EP2543111B1 - Antenna structure with dipoles - Google Patents

Description

The present invention relates to a broadband antennal structure of the type comprising a dipole with two metal cones. The antennal structure may be used in reception for the interception of radio signals, or in transmission for broadcasting radio signals, particularly in the VHF and UHF bands.

Directional antenna structures having several dipoles are known. For example, such a dual polarization antennal structure comprises two first parallel strand dipoles forming a directional assembly with a vertical rectilinear polarization and two second parallel stranded dipoles forming another directional assembly for a horizontal rectilinear polarization. The strands of each dipole are collinear. The four dipoles are arranged along the sides of a square and fixed by four feet above a grid reflector panel much wider than the square. The operating frequency band of this antenna structure is narrow, for example between 174 MHz and 240 MHz, and thus much less than an octave. When several antennal structures are grouped, the radiation and gain performances of the antennal structures are strongly degraded over an octave.

The reflector has dimensions much larger than the half-wavelength associated with the low frequency of operation. For example, the reflector is a square having sides of length 1300 mm significantly larger than said half wavelength λ / 2 = 862 mm. If these antennal structures are grouped for example in a cube, the dimensions of the cubic group of antenna structures then become too large to obtain an omnidirectional diagram for the double frequency of the low operating frequency.

The object of the invention is to provide an antenna structure with two dipoles associated with a linear polarization, adapted to a wider operating frequency band and a smaller footprint for a predetermined low frequency of operation. The antenna structure is usable in a group of antenna structures having an omnidirectional radiation pattern for the entire operating frequency band.

To achieve this objective, an antenna structure according to the invention comprising a flat reflector, two dipoles symmetrical with respect to a plane of symmetry perpendicular to the reflector and a dipole supply network, is characterized in that
the length of the sides of the reflector is at most equal to half the wavelength corresponding to the low frequency of an operating frequency band,
each of the dipoles has two metal cones forming a vee,
hollow metal feet are fixed transversely to the horns and on the reflector so that projections of the dipoles orthogonal to the reflector are contained in the reflector,
the horns and the feet have lengths less than a quarter of the wavelength, and
the supply network extends for each dipole in the foot of one of the horns and in said each dipole between the foot of said one horn and the small base of the other horn to feed the dipoles symmetrically.

The antenna structure of the invention has dipoles and a reflector carrying the dipoles having small dimensions vis-à-vis the wavelength associated with the low frequency of operation. This characteristic associated with the vee shape of the horned dipoles confers a small footprint on the antenna structure and thus allows a grouping of several antennal structures according to the invention on a pylon, while offering a better omnidirectionality of the radiation patterns over a wide area. frequency band of at least one octave.

The low losses and the good high power performance of the dipole horns, the vee shape, the positioning and spacing of the dipoles and their distances to the reflector as well as the orientation of the cones of the dipoles, for example frustoconical or truncated pyramidal, above the reflector contribute to obtaining radiation patterns and controlled gains on octave bandwidths. Various characteristics of the antenna structure set forth below in combination with those set forth above contribute to the improvement of these results.

Each dipole may comprise a dielectric bent sleeve attached to the small bases of the horns, sealing one end of the supply network.

According to a first embodiment, the axes of the horns of each dipole and the axes of the feet of the horns may be situated on a plane perpendicular to the reflector, and the large bases of the horns may be closer to the reflector than the vertices of the ves formed by the dipoles. .

According to a second embodiment, the axis of one of the horns of each dipole and the axis of the foot of said one horn may be coplanar to the half-plane of an obtuse dihedron whose other half-plane is coplanar to the axis of the other horn of said each dipole and the axis of the foot of the other horn, and the large bases of two horns of the dipoles oriented towards one side of the reflector may be further apart than the vertices of the vés formed by the dipoles and may be closer to the reflector than the tops of the ves.

Moreover, the supply network can be advantageously embedded in the antenna structure without the use of visible cables or wires and while retaining the coaxial nature of the cable feeding the antenna structure and winding for example in a pylon whose top supports the antennal structure . In this regard, parallel metal hollow sleepers may be attached to both sides of the reflector and each support the feet of two horns, and the feed network may extend from a connector attached to one of the cross between the two feet supported by said one cross member, to the dipoles through said one cross member and said two legs. Although the power supply network may comprise coaxial cables combined with impedance matching means and housed at inside said one crossbar and said two feet, the impedance matching of the dipoles and the supply network with the characteristic impedance of the coaxial cable feeding the antenna structure is facilitated when the power supply is in coaxial line having, as external conductor, the walls of said one crossbeam, said two feet and the horns supported by said two feet and, as inner conductor, metal cores centered in said one crossbeam and said two feet, and for each dipole a bent core passing centrally through the dipole from the level of the foot of the horn acting as an external conductor to at least one metal return element fixed in the vicinity of the small base of the other horn of the dipole.

The impedance matching in each dipole can be achieved in particular by means of an impedance matching metal washer, acting as an iris at the small base of one of the horns, traversed without contact by the horn. bent core of the supply network in the dipole. The adaptation can also be done by successive modifications of the diameter of the metal core in different places inside the metal sleepers.

To lighten the reflector and make it less resistant to wind, the reflector may comprise a metal frame, for example square, and a wire mesh attached to the sides of the frame.

The antennal structure can be used as a single unit as was presented above. However, its performance is optimized for use in bundling to provide an omnidirectional diagram. Several antenna structures according to the invention may be grouped by connecting two parallel sides of the reflector of each antenna structure to sides of the reflectors of two other antenna structures to form a polyhedral cage, having two parallel hollow faces, the dipoles being positioned at the outside the cage and oriented globally in a common direction. For example, the hollow parallel faces are triangular equilateral for a group of three antennal structures or are square for a group of four antenna structures.

An antenna structure according to the invention may be intended for operation with simple rectilinear polarization, as presented above. However, it may be intended for operation with two cross-polarized or circular or elliptically polarized polarizations, while covering a bandwidth of at least one octave.

In this case, the antenna structure comprising a square flat reflector, two first dipoles symmetrical with respect to a first plane of symmetry perpendicular to the reflector, two second dipoles symmetrical with respect to a second plane of symmetry perpendicular to the reflector and the first plane of symmetry , a first power supply network of the first dipoles and a second supply network of the second dipoles, is characterized in that
the length of the sides of the reflector is at most equal to half the wavelength corresponding to the low frequency of an operating frequency band,
each of the first dipoles has two metal cones forming a vee and supported transversely by first hollow metal feet fixed on the reflector so that projections of the first dipoles orthogonal to the reflector are contained in the reflector,
each of the second dipoles has two metal cones forming a vee and supported transversely by second hollow metal feet fixed on the reflector so that projections of the second dipoles orthogonal to the reflector are contained in the reflector,
the horns and feet of all the dipoles have lengths less than a quarter of the wavelength,
the first feed network has the first plane of symmetry and extends for each first dipole in the foot of one of said each first dipole horns and in said each dipole between the foot of said one horn and the small base of the another horn of said first dipole, for supplying phase to the first dipoles, and
the second feed network has the second plane of symmetry and extends for each second dipole in the foot of one of said each second dipole cones and in said each dipole between the foot said one horn and the small base of the other horn of said each second dipole, for supplying phase to the second dipoles.

The antenna structure with four dipoles can be such that
for each first dipole, the axes of the horns and the axes of the feet of the horns are located on a plane perpendicular to the reflector, and the large bases of the horns are closer to the reflector than the vertex of the vé formed by said each first dipole, and
for each second dipole, the axis of one of the horns and the axis of the foot of said one horn are coplanar to the half-plane of an obtuse dihedron whose other half-plane is coplanar to the axis of the another horn and the axis of the foot of the other horn, and the large bases of two horns of the second dipoles oriented towards one side of the reflector are farther apart than the vertices of the vés formed by the second dipoles and are closer to the reflector that the vertices of vés formed by the second dipoles.

For the first power supply network in the antenna structure with four dipoles may be provided first metal hollow sleepers fixed perpendicularly to two parallel sides of the reflector on one side of the reflector and each supporting the first feet of two horns of the first dipoles. The first power supply network can then extend from a connector fixed on one of the first cross members between the first two feet supported by said first cross member, to said first dipoles through said first cross member and said first two feet. . In a manner similar to the first supply network, second metal sleepers may be provided for the second supply network fixed perpendicularly to two other parallel sides of the reflector on another face of the reflector and each supporting the second feet of two horns. second dipoles. The second supply network can then extend from a connector attached to one of the second cross members between the two second feet supported by said second cross member, to said second dipoles through said second cross member and said two second legs. .

• la figure 1 est une vue en perspective d'une structure antennaire à deux premiers dipôles et deux seconds dipôles pour des polarisations linéaires orthogonales, selon l'invention;
• la figure 2 est une vue de dessus de la structure antennaire montrant en détail un premier réseau d'alimentation pour les premiers dipôles et un second réseau d'alimentation pour les seconds dipôles;
• la figure 3 est une vue de la structure antennaire avec les réseaux d'alimentation, du côté de l'un des premiers dipôles;
• la figure 4 est une vue de la structure antennaire avec les réseaux d'alimentation, du côté de l'un des seconds dipôles;
• les figures 5 et 6 sont des vues en perspective et de dessus d'une cage groupant quatre structures antennaires;
• la figure 7 est une vue en perspective d'un groupe de quatre cages selon la figure 5 ; et
• les figures 8 et 9 illustrent schématiquement deux structures antennaires à deux dipôles conformes à la présente invention.

Other features and advantages of the present invention will emerge more clearly on reading the following description of several preferred embodiments of the invention, given by way of non-limiting examples, with reference to the corresponding appended drawings in which:

• the figure 1 is a perspective view of an antenna structure with two first dipoles and two second dipoles for orthogonal linear polarizations, according to the invention;
• the figure 2 is a top view of the antenna structure showing in detail a first power supply network for the first dipoles and a second power supply network for the second dipoles;
• the figure 3 is a view of the antennal structure with the supply networks, on the side of one of the first dipoles;
• the figure 4 is a view of the antennal structure with the supply networks, on the side of one of the second dipoles;
• the Figures 5 and 6 are perspective and top views of a cage grouping four antenna structures;
• the figure 7 is a perspective view of a group of four cages according to the figure 5 ; and
• the figures 8 and 9 schematically illustrate two antenna structures with two dipoles in accordance with the present invention.

As shown to Figures 1 to 4 , a dual-polarization antenna structure SA and two pairs of dipoles according to the invention comprises two first horned vee dipoles 11-12 for a first rectilinear polarization of the electric field, two second horned vee dipoles 21-22 for a second rectilinear polarization of the electric field orthogonal to the first polarization, and a square flat metal reflector 3 supporting the dipoles 11-12 and 21-22 by tubular metal legs 13, 14 and 23, 24 fixed on square metal hollow sleepers 15 , 16 and 25, 26. The structure SA composed of the aforementioned elements is symmetrical with respect to an axis of symmetry ZZ perpendicular and central to the reflector. The first dipoles 11-12 are arranged symmetrically with respect to a first plane of symmetry YY-ZZ perpendicular to the reflector and parallel to the first sides of the reflector. The seconds dipoles 21-22 are arranged symmetrically with respect to a second plane of symmetry XX-ZZ perpendicular to the reflector and the first plane of symmetry YY-ZZ and parallel to the second sides of the reflector. A supply network 4 of the dipoles 11-12 and a supply network 5 of the dipoles 21-22 are included in the feet 13 and 23 and the crosspieces 15 and 25. The structure is intended to operate in transmission and / or reception in a predetermined frequency band of one octave [f, 2f], included in the VHF and UHF bands, such as for example [100, 200] MHz or [200, 400] MHz.

Each dipole 11-12, 21-22 as an antenna comprises two vee arms formed by identical metal cone cones 11 and 12, 21 and 22 and a dielectric elbow 17, 27 between the horns. The small bases of the frustoconical cones 11 and 12, 21 and 22 are fitted into the ends of the sleeve 17, 27. The sleeves 17 and 27 are made for example of fiberglass, or of any other non-conductive electrical material that does not degrade the performance in impedance matching of the antennal structure. According to the embodiment illustrated, the sleeves consist of two half-shells having ends forming clamps around the small bases of the frustoconical cones. The sleeves participate in sealing the ends of the supply networks 4 and 5 in the cones. The large bases of the cones 11 and 12, 21 and 22 located opposite the sleeves 17, 27 are closed by thin circular plugs 18, 28. The plugs can be dielectric or metal and also participate in the sealing of the networks of feeding in the cornets.

The horns 11 and 12 of the first dipoles have dimensions different from those of the horns 21 and 22 of the second dipoles. The horns 11 and 12 and the ends of the feed system 4 therein are sized and oriented so that the first dipoles radiate or pick up waves with the first rectilinear polarization in the frequency band [f, 2f]. The horns 21 and 22 and the ends of the feed system 5 therein are dimensioned and oriented so that the second dipoles radiate or pick up waves with the second straight polarization in the band. frequency [f, 2f]. The horns 11 and 12 have small bases and large bases respectively larger than those of the horns 21 and 22 and are shorter than the horns 21 and 22. The ends of the tubular legs 13 and 14, 23 and 24 are welded to the horns 11 and 12, 21 and 23 at orifices substantially mid-length of the horns. The length of the horns and feet is less than λ / 4, λ being the wavelength corresponding to the low operating frequency f of the band [f, 2f]. The length of the dielectric sleeves 17 and 27 is very small compared to the wavelength λ. At the high frequencies of the band [f, 2f], the waves relative to each dipole 11-12, 21-22 are further confined around the narrow portions of the dipole horns connected by the sleeve 17, 27 which is transparent to the radio waves and which seals the end of the supply network included in the dipole.

The horns and sleeves may have a regular circular or polygonal cross section, for example hexagonal or octagonal. The cones can be made by rolling or folding a metal sheet or by discretizing the frustoconical profile by means of a frustoconical sheet of metal son stretched and welded to the bases of the cones. The horns, the reflector, the feet and the cross members may be for example aluminum or a light alloy so that the mass of the antennal structure is low.

The orientations of the horns of the dipoles in vee 11-12 and 21-22 vis-à-vis the flat reflector 3 are also different to optimize the omnidirectionality of the radiated waves or picked up by the pairs of dipoles 11-12 and 21-22 for the two polarizations. The obtuse angle α 1 of the bent sleeve 17 between the axes of revolution of the frustoconical cones 11 and 12 is larger than the obtuse angle α 2 of the bent sleeve 27 between the half-planes of an obtuse dihedral each containing the axis of a horn 21, 22 and the axis of the foot 23, 24 supporting the horn 21, 22. The axes of the frustoconical cones 11 and 12 of each first dipole and the axes of the feet 13 and 14 supporting these horns are located on a perpendicular plane to the reflector 3 and parallel to the plane of symmetry YY-ZZ, as shown in FIG. figure 2 . The large bases 18 of the horns are closer to the reflector 3 than the sleeve 17 to the vertex of the first dipole, as shown in figure 3 . The feet 13 and 14 of each first dipole 11-12 are perpendicular respectively to the axes of the horns 11 and 12 of the dipole and form between them an angle of 180 ° - α1. The dipole 11-12 with its two feet is symmetrical with respect to the bisecting plane of the sleeve 17 perpendicular to the reflector 3. The axes of the horns 11 and 12 of the two first dipoles and their feet 13 and 14 are thus contained in parallel planes between them and perpendicular to the reflector. The axes of the feet 23 and 24 supporting the horns 21 and 22 of each second dipole 21-22 are perpendicular to the reflector 3 and each form with the axis of the overlying horn 21, 22 an acute angle β2 greater than α2 / 2, as shown in figure 4 . For each second dipole, the axes of the horns 21 and 22 are contained in a secant plane of the reflector, and the sleeve 27 at the top of the vee of the second dipole is further from the reflector than the large bases 28 of the horns 21 and 22. axial axes of the two second dipoles 21-22 are inclined symmetrically with respect to the plane of symmetry XX-ZZ of the antenna structure above the reflector. The distance between the sleeves 27 of the second dipoles is smaller than the distance between the plugs 28 in the large bases of the horns 21, 22 of the second dipoles oriented towards a common side of the reflector and substantially equal to the distance between the sleeves 17 of the first dipoles. The spacing and the distance between the dipoles and the horn orientations make it possible to optimize the gain and the omnidirectionality of the grouping of each pair of dipoles 11-12, 21-22 over the entire operating frequency band [f, 2f ]. The angled shape of the dipoles also contributes to reducing the size of the antennal structure SA.

The reflector 3 comprises a rigid square metal frame 31 formed by brackets and a wire mesh 32 fixed to the sides of the frame. The length of the sides of the frame is at most equal to λ / 2. The mesh sides of the grid 32 are very small relative to the wavelength λ so as to minimize the radiation towards the rear of the SA structure opposite the cones with respect to the reflector, while limiting the wind resistance and the weight of the structure. The mesh pattern of the grid 32 may be of any shape. Alternatively, the reflector is made by a solid metal surface, for example by a sheet. The antennal structure can be protected by a radome ensuring its sealing.

The two crosspieces 15 and 16 and the two crosspieces 25 and 26 are perpendicular to each other and arranged parallel to the sides of the frame 31 respectively on the front and rear faces of the grid 32, and have fixed ends, for example screwed, alongside the frame which stiffens the grid and the frame. The ends of the feet 13 and 14 are welded at orifices respectively on the hollow crosspieces 15 and 16, and the ends of the feet 23 and 24 are welded at orifices respectively on the hollow cross-members 25 and 26. With a view to above, as shown in figure 2 , the sleeves 17 and 27 are located in the middle of sides of a rectangle close to a square, coaxial with the frame and having sides parallel to the sides of the frame 31 and smaller than these so that the projections of all the dipoles 11-12 and 21-22 and all the feet 13, 14 and 23, 24 orthogonal to the plane of the reflector 3 are contained in the frame 31. This arrangement of the dipoles does not protrude from the sides of the reflector and the bent form of the dipoles makes the antenna structure SA more compact and allows a grouping of several antenna structures by arranging their reflectors side by side, as for example according to the antenna structure groupings shown in FIGS. Figures 5 to 7 .

The supply networks 4 and 5 of the dipoles are now described with reference to the figures 2 , 3 and 4 . The supply networks 4 and 5 are connected to the outputs of a directional coupler by coaxial cables whose ends are connected to coaxial connectors 41 and 51 having bases welded in the middle of the rear sides of the cross member 15 supporting one 13 feet of each first dipole and the cross member 25 supporting one 23 of the feet of each second dipole. In a variant, the directional coupler is replaced by a 90 ° and wideband hybrid coupler connected to the coaxial connectors 41 and 51 by the coaxial cables so that the antenna structure operates with a circular polarization.

From the connectors 41 and 51, the supply networks 4 and 5 are in coaxial lines respectively having, as external conductors, the walls of the cross-members with square section 15 and 25, the circular section legs 13 and 23 of the dipoles 11- 12 and 21-22 as well as frustoconical cones 11 and 21 and, as internal conductors, cylindrical cores made of metal, for example aluminum, centered in these crosspieces and feet by dielectric line centralizers 151, 131 and 251, 231. The section of the metal cores of the coaxial lines in the crosspieces 15 and 25 and the feet 13 and 23 vary regularly to improve the impedance matching in the frequency band [f, 2f]. Alternatively, the coaxial lines formed in the crosspieces 15 and 25 and the legs 13 and 23 may be replaced by coaxial cables combined with impedance matching means and housed within the cross members 15 and 25 and feet 13 and 23.

The part of the supply network 4 for a first dipole 11-12 comprises from the connector 41 a coaxial line section 42 with a core passing through the centralizers 151 in the crosspiece 15, a coaxial line section 43 with a core passing through the centralizers 131 in the foot 13 of the dipole 11-12 and having an end opening into the horn 11 of the dipole 11-12, a coaxial line section 44 with a core passing through a dielectric centerer 111 in the horn 11 located between the foot 13 and the sleeve 17, an impedance matching metal washer 171 located at the junction of the horn 11 and the dielectric sleeve 17, and a bent core 45 passing centrally through the dielectric sleeve 17 of the dipole 11-12 and having an end fixed to the center of a metal return element 121 fixed in the vicinity of the small base of the other horn 12 of the dipole 11-12. The metal mass return element 121 constitutes a coaxial line termination and may be a transverse metal disk, or regularly distributed diametrical metal wires, fixed inside the horn 12. The part of the supply network 4 for the other first dipole 11-12 is identical to that described above and symmetrical thereof relative to the plane of symmetry YY-ZZ passing through the connector 41 fixed to the cross member 15.

The part of the supply network 5 for a second dipole 21-22 comprises, since the connector 51, a coaxial line section 52 with a core passing through the centralizers 251 in the crosspiece 25, a coaxial line section 53 with a core passing through the centralizers 231 in the foot 23 of the dipole 21-22 and having an end opening into the horn 21 of the dipole 21-22, a coaxial line section 54 with a core passing through a dielectric center 211 in the horn 21 located between the foot 23 and the sleeve 27 and an impedance matching metal washer 271 located at the junction of the horn 21 and the dielectric sleeve 27, and a bent core 55 passing through the dielectric sleeve 27 of the dipole 21-22 and having an end fixed in the center of a metal return element 221 fixed in the vicinity of the small metal base of the other horn 22 of the dipole 21-22. The metal mass return element 221 is similar to the ground return element 121 and constitutes a coaxial line termination. The part of the supply network 5 for the other second dipole 21-22 is identical to that described above and symmetrical thereof with respect to the plane of symmetry XX-ZZ passing through the connector 51 fixed to the crossbar 25 .

The metal washers 171 and 271 are respectively fixed to the small bases of the dipoles 11 and 21 and crossed without electrical and mechanical contact by the ends of the webs of the supply networks 4 and 5. The metal washers are dimensioned to optimize the adaptation of impedance of the power supply networks in addition to the impedance matching by the various sections of the cores of the coaxial line sections in the feet 13 and 23 and the crosspieces 15 and 25. The diameters of the holes in the metal washers 171 and 271 for the free passage of the extreme souls of the supply networks are also determined for a significant modification of the complex impedance of the dipoles over the entire predetermined frequency band [f, 2f].

Given the symmetry of the power networks 4 and 5 for each of the two polarizations, the first two dipoles 11-12 are supplied in phase and the two second dipoles 21-22 are also supplied in phase.

As shown in Figures 5 and 6 four four-pole antenna structures SA1 to SA4 each with a pair of first dipoles 11-12 and a pair of second dipoles 21-22, such as the antenna structure SA, are grouped together to form a cubic cage CG. The reflectors 3 of the antenna structures SA1 to SA4 are arranged on two pairs of parallel faces of the cubic cage. The arrangement of the reflectors 3 of the antenna structures SA1 to SA4 is maintained by arrangement of rods and bars (not shown) supporting the reflectors and fixed for example at the top of a pylon. Two parallel sides of the reflector frame 31 of each antenna structure may be connected to sides of the reflector frames of two other antenna structures. Two parallel faces of the cubic cage CG are hollow. The dipoles of the antennal structures SA1 to SA4 are outside the cage CG. The edges of the cage have a length substantially equal to the half-wavelength λ / 2. For example, the reflectors 3 of the structures SA1 to SA4 are placed vertically at the top of the pylon so that the hollow faces of the cage CG are horizontal, the first dipoles 11-12 are oriented globally vertically to radiate or pick up omnidirectionally waves with a vertically rectilinear polarization, and the second dipoles 21-22 are oriented horizontally horizontally to radiate or pick up omnidirectionally waves with a horizontal linear polarization.

The figure 7 shows a group of four cages CG1 to CG4 each having four antenna structures SA1 to SA4 and hollow faces stacked coaxially with their fixed edges to each other. This stack of cages side by side is compact thanks to the angled shape of the dipoles and the arrangement of the dipoles not overlapping the sides of the reflectors of each antennal structure. To the figure 7 is also shown a typical radiation pattern DR cage grouping orthogonal to the axis of symmetry common to the cages. The omnidirectional character of the group of cages appears in the central annular zone lighter than the zones lateral of the DR diagram between the two pairs of cages CG1-CG2 and CG3-CG4.

Although the embodiments described above relate to antenna structures with a double polarization, the invention also relates to any antenna structure having a single polarization having two horn dipoles 11-12 or 21-22 fixed on a reflector 3 and any array of several antennal structures with single polarization.

The two second turbocharged dipoles 21-22, the feet 23 and 24, the crosspieces 25 and 26 and the supply network 5 are removed in the Figures 1 to 4 when the single polarization antennal structure comprises only the first two horned vee dipoles 11-12 (see FIG. figure 8 ).

The first two horned vee dipoles 11-12, the feet 13 and 14, the crosspieces 15 and 16 and the feed network 4 are removed in the Figures 1 to 4 when the single polarization antenna structure comprises only the two second horned vee dipoles 21-22 (see FIG. figure 9 ).

Claims (11)

1. Antenna structure comprising a flat reflector (3), two dipoles (11-12; or 21-22) which are symmetrical about a plane of symmetry (YY-ZZ; or XX-ZZ) which is perpendicular to the reflector, and a network (4; or 5) for supplying the dipoles, characterised in that
   the length of the sides of the reflector (3) is at most equal to half of the wavelength which corresponds to the low frequency of an operating frequency band,
   each of the dipoles (11-12; or 21-22) has two metal horns forming a V,
   hollow metal feet (13, 14; or 23, 24) are fixed transversely to the horns and on the reflector so that projections of the dipoles which are orthogonal to the reflector are held in the reflector,
   the horns and the feet have lengths which are less than a quarter of the wavelength, and
   the supply network (4; or 5) extends for each dipole into the foot (13; or 23) of one (11; or 21) of the horns and into said each dipole between the foot of said one horn and the narrow end of the other horn (12; or 22) in order to supply the dipoles in a symmetrical manner.
2. Antenna structure according to claim 1, wherein each dipole comprises a bent dielectric sleeve (17; or 27) which is fixed to the narrow ends of the horns (11, 12; or 21, 22).
3. Antenna structure according to either claim 1 or claim 2, wherein the axes of the horns (11, 12) of each dipole and the axes of the feet (13, 14) of the horns are located on a plane which is perpendicular to the reflector (3), and the wide ends (18) of the horns are closer to the reflector (3) than the peaks (17) of the Vs formed by the dipoles.
4. Antenna structure according to either claim 1 or claim 2, wherein the axis of one (21) of the horns of each dipole and the axis of the foot (23) of said one horn (21) are coplanar with the half-plane of an obtuse dihedral, the other half-plane of which is coplanar with the axis of the other horn (22) and the axis of the foot (24) of the other horn (22) of said each dipole, and the wide ends (28) of two horns of the dipoles oriented towards one side of the reflector are spaced further apart than the peaks (17) of the Vs formed by the dipoles and are closer to the reflector (3) than the peaks (17) of the Vs.
5. Antenna structure according to any of claims 1 to 4, comprising parallel hollow metal cross members (15, 16; or 25, 26) which are fixed to two sides of the reflector (3) and each support the feet (13, 14; or 23, 24) of two horns, and the supply network (4; or 5) extends from a connector (41; or 51) which is fixed on one (15; or 25) of the cross members between the two feet (13; or 23) which are supported by said one cross member, as far as the dipoles through said one cross member and said two feet.
6. Antenna structure according to claim 5, wherein the supply network (4; or 5) is in a coaxial line having, as an external conductor, the walls of said one cross member (15; or 25) and of said two feet (13; or 23) and, as an internal conductor, metal cores (42, 43; or 52, 53) which are centred in said one cross member, said two feet and the horns supported by said two feet, and for each dipole (11-12; or 21-22), a bent core (44, 45; or 54, 55) passing through the centre of the dipole from the foot (13; or 23) of the horn (11; or 21) acting as an external conductor as far as at least one metal earth return member (121; or 221) which is fixed in the vicinity of the narrow end of the other horn (12; or 22) of the dipole.
7. Antenna structure according to claim 6, wherein each dipole (11-12; or 21-22) comprises a metal impedance-matching wafer (171; or 271) which is passed through in a contact-free manner by the bent core (44, 45; or 54, 55).
8. Group of a plurality of antenna structures (SA1-SA4) according to any of claims 1 to 7, wherein two parallel sides of the reflector (3) of each antenna structure (SA1) are connected to sides of the reflectors of two other antenna structures (SA2, SA3) in order to form a polyhedral cage (CG) which has two parallel hollow faces, and the dipoles (11-12; or 21-22) are positioned outside the cage and are oriented overall in a common direction.
9. Antenna structure comprising a square flat reflector (3), two first dipoles (11-12) which are symmetrical about a first plane of symmetry (YY-ZZ) which is perpendicular to the reflector, two second dipoles (21-22) which are symmetrical about a second plane of symmetry (XX-ZZ) which is perpendicular to the reflector and to the first plane of symmetry, a first network (4) for supplying the first dipoles and a second network (5) for supplying the second dipoles, characterised in that
   the length of the sides of the reflector (3) is at most equal to half of the wavelength which corresponds to the low frequency of an operating frequency band,
   each of the first dipoles (11-12) has two metal horns forming a V and supported transversely by first hollow metal feet (13, 14) which are fixed on the reflector so that projections of the first dipoles which are orthogonal to the reflector are held in the reflector,
   each of the second dipoles (21-22) has two metal horns forming a V and supported transversely by second hollow metal feet (23, 24) which are fixed on the reflector so that projections of the second dipoles which are orthogonal to the reflector are held in the reflector,
   the horns and the feet of all of the dipoles have lengths which are less than a quarter of the wavelength,
   the first supply network (4) has the first plane of symmetry (YY-ZZ) and extends for each first dipole into the foot (13) of one (11) of the horns of said each first dipole and into said each dipole between the foot of said one horn and the narrow end of the other horn (12) of said each first dipole in order to supply the first dipoles in phases, and
   the second supply network (5) has the second plane of symmetry (XX-ZZ) and extends for each second dipole into the foot (23) of one (21) of the horns of said each second dipole and into said each dipole between the foot of said one horn and the narrow end of the other horn (22) of said each second dipole in order to supply the second dipoles in phases.
10. Antenna structure according to claim 9, wherein, for each first dipole, the axes of the horns (11, 12) and the axes of the feet (13, 14) of the horns are located on a plane which is perpendicular to the reflector (3), and the wide ends (18) of the horns are closer to the reflector (3) than the peak of the V (17) formed by said each first dipole, and
    for each second dipole, the axis of one (21) of the horns and the axis of the foot (23) of said one horn (21) are coplanar with the half-plane of an obtuse dihedral, the other half-plane of which is coplanar with the axis of the other horn (22) and the axis of the foot (24) of the other horn (22), and the wide ends (28) of two horns of the second dipoles which are oriented towards a side of the reflector are spaced further apart than the peaks of the Vs (27) formed by the second dipoles and are closer to the reflector (3) than the peaks of the Vs formed by the second dipoles.
11. Antenna structure according to either claim 9 or claim 10, comprising
    first hollow metal cross members (15, 16) which are fixed perpendicularly to two parallel sides of the reflector (3) on one face of the reflector and each support the first feet (13) of two horns (11) of the first dipoles, the first supply network (4) extending from a connector (41) which is fixed on one (15) of the first cross members between the two first feet (13) supported by said one first cross member, as far as said first dipoles (21-22) through said first cross member and said two first feet, and
    second hollow metal cross members (25, 26) which are fixed perpendicularly to two other parallel sides of the reflector (3) on another face of the reflector and each support the second feet (23) of two horns (21) of the second dipoles, the second supply network (5) extending from a connector (51) which is fixed on one (25) of the second cross members between the two second feet (23) which are supported by said one second cross member, as far as said second dipoles (21-22) through said one second cross member and said two second feet.

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