Abstract:
The present invention relates to a method for producing an antenna comprising a conductive wire, formed by a wire, made of a conductive material, whose path has a helical shape. In accordance with the invention, such a method is characterised in that a print is formed in relief on one side of an element of an electrically insulating material such that the helical shape is generated by depositing the conductive material in this print.

Description:
[0001]     The present invention relates to a method for manufacturing an antenna and/or a network of antennas, as well as an antenna and/or network of antennas manufactured according to such a method. It particularly relates to helical antennas or networks of helical antennas.  
       BACKGROUND OF THE INVENTION  
       [0002]     An antenna is a unit for transmitting and/or receiving electromagnetic radiation. It is used in numerous communication systems that require one or more of these radiating units.  
         [0003]     When several antennas are associated for a single common function, the set of these antennas is known as a network of antennas. A network of antennas provides certain advantages in relation to one antenna, such as providing greater directivity, the equivalent radiating aperture of a network being greater than that of a single antenna within this network.  
         [0004]     Helical antennas, i.e. antennas formed by a conductive wire whose path describes a helix, are used in many systems such as mobile phones, sources for the focal points of a concentrating system or the major antenna networks.  
         [0005]      FIG. 1   a  shows the projection of a helical antenna  100  onto a plane comprising the longitudinal axis  104  of the antenna and  FIG. 1   b  shows the projection view of the antenna  100  on a plane perpendicular to the longitudinal axis  104  of the antenna.  
         [0006]      FIG. 1   b  enables the interior  105  of the helix to be defined as the volume generated by the cylinder of an axis and radius equal to the axis and radius of the helix, the exterior  107  of the helix being the volume surrounding the interior  105 .  
         [0007]     A helical antenna  100  comprises one or more conductive wire segments  102  of a helical or spiral form. Each segment  102  can be defined by its length  108  according to the longitudinal axis  104 , by the number of turns  110 , by the cross-section  112  of the wire, by the pitch  114  of the helix, by the angle of inclination  116  of the helix (angle between a tangent to the helix and a plane perpendicular to the axis  104  of the antenna), by the radius  106  of the helix and by the nature of the conductive material forming the wire.  
         [0008]     Moreover, it must be noted that these parameters can be variable or constant along the axis  104  of the helix, and particularly along a segment  102 .  
         [0009]     Such a helical antenna  100  has several operating modes that depend on various parameters such as its radius  106 , its angle  116  of inclination of the turns  110  and its pitch  114 .  
         [0010]     According to a first polarisation mode, a helix can be polarised along its longitudinal axis  104  (linear polarisation) in a mode called normal, this linear polarisation being used particularly in mobile phones.  
         [0011]     According to a second mode, a helix can also be polarised in a circular manner (circular polarisation in a mode known as axial) around the helix axis when the helix radius is in the order of the wavelength of the waves to be transmitted or received, this circular polarisation being generally used, for example, for the helical antennas present at the focal points of parabolic antennas used to receive and transmit electromagnetic waves coming from satellites.  
         [0012]     In this latter case, the quality of the circular polarisation depends on the number of turns  110 , whereas the directivity depends on the length  108  of the antenna.  
         [0013]     It is known that a helical antenna is manufactured according to several distinct methods, these methods being described hereafter:  
         [0014]     A first manufacturing technique consists in conferring a helical profile on a conductive wire by forming, in a manner analogous to the manufacture of a spring.  
         [0015]     A second manufacturing technique consists in winding a conductive wire around an insulating element such as a plastic tube or a foam block, the insulating element being used to support the helix.  
         [0016]     The insulating element can subsequently be left or removed if the strength of the helical antenna is great enough to retain its shape without the support.  
         [0017]     A third technique consists in printing one or more conductive lines diagonally on a substrate, for example, a sheet of an insulating element that is folded onto itself, thus forming a helix whose angle of inclination is the angle of the diagonal on the substrate.  
         [0018]     Moreover, when antennas are manufactured individually, it may be necessary to group these antennas into a network, in which case these antennas must be linked together by a rigid conductive element and a driver circuit.  
         [0019]     These methods for manufacturing antennas have various drawbacks. Hence, the forming technique requires sufficient rigidity from the conductive element whereas the manufacturing methods according to the second and third techniques, i.e. by winding around a support or by folding a substrate, are relatively complex and costly to implement, particularly owing to the required deformation of the conductive wire (for winding) or the substrate (for folding).  
         [0020]     Finally, these known methods only apply to the manufacture of individual antenna of such a sort that grouping several antennas into a network requires other specific operations, thus increasing the cost of manufacturing a network.  
         [0021]     During these operations specific to grouping antennas into a network, it must be noted that each antenna must be connected to the entire network with a mechanical rigidity sufficient to retain, over a long period, the polarisation characteristics of the antenna network in its entirety, which is particularly complex and costly.  
         [0022]     Now, it is known that, in a network, the antennas must keep their directions with respect to each other and with respect to the driver circuit, so that the driver circuit can maintain its performances.  
       BRIEF SUMMARY OF THE INVENTION  
       [0023]     The present invention solves at least one of the problems cited above. It particularly results from the observation that a helix cannot be superimposed when its shape is projected onto a surface parallel to its axis, as shown in  FIG. 1   a.    
         [0024]     This is the reason that the invention relates to a method for manufacturing an antenna comprising a wire, made of a conductive material, whose path has a helical shape characterised in that a print is formed in relief on one side of an element made of an electrically insulating material such that the helical shape is generated by depositing the conductive material in this print.  
         [0025]     Thanks to the invention, it is possible to manufacture an antenna with a limited cost, because simply depositing the conductive wire onto a bearing surface according to the invention enables such an antenna to be generated.  
         [0026]     In particular, it is unnecessary to shape a conductive wire, according to the winding and forming methods described above, or to fold a substrate to obtain the helical shape, which simplifies the manufacture of the antenna.  
         [0027]     Moreover, the invention enables a large number of different helical antennas to be manufactured with a great flexibility in design, as the variation in physical parameters between the antennas to manufacture can be taken into account merely by varying the bearing surfaces of these antennas according to these parameters.  
         [0028]     Hence, many parameters can be modified easily with a manufacturing method according to the invention. For example, the material of the insulating element, its dielectric index (which enables the radioelectric characteristics to be modified), the conducting material and the different helical profiles possible for the antennas.  
         [0029]     The invention also relates to a method for manufacturing a network of antennas characterised in that several antennas are manufactured on a single insulating element by the helical antenna manufacturing method of the invention.  
         [0030]     Thanks to this invention, it is possible to group helical antennas into a network in a simple, reliable, reproducible and rapid manner that is analogous to the method of manufacturing each of the antennas forming the network.  
         [0031]     Moreover, the entire antenna network is rigid thanks to the common support, which enables it to maintain the relative direction of the antennas with respect to each other, and therefore to maintain the performances of the network.  
         [0032]     It is also possible to associate the driver circuit in a single industrial operation, as in for example the printing of coplanar lines or the addition of a microstrip with substrate and ground plane on the same insulating element where the antennas of the network are manufactured. This method is, like the method for manufacturing the antennas, a simple method to implement and requiring limited production costs.  
         [0033]     The invention also relates to a helical antenna or a network of helical antennas manufactured according to a manufacturing method in accordance with the invention.  
         [0034]     In one embodiment, the contact surface between the conductive material and the insulating element in the print is located within the helix for some sections and outside the helix for other sections.  
         [0035]     According to one embodiment, the material of the electrically insulating element is obtained by moulding or by forming.  
         [0036]     In one embodiment, the conductive element is formed by depositing a metallic material on the insulating element.  
         [0037]     According to one embodiment, the insulating element is foam.  
         [0038]     In one embodiment, the print is generated by the mechanical pressure of a helical die on the side of the insulating element.  
         [0039]     According to one embodiment, the helical die generating the print is a model of the antenna to manufacture.  
         [0040]     In one embodiment, the print comprises a regular series of grooves along a longitudinal axis, corresponding to the axis required for the helical antenna, practically parallel to each other and uniformly spaced by a peak to peak distance equal to the pitch required for the helical antenna, the angle between the direction of the grooves and the perpendicular of the antenna axis corresponding to the angle of inclination required for helical antenna.  
         [0041]     According to one embodiment, the conductive wire is deposited by spraying metallic particles into the print by means of a stencil.  
         [0042]     In one embodiment, the stencil has a relief corresponding to the relief of the bearing surface.  
         [0043]     The invention also relates to a method for manufacturing a network of antennas comprising at least two antennas with a helical shape, characterised in that at least two of these antennas are manufactured according to a method in accordance with one of the above embodiments.  
         [0044]     In one embodiment, the antennas are manufactured using a single electrically insulating element.  
         [0045]     According to one embodiment, the antennas are connected by a conductive circuit printed onto the electrically insulating element.  
         [0046]     In one embodiment, a driver circuit is integral with the electrically insulating element.  
         [0047]     According to one embodiment, the driver circuit is printed or engraved onto a substrate glued to the electrically insulating element.  
         [0048]     The invention also relates to a helical antenna characterised in that it is manufactured by a manufacturing method compliant with one of the aforementioned antenna manufacturing method embodiments.  
         [0049]     Finally, the invention also relates to a network of antennas characterised in that it is manufactured by a manufacturing method in accordance with one of the aforementioned antenna manufacturing method embodiments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0050]     Other characteristics and advantages of the invention will emerge with the description made below as an example, which is descriptive and non-restrictive, and refers to the figures herein where:  
         [0051]      FIGS. 1   a  and  1   b,  described above, show two diagrammatic views of a simple helical antenna,  
         [0052]      FIGS. 2   a  and  2   b  diagrammatically show two stages of the method for manufacturing an antenna according to a first preferred embodiment of the invention,  
         [0053]      FIGS. 3   a  and  3   b  are diagrams of an insulating element used to manufacture an antenna according to a second embodiment of the invention, and  
         [0054]      FIG. 4  is a diagram of an embodiment of a network of helical antennas in accordance with the invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0055]     In the preferred embodiment of the invention described hereafter, in accordance with the invention, a print is used to deposit a conductive wire on the bearing surface such that the path of the wire according to this print is helical.  
         [0056]     To this purpose, an electrical insulating material is chosen, selected according to the dielectric index required for the antenna, considering that the radiating body of the antenna (the conductive wire) is brought into contact with this material.  
         [0057]     Moreover, this material has a capacity to be shaped, which is why in this embodiment, the material comprises foam, for example in polymethacrylimide or expanded polystyrene. The known usable polymethacrylimide foams have, for example, a permittivity Er varying between 1.07 and 1.08 and a loss tangent varying between 0.0002 and 0.0003.  
         [0058]     The expanded polystyrene foams have, for example, a permittivity Er in the order of 1.56 and a loss tangent in the order of 0.002.  
         [0059]     Subsequently, one of the two methods described above is implemented, producing a print on the foam.  
         [0060]     A first method, described with  FIGS. 2   a  and  2   b , is applied when a helical antenna die or model  202  can be realised similar to the antenna to be manufactured showing sufficient rigidity, and particularly a cross-section large enough so that the antenna will not become deformed when it is pushed into a foam element as described hereafter.  
         [0061]     Indeed, this helix model  202  is used to produce a print  210  of this model on one face of a volume  208  of foam by laterally pushing, according to a direction  206 , the model  202  into this volume until this model  202  is fully inserted into the foam.  
         [0062]     Hence, it appears that the cross-section of the model  202  must be large so that this model  202  has sufficient mechanical strength to conform to the volume  208  of foam by marking a print such that, subsequently, conductive material can be printed in the print thus produced as described hereafter.  
         [0063]     In fact, on removing the model  202 , a three-dimensional print is left in the foam  208 , shown diagrammatically in the view according to the arrow  200  by  FIG. 2   b.    
         [0064]     When it is not easy to produce a low-cost model having sufficient mechanical characteristics so as not to change shape, or if the conductive wire cross-section is small, it is difficult to print a conductor in the relief of the print thus created.  
         [0065]     This is why, according to a second manufacturing method, the print is made in the insulating element  300  by marking onto the surface of this insulating element a print  302 , comprising a regular series of parallel grooves  304  along a longitudinal axis  306 , called the axis of the helical antenna.  
         [0066]     The cross-section of these grooves is determined mathematically beforehand or empirically by calculation methods, for example, by considering that this print corresponds to the lateral projection of an imaginary ideal helical antenna serving as a model for the antenna to be manufactured.  
         [0067]     These grooves have a peak to peak pitch  308  equal to the pitch required for the helical antenna to be manufactured. The angle  310  between the longitudinal axis  312  of the grooves and the perpendicular  314  to the antenna axis is the required angle of inclination for the helical antenna to manufacture.  
         [0068]     The grooves  304 , shown in detail in  FIG. 3   b , have a cross-section such that a curve  318  is obtained describing the same helix, in three dimensions, as the imaginary ideal helix, by projecting the latter helix onto these grooves according to the direction  312 , perpendicular to the surface of the insulating element  300 .  
         [0069]     After the creation of the print according to one or other of the methods described below, a previously manufactured stencil that follows the shape of the print is positioned.  
         [0070]     This stencil is, for example, a sheet of moulded metal such as a strip of a width equal to that of the print that is required to be produced, which is cut out in the stencil. This cutting to shape can be achieved for example by a pressure water jet or by a laser perpendicular to the sheet and describing the helix projection on a plane parallel to the helix axis.  
         [0071]     When the stencil is in position, a conductive material (e.g. metal) is sprayed according to certain processing parameters, such as time and/or density of spraying, to obtain the required thickness of the conductor on the print and, if necessary, other parts of the insulating element to produce the connection to the helix to be produced. The conductor deposit thus produced on the insulating element describes the required helix in space.  
         [0072]     A property of the helix thus formed by depositing conductive material on the insulating element can be noted: the contact surface between the conductive material and the insulating element in the print is located inside the helix for some sections (e.g. at the points  212  of  FIG. 2  or  322  of  FIG. 3 , which corresponds to the peaks of the print) and outside the helix for other sections (e.g. at points  214  of  FIG. 2  or  320  of  FIG. 3 , which corresponds to the grooves of the print).  
         [0073]     A network of helixes  400  ( FIG. 4 ) can also be produced by using the method for manufacturing helical antennas to manufacture several antennas on the same insulating element  402 , as for example a foam block.  
         [0074]     In the embodiment described below using  FIG. 4 , four helixes  404 ,  406 ,  408  and  410  were produced on the same foam block  402  by using the method of the invention.  
         [0075]     Hence, it is possible to combine four helixes on a substrate  412  with a ground plane and an excitation network  414  that can be printed or engraved, for example, such that the entire antenna network is rigid.  
         [0076]     This invention can have many variants relating, among others, to the different profiles that can be produced for the radiating elements, to the possible addition of a driver circuit (e.g. a microstrip with substrate and ground plane) and to the materials supporting the antennas according to the dielectric index required.