direction of this plane. 
   Systems also exist in the prior art which comprise three resonant antennas each oriented in a different spatial direction. These antennas are connected to the input of a signal processing computer. The computer is adapted to process the signals received at the input in such a way as to restore at the output one single signal similar to that of an omnidirectional resonant antenna operating in all spatial directions. 
   However, these systems are difficult to integrate into industrial applications, particularly because of the presence of the computer. 
   Therefore no resonant antennas exist at present which have the simplicity of the antennas formed with one single electrical conductor whilst being omidirectional in a half-space or all of space. 
   SUMMARY OF THE INVENTION 
   Therefore the object of the present invention is to fill this gap by creating an omnidirectional resonant antenna operating in a half-space or in all of space. 
   It therefore relates to an omnidirectional resonant antenna operating in a half-space or all of space having one single radiating electric conductor formed of at least three strands placed end to end, the length of each strand and the orientation of the strands with respect to one another contributing to determining the global radiation of the electric conductor, characterised in that the strands are oriented in at least three different spatial directions and that the lengths of the strands are determined in such a manner as to obtain an omnidirectional global radiation of the electric conductor operating in a half-space or in all of space. 
   According to other characteristics of the invention, it may also comprise one or several of the following characteristics:
         the radiating electric conductor has two parts which are symmetrical with respect to a plane of symmetry in order to obtain radiation of the electric conductor which is omnidirectional in all of space;   the radiating electric conductor is composed of a first, a second, a third, a fourth and a fifth strand, the fourth and fifth strands being respectively the images by symmetry of the second and the first strands with respect to the median plane of symmetry of the third strand;   a strand at the end of the radiating electric conductor is positioned perpendicular to a mass plane;   the dimensions of the mass plane are less than the wavelength λ in order to obtain omnidirectional radiation of the electric conductor in all of space;   the dimensions of the mass plane are several times greater than the wavelength λ in order to obtain omnidirectional radiation of the electric conductor operating in a half-space;   it has mass elements and in that the strands of the radiating electric conductor are respectively coplanar therewith;   the radiating electric conductor has a first end connected to a wave emitter/receiver and a second end connected to the mass plane;   the radiating electric conductor has a first end connected to a wave emitter/receiver and a second end connected to the mass elements;   the radiating electric conductor is connected to the wave emitter/receiver by means of an electromagnetic coupling zone;   the dimensions of the electromagnetic coupling zone partially determine the real impedance of the antenna;   the radiating electric conductor is composed of a first, a second and a third strand;   the consecutive strands of the radiating electric conductor are oriented in two directions which are orthogonal with respect to one another;   the strands are each formed by a band of which the width is determined in such a manner as to adapt, at least partially, the real impedance of the antenna to the impedance of a wave emitter/receiver intended to be connected to the antenna;   the radiating electric conductor is composed of wire strands;   the radiating electric conductor has a first end connected to a wave emitter/receiver and a second free end;   the radiating electric conductor is associated with a dielectric material reducing the dimensions of the antenna;   the radiating electric conductor is embedded in a dielectric material reducing the dimensions of the antenna; and   the radiating electric conductor is positioned on the surface of a dielectric material reducing the dimensions of the antenna.       

   The invention also relates to a device for receiving and emitting electromagnetic radiation in a half-space or in all of space, characterised in that it has a plurality of omnidirectional resonant antennas as claimed in any one of the preceding claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood upon reading the following description which is given solely by way of example and with reference to the accompanying drawings, in which: 
       FIG. 1  shows schematically an electric conductor connected by a first end to a wave emitter/receiver and by a second end to a mass, as well as a graph illustrating the distribution of the surface current density along this conductor. 
       FIG. 2  shows schematically in perspective a first embodiment of an omnidirectional resonant antenna operating in space according to the invention, dimensioned on the basis of the graph of  FIG. 1 . 
       FIG. 3  shows in perspective a second embodiment of an omnidirectional resonant antenna operating in space according to the invention. 
       FIG. 4  shows an electric conductor connected by a first end to a wave emitter/receiver, the second end thereof being free, as well as a graph illustrating the distribution of the surface current density along this conductor. 
       FIG. 5  shows in perspective a third embodiment of an omnidirectional resonant antenna operating in space according to the invention, dimensioned on the basis of the graph of  FIG. 4 ; and 
       FIG. 6  shows in perspective a fourth embodiment of an omnidirectional resonant antenna operating in space according to the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows an electric conductor  4  forming a monopole extending along the axis of the co-ordinates of the graph. In a conventional manner this is a “quarter-wave” electric conductor, that is to say an electric conductor of which the total length is equal to quarter of a wavelength, denoted by λ, of a predetermined frequency. The predetermined frequency is hereafter called the “working frequency”. A constructive resonance phenomenon is produced in the electric conductor  4  when electromagnetic radiation of which the wavelength is λ is emitted and/or received. The electric conductor  4  is formed here by a current-conducting band of constant width. The electric conductor  4  has a first end  6  connected to a mass and a second end  8  connected to a wave emitter/receiver  10  such as a conventional microwave emitter/receiver. In the following description the term “wave emitter/receiver” is used to mean an emitter/receiver capable of emitting and/or receiving electromagnetic radiation at a given frequency which is connected to an electric conductor. A curve  12  represents the distribution of the surface current density along the electric conductor at the working frequency. This curve is determined for example with the aid of conventional software for simulation of electromagnetic radiation of electric conductors. The area between the curve  12  and the electric conductor  4  is divided into three areas  14 ,  16  and  18  of equal surface area and of which the interesting features will become apparent later in the description. A point  20  on the electric conductor  4  marks the limit separating the area  14  from the area  16 ; equally a point  22  on the electric conductor  4  marks the limit separating the area  16  from the area  18 . Thus the points delimit two strands placed end to end on the electric conductor  4 . 
   The areas  14 ,  16  and  18  are respectively proportional at the level of radiation of the strands of the electric conductor  4  between the end  8  and the point  20 , between the points  20  and  22  and between the point  22  and the end  6 . It will be appreciated therefore that with the aid of  FIG. 1  it is possible to determine the length of an electric conductor so that it has a predetermined level of radiation. 
     FIG. 2  shows a first embodiment of an omnidirectional resonant antenna on the basis of the graph of  FIG. 1 . This includes an electric conductor forming a monopole similar to that of  FIG. 1 . Thus the electric conductor  26  has a distribution of surface current density and per unit of length which is similar to that of  FIG. 1 . It is composed of three strands  28 ,  30  and  32  which are placed end to end and are orthogonal with respect to one another. The strand  28  has a length equal to that of the strand between the end  8  and the point  20  of  FIG. 1 . The strand  30  has a length equal to that of the strand between the points  20  and  22  of  FIG. 1 . The strand  32  has a length equal to that of the strand between the point  22  and the end  6  of  FIG. 1 . The free end of the strand  28  is connected by means of an electromagnetic coupling zone  34  to a terminal  36  of a wave emitter/receiver  37 . The length of the coupling zone  34 , that is to say the space between the free end of the strand  28  and the terminal  36 , is determined by simulation or experimentally in order to adapt the real impedance of the antenna to the impedance of the wave emitter/receiver  37 . It will be noted that it is equally possible to act on the width of each strand of the electric conductor  26  in order to adapt the real impedance of the antenna to the impedance of the wave emitter/receiver  37  in such a way as to limit the reflection phenomena at the interface of these two devices  26  and  37 . The free end of the strand  32  is connected perpendicularly to a mass plane  38  of which the dimensions are less than the wavelength λ of the working frequency. In these conditions the mass plane  38  does not form a screen to the radiation of the electric conductor  26 . On the other hand, the different parameters of the strands (length, width, orientation, . . . ) must be adjusted in order to compensate for the effects of the edge of the mass plane  38 . 
   As a variant, the mass plane  38  is a plane of which the width and the length are several times greater than the wavelength λ of the working frequency of the electric conductor  26 . Then it is said that the mass plane is infinite. It will be noted that an infinite mass plane forms a screen to the electromagnetic radiation of an electric conductor such as the conductor  26  and that consequently the resonant antenna is omnidirectional in a half-space. In this case the lengths of the strands such as the strands  28 ,  30  and  32  are respectively less than 
             λ   5     ,     λ   10     ,     and   ⁢           ⁢     λ   80       ,         
where λ is the wavelength of the working frequency.
 
   Thus for example for a wavelength λ=314 mm and for an electric conductor formed with a band of 5 mm width, the lengths of each of the strands corresponding to the strands  28 ,  30  and  32  are respectively 53 mm, 30 mm and 3 mm. Furthermore, in this example the width of the coupling zone such as the zone  34  is 1 mm, the terminal  36  has a length of 4 mm and the diameter of the wire for connection to the emitter/receiver is 0.2 mm. 
     FIG. 3  shows a second embodiment of an omnidirectional resonant antenna operating in space according to the invention in which the resonant antenna is formed by an electric conductor  50  forming a monopole. This electric conductor has five strands  52 ,  54 ,  56 ,  58  and  60  which are placed end to end and are disposed in such a way as to form a first and a second part which are the image of one another with respect to a plane of symmetry  62 . The strands  52 ,  54  and  56  are rectilinear and orthogonal in pairs with respect to one another. The first part is composed of the strands  52  and  54  and a half-strand  64 . The half-strand  64  represents the upper half of the strand  56 . The strands  52 ,  54  and  64  form an electric conductor similar to the electric conductor  26  described with regard to  FIG. 2 . The total length of the electric conductor formed by the strands  52 ,  54  and by the half-strand  64  is equal to the wavelength of the working frequency divided by four. More precisely, the length of the strand  52  is equal to that of the strand between the end  8  and the point  20  of  FIG. 1 . The length of the strand  54  is equal to that of the strand between the points  20  and  22  of  FIG. 1 . The length of the half-strand  64  is equal to that of the strand between the point  22  and the end  6  of  FIG. 1 . The second part of the electric conductor  50  is composed of the strands  58 ,  60  and a half-strand  66 . The half-strand  66  represents the lower half of the strand  56 . The dimensions of the strands  58 ,  60  and of the half-strand  66  are respectively the same as those of the strands  54 ,  52  and the half-strand  64 . The second part of the electric conductor  50  is intended to produce an electric image of the first part in such a way as to simulate the existence of a mass plane. Thus the second part fulfils the functions of a mass plane such as the mass plane  38  of  FIG. 2  for the first part, and vice versa. This is why the dimensions of the strands of the first part are determined in the same way as in the embodiment according to  FIG. 2 . The free end of the strand  52  is connected to a first terminal of a wave emitter/receiver  68  and the free end of the strand  60  is connected to a second terminal of the wave emitter/receiver  68 . This first and this second terminal are equally the image of one another with respect to the plane of symmetry  62  in such a way that a phase displacement is not introduced between the signals transmitted/received by the wave emitter/receiver  68 . 
     FIG. 4  shows an electric conductor  68  forming a monopole extending along the axis of the co-ordinates of the graph. This electric conductor is formed here by a current-conducting band of constant width, but other forms may be used in other embodiments. A first end of this electric conductor is connected to a wave emitter/receiver  69 . The second end remains free. A curve  70  represents the surface current density along the electric conductor  68  at the working frequency. This curve is obtained for example with the aid of conventional simulation software. In this example, and in a similar manner to that described with regard to  FIG. 1 , the area between the curve  12  and the electric conductor  68  is divided into three areas  72 ,  74  and  76  of equal surface area. Once these areas are defined, a point  78  is placed on the electric conductor  68  to mark the limit between the area  72  and the area  74 . Equally a point  80  on the electric conductor  68  marks the limit between the area  74  and the area  76 . The points  78  and  80  cut the electric conductor  68  into three strands of respective lengths L 1 , L 2  and L 3 . The surfaces of the areas  72 ,  74  and  76  are respectively proportional to the levels of radiation of the strands of length L 1 , L 2  and L 3 . 
     FIG. 5  shows a resonant antenna dimensioned according to the graph of  FIG. 4 . This antenna has an electric conductor  86  forming a monopole similar to the electric conductor  68  of  FIG. 4 . The electric conductor  86  is connected by a first end to a terminal  87  of a wave emitter/receiver  88 . A second end of the electric conductor  68  remains free. This electric conductor  86  is composed of three strands  90 ,  92  and  94  placed end to end. These strands are rectilinear and orthogonal in pairs with respect to one another. The length of each of these strands is determined in accordance with  FIG. 4 , that is to say that the strand  94  has a length L 1 , the strand  92  has a length L 2  and the strand  90  has a length L 3 . The free end of the strand  94  is connected to the wave emitter/receiver  88  whilst being perpendicular to a mass plane  96  of which the dimensions are less than the wavelength λ of the working frequency. The whole of the antenna constituted by the electric conductor  86  and the mass plane  96  is embedded in a dielectric material  98  in order to reduce the dimensions of the antenna. In effect, embedding the electric conductor of an antenna in a dielectric material or disposing it on the surface of a dielectric material makes it possible to reduce the dimensions required for the electric conductor and therefore for the antenna. 
   The resonant antenna of  FIG. 6  has an electric conductor  110  formed by a band of current-conducting material of constant width. This electric conductor is composed of three strands  112 ,  114  and  116  which are placed end to end and are orthogonal in pairs with respect to one another. The antenna also has two mass elements  120  and  122 . These mass elements  120  and  122  are each formed by a band of current-conducting material of constant width. The first element  120  has three strands  124 ,  126  and  128  placed end to end. The second mass element  122  also has three strands  130 ,  132  and  134  placed end to end. These two mass elements  120  and  122  are respectively disposed to the right and to the left of the electric conductor  110 . The strands  124  and  130  of the mass elements are parallel to and coplanar with the strand  112  of the electric conductor  110 . Equally, the strands  126  and  132  and the strands  128  and  134  are respectively parallel to and coplanar with the strands  114  and  116  of the electric conductor  110 . The ends of the strands  128 ,  116  and  134  opposite the strands  126 ,  114  and  132  are connected to one another by a current-conducting element  136 . The free end of the strand  112  is connected to a wave emitter/receiver  138 . The lengths of the strands  112 ,  114  and  116  are determined as a function of the distribution of the surface current density along the electric conductor  110  in a similar manner to that described with regard to  FIGS. 1 and 2 . The width of the gaps  140 ,  142  separating the strands of the mass elements from the strands of the electric conductor  110 , that is to say the width of the bands forming the mass elements, are determined by simulation or by experimentation in order to adapt the real impedance of the antenna to that of the wave emitter/receiver  138 . Such an antenna is typically produced by cutting slots of constant width in a metal sheet which is then bent at right angles. 
   The operation of the resonant antenna which is omnidirectional in space will now be described with the aid of  FIGS. 1 and 2 . 
   During the emission of electromagnetic radiation at the working frequency with the aid of the antenna of  FIG. 2 , the wave emitter/receiver  37  generates, by electromagnetic coupling in the electromagnetic coupling zone  34 , a surface current density in the electric conductor  26 . The surface current density thus created is distributed along the electric conductor  26  as illustrated on the graph of  FIG. 1 . 
   The length of the strands  28 ,  30  and  32  is determined so that the areas  14 ,  16  and  18  have an equal surface area. Consequently the levels of radiation of each of the strands of the electric conductor  26  are the same. 
   Moreover, the level of radiation emitted at any point in space is practically the vectorial sum of the radiation emitted by each of the strands  28 ,  30  and  32 . These strands are orthogonal with respect to one another and as the radiation emitted by a strand is parallel to the direction thereof it will be appreciated that the radiation emitted by one strand does not interfere with that of the others. Thus it will be noted that the orthogonal strands optimise the gain of the antenna whilst avoiding destructive interference phenomena. Thus it will be appreciated that this antenna does not favour any particular direction in space, since the strands are orthogonal and the level of radiation of each strand is the same. Consequently, the antenna thus produced is practically omnidirectional. It is considered here that the radiation is practically omnidirectional in a predetermined region of space, if the level of radiation emitted/received by the antenna in any two directions of this region of space does not vary by more than 50%. 
   It will be noted that the mass plane  38  does not constitute a screen to the electromagnetic radiation and that consequently the radiation of the preceding antenna is omnidirectional in all of space. 
   During the reception of electromagnetic radiation at the working frequency with the aid of the antenna of  FIG. 2 , the levels of radiation received in the directions of the strands  28 ,  30  and  32  are respectively proportional to the areas  14 ,  16  and  18  and therefore determined by the respective lengths of each strand. In the particular case of the first embodiment, the length of each strand has been chosen so that the areas  14 ,  16  and  18  are equal. Consequently the level of radiation received for a given radiation parallel to a strand will be the same regardless of whether this radiation is parallel to the strands  28 ,  30  or  32 . Radiation from any direction can always be broken down into three components respectively parallel to the tree strands  28 ,  30  and  32 , and therefore the global level of radiation received by the antenna is unchanged irrespective of the direction of this radiation. It will be noted that, like the emission, the reception is not limited by the mass plane  38  to a half-space if the dimensions thereof in terms of width and length are less than λ. 
   The operation of the antenna shown in  FIG. 3  follows from what has already been described. 
   In effect, the second part of the electric conductor  50  of the antenna formed by the strands  58 ,  60  and the half-strand  66  fulfils the functions of an mass plane extending along the plane of symmetry  62  for the first part formed by the strands  52 ,  54  and the half-strand  64 . Consequently the study of the operation of the first part of the antenna leads to the study of the operation of an electric conductor connected perpendicularly to a mass plane merging with the plane of symmetry  62 . The operation of such a structure has already been described with regard to  FIG. 2 . 
   Conversely, the first part of the antenna fulfils the functions of a mass plane merging with the plane of symmetry  62  for the second part of the antenna. Consequently, in a manner similar to that which has just been described above, the operation of the second part of the antenna leads to the study of an antenna of which the structure is similar to that described with regard to  FIG. 2 . 
   The operation of the resonant antennas shown respectively in  FIGS. 5 and 6  may be deduced easily from the operation of the antenna described with regard to  FIG. 2 . 
   As a variant, the electric conductor of the preceding embodiments is composed of strands formed by wire elements instead of strands in the form of bands. The diameter of the wire forming each strand is determined so as to adjust the real impedance of such an antenna to that of the wave emitter/receiver. 
   As a variant, the electric conductor of the preceding embodiments is composed of strands of any form in respect of which it is possible to calculate the surface current density at the working frequency. 
   Advantageously a device for receiving and emitting electromagnetic radiation has a plurality of omnidirectional resonant antennas operating in a half-space or in all of space such as those described above, each adapted so as to receive and emit a predetermined wavelength. Thus the device for reception and emission is simultaneously omnidirectional in a half-space or in all of space, and capable of receiving and emitting at different wavelengths.