AU2013200058B2 - A wideband compact antenna of very small thickness and with dual orthogonal linear polarization operating in the v/uhf bands - Google Patents

Abstract

A wideband compact antenna with very small thickness and with dual orthogonal linear polarization, operating in the V/UHF bands An antenna (2) for emitting/receiving electromagnetic waves of the type comprising two dipoles (16A, 16B) orthogonal to each other, each dipole (16A, 16B) comprising two 5 radiating elements (4), a metal plate (8), and an absorptive structure (6). The radiation elements (4) are all substantially planar, both dipoles (16A, 16B) being substantially comprised in a same plane (P), and the absorptive structure (6) is interposed between the metal plate (8) and the dipoles (16A, 16B) and is laid out in contact with the metal plate (8). Fig. 1 iS K- | A! 19 j 4 1 122 10 282 6 30 34- ' 34'-30 23 "3 21 2331 24 36 6 24 31 2

Description

A wideband compact antenna of very small thickness and with dual orthogonal linear polarization operating in the V/UHF bands

This application claims priority from French Application No. 11 04121 filed on 27 December 2011, the contents of which are to be taken as incorporated herein by this reference.

The invention relates to a wideband compact antenna of very small thickness and with dual orthogonal linear polarization operating in the V/UHF bands.

More particularly, the invention relates to an antenna for emitting/receiving electromagnetic waves, of the type comprising: - two dipoles orthogonal to each other, each dipole comprising two radiating elements, - a metal plate, and - an absorptive structure.

The invention is located in the field of antennas and wideband compact antenna systems. These systems are dedicated to applications for receiving and emitting , electromagnetic waves in a very wide band of frequencies. For example, the compact antenna according to the invention is intended to operate in the VHF and UHF bands, i.e. at frequencies comprised between 30 MHz and 3 GHz, and more particularly at frequencies comprised between 30 MHz and 500 MHz.

Such antennas are used for various purposes, for example in the field of radio communications and are notably intended to be integrated to a vehicle, whether this be a land, airborne or naval vehicle.

Consequently, these antennas are subject to many constraints.

Thus, for example, they have to: • have reduced size, • have low visual discretion or low RES, for Radar Equivalent Surface, • have high radio-electric performances such as low SWR, for Stationary Wave Ratio, high gain, etc., • be suitable for emitting or receiving electromagnetic waves regardless of their polarization (linear polarization, circular polarization and elliptical polarization), and • have unidirectional radio-electric coverage.

Finally, they should be compliant with the roadway clearance of land vehicles and not degrade the aerodynamics of airborne vehicles to which they are integrated and have independent radio-electric performances with respect to the latter.

Further, these antennas should have a very small thickness so as to be either laid out directly on one of the surfaces of a vehicle or in a cavity provided for this purpose in said vehicle, for example so that they are flush with a surface which it comprises.

Thus, the document « A novel compact dual-linear Polarized UWB Antenna for VFIF/UFIF applications » describes a wideband compact antenna of the aforementioned type. The radiating elements of the antenna are curved and have meanders so as to increase the electric length of the antenna and to thereby optimize low frequency radio-electric performances. Further, the metal plate of the antenna is positioned on a disc made with a ferrite material. It is found at a distance from the radiating elements so that it reflects the electromagnetic waves emitted or received by the high frequency antenna.

Flowever, this solution does not give entire satisfaction.

Firstly, this antenna does not allow use from 30 MFIz with an acceptable SWR.

Secondly, because of the curved shape of the radiating elements, the antenna forms a large protuberance protruding from the vehicle when it is laid out on a surface of the latter, or imposes over-dimensioning of the cavity in which it is laid out, which in particular proves to be a penalty for certain vehicles.

Thirdly, taking into account the design of the antenna, the low frequency radio-electric properties of this antenna have to vary according to the vehicle on which it is laid out and the latter will be particularly impacted in the case when this antenna is positioned in a metal cavity.

The discussion of the background to the invention included herein including reference to documents, acts, materials, devices, articles and the like is included to explain the context of the present invention. This is not to be taken as an admission or a suggestion that any of the material referred to was published, known or part of the common general knowledge in Australia or in any other country as at the priority date of any of the claims.

The present invention may at least alleviate these problems.

According to an aspect of the present invention, there is provided an antenna for emitting/receiving electromagnetic waves, of the type comprising: - two dipoles orthogonal to each other, each dipole comprising two radiating elements, a metal plate, and an absorptive structure, wherein the metal plate provides the functions of a ground plane as well as of a mechanical and electrical interface between the antenna and the structure on which the antenna is intended to be integrated and in that the radiating elements are all substantially planar, both dipoles being substantially comprised in a same plane and in that the absorptive structure is interposed between the metal plate and the dipoles and is laid out in contact with the metal plate.

For this purpose, the invention relates to an antenna of the aforementioned type, characterized in that the radiating elements are all substantially planar, the two dipoles being substantially comprised in a same plane, and in that the absorptive structure is interposed between the metal plate and the dipoles and is laid out in contact with the metal plate.

According to other aspects of the invention, the wideband compact antenna comprises one or more of the following features, taken alone or according to all technically possible combination(s): - each radiating element has a general disc sector shape; - said plane is at a distance d from the absorptive structure comprised between 1 mm and 2 mm; - it comprises an impedance matching circuit made in printed technology; - the metal plate comprises a sole, the impedance matching circuit being laid out in said sole; - it also comprises a protective radome; - the absorptive structure has a general cylindrical shape; - the height of the absorptive structure is comprised between 20 mm and 21 mm, and advantageously has the value of 20 mm, and its diameter is comprised between 330 mm and 334 mm, and advantageously has the value of 330 mm; - the dipoles and the absorptive structure are integrally comprised in a cylinder with a diameter substantially equal to 330 mm and with a height substantial equal to 22 mm; - the electromagnetic waves which it is capable of emitting and receiving have frequencies comprised in the whole range of frequencies 30 MHz - 500 MHz, and advantageously in the whole range of frequencies 30 MHz - 700 MHz; - it is capable of emitting and receiving electromagnetic waves having any polarization from among a linear polarization, a circular polarization or an elliptical polarization, each dipole being capable of emitting/receiving electromagnetic waves having a horizontal linear polarization for one of the dipoles and a vertical linear polarization for the other dipole, respectively.

Further, the invention relates to a land, airborne or naval vehicle of the type including: - a planar surface and/or a cavity, - an antenna as described above and laid out on said surface and/or in said cavity.

According to other aspects of the invention, the vehicle comprises one or more of the following features, taken alone or according to all technically possible combination(s): - the planar surface and/or the cavity are made in a metal material.

The invention will be better understood by means of the description which follows, only given as an example and made with reference to the appended drawings wherein: - Fig. 1 is a perspective view of a wideband compact antenna according to a first embodiment of the invention; - Fig. 2 is a sectional view of the antenna of Fig. 1 along the plane II; - Fig. 3 is a curve illustrating the Stationary Wave Ratio of one of the two dipoles of a wideband compact antenna according to the invention versus the frequency in MHz; - Fig. 4 is a curve illustrating the insulation between the two dipoles of a wideband compact antenna according to the invention versus the frequency in MHz; - Fig. 5 is a curve illustrating the gain of one of the two dipoles of a wideband compact antenna according to the invention versus the frequency in MHz; - Fig. 6 illustrates radiation diagrams along the azimuthal plane of one of the two dipoles of a wideband compact antenna according to the invention for frequencies having the values of 30 MHz, 50 MHz, 100 MHz, 300 MHz and 500 MHz respectively; - Fig. 7 is a side view of a wideband compact antenna according to a second embodiment of the invention; - Fig. 8 is a side view of a wideband compact antenna according to the invention comprising a protective radome; and - Fig. 9 is a schematic illustration of the antenna of Fig. 8 laid out in a cavity made in a vehicle.

In all the following, the expressions of «lower» and « upper» are used with reference to the figures and in a non-limiting way.

The antenna according to the invention is intended to emit and receive electromagnetic waves, the frequencies of which are preferentially comprised in the whole range of frequencies 30 MHz - 500 MHz. Advantageously, it is intended to emit and receive electromagnetic waves, the frequencies of which are comprised in the whole range of frequencies 30 MHz - 700 MHz.

With reference to Figs. 1 and 2, the antenna 2 comprises radiating elements 4, an absorptive structure 6 and a metal plate 8. Further, it comprises means 10 for impedance matching and for powering the radiating elements.

The radiating elements 4 are capable of emitting and receiving electromagnetic waves.

For this purpose, the radiating elements 4 are made in an electrically conducting material.

In the example of Fig. 1, the radiating elements 4 are made in printed technology known to one skilled in the art.

The antenna 2 thus comprises four substantially planar radiating elements 4 with a general triangular shape, and more specifically each with the shape of a disc sector. Each radiating element 4 thus has a rounded edge 12 and an apex 14 opposite to said rounded edge 12. Each radiating element 4 has an aperture angle a at its apex 14, the value of which is substantially 45°.

With this value of the aperture angle a, it is possible to optimize the impedance and gain performances of the antenna 2 over the covered bandwidth, while minimizing its size.

The radiating elements 4 are substantially included in a circle C of centre O, the rounded edge 12 of each radiating element substantially belonging to said circle C. Further, the apices 14 opposite to these rounded edges all substantially point towards the point 0.

The radiating elements 4 are all substantially comprised in a same plane P and substantially have the same dimensions.

The radiating elements 4 are distributed in two dipoles 16A, 16B each comprising two diametrically opposite radiating elements 4. Each dipole 16A, 16B is symmetrical relatively to said point 0 and has an axis of symmetry 17A, 17B comprised in the plane P, passing through 0 and coinciding with the bisecting line of the angle at the apex 14 of each of its radiating elements 4.

Each of the two dipoles 16A, 16B is capable of emitting and receiving electromagnetic waves having vertical linear polarization for one of them and a horizontal linear polarization for the other. Emission and reception of electromagnetic waves having any polarization (linear polarization, circular polarization or elliptical polarization) are then obtained by combining both linear polarizations either in an analog way for example by adding a coupling function, or by digital processing, this being known to one skilled in the art.

For this purpose both dipoles 16A, 16B are orthogonal, i.e. their axes of symmetry · 17A, 17B are orthogonal. Further, each radiating element 4 of a dipole 16A adapted for emitting/receiving waves of given linear polarization is then laid out between both radiating elements 4 of the dipole 16B adapted for emitting/receiving electromagnetic waves of the complementary linear polarization, as illustrated in Fig. 1.

With reference to Figs. 1 and 2, the preferential radiation direction of the antenna 2 corresponds to an axis A-A’ perpendicular to the plane P of the radiating elements 4 and passing through the point 0.

Always with reference to Fig. 1, the dipoles 16A, 16B are substantially included in the circle C.

The diameter of the circle C is equal to a fraction of the length of an electromagnetic wave, i.e. the diameter is equal to , wherein λ is the wavelength and n is a strictly positive number.

For an ideal antenna with a small bandwidth centered around a wavelength λ , n is typically selected to be equal to 2.

The dimensioning of the dipoles of this antenna is then, as a general rule, determined by the ratio

independently of the resulting size.

Now, the constraints on size and bandwidth which the antenna 2 is intended to meet, are expressed by a large deviation with this scenario.

In the relevant embodiment, the diameter of the circle C is taken to be substantially equal to 330 mm, n then being comprised approximately between 30 and 1.8 respectively for electromagnetic waves with a frequency ranging from 30 MHz to 500 MHz.

The geometry of the dipoles 16A, 16B notably have the effect of minimizing the volume which they occupy, while having capability of emitting and receiving electromagnetic waves of any polarization from a single antenna 2.

The absorptive structure 6 is capable of improving the impedance matching level , of the antenna 2 and of increasing its directivity by absorbing a portion of the back radiation from the dipoles 16A, 16B of the antenna 2, i.e. radiation emitted in the direction opposite to its preferential radiation direction. Therefore it is capable of optimizing the gain of the antenna, particularly for low frequencies of its frequency band, for example at frequencies comprised between 30 MHz and 200 MHz. Further, it is capable of minimizing bulkiness as regards diameter and thickness of the antenna 2.

For this purpose, the absorptive structure 6 is interposed between the radiating elements 4 and the metal plate 8. It is then both located in proximity to the radiating elements 4 and in contact with the metal plate 8. Further, it comprises an assembly of tiles made from a ferrite type material known to one skilled in the art.

The absorptive structure 6 has a general cylindrical shape with an axis A-A’ and with a diameter substantially equal to the diameter of the circumscribed circle C to the dipoles 16A, 16B, and more particularly comprised between 330 mm and 334 mm.

In the example of Figs. 1 and 2, it has a diameter substantially equal to 330 mm.

Moreover, the absorptive structure 6 has a height substantially comprised between 20 mm and 21 mm, and advantageously substantially equal to 20 mm.

This value corresponds to a good compromise between the radio-electric performances of SWR and of gain between low and high frequencies, the resulting size of the antenna 2, and the absorption properties related to the complex permittivity and complex permeability characteristics of the material of the absorptive structure 6.

The arrangement of the absorptive structure 6 in proximity to the radiating elements 4 and in contact with the metal plate 8 gives the possibility of significantly reducing the influence of the vehicle, to which the antenna 2 is integrated, on the radio-electric performances at low frequencies, notably in the case when the antenna 2 is laid out in a metal cavity.

The absorptive structure 6 is delimited vertically by a substantially planar upper surface 18 and a lower surface 21 both parallel to the plane P. Said plane P is then located at a distance d from said upper surface 18 comprised between 1 mm and 2 mm. Further, the lower surface 21 is positioned in contact with the metal plate 8.

This low value of the distance d has the effect of self-matching the antenna 2 via the matching and supply means 10, and therefore of generating a reduction in the value of the Stationary Wave Ratio of the antenna 2 in particular at low frequencies of its frequency bands, for example at frequencies comprised between 30 MHz and 200 MHz.

The circle C and the absorptive structure 6 both have the same axis of revolution A-A’. In the embodiment of Fig. 1, the dipoles 16A, 16B and said absorptive structure 6 are thus comprised in a cylinder of axis A-A’ with a diameter substantially equal to 330 mm and a height substantially equal to 22 mm.

When the antenna 2 is laid out in a cavity, this notably gives the possibility of minimizing the dimensions of said cavity, as this will be seen subsequently.

The absorptive structure 6 is adapted for letting through the impedance and supply means 10. For this purpose, the absorptive structure 6 delimits a passage orifice 19 for letting through impedance matching and supply means 10 as this will be seen subsequently. This orifice has a general cylindrical shape of axis A-A’ and with a small diameter as compared with the diameter of the absorptive structure 6.

The metal plate 8 provides the functions of a ground plane as well as of a mechanical and electrical interface between the antenna 2 and the structure on which the , antenna 2 is intended to be integrated.

The metal plate 8 is capable of providing a ground reference to the different members of the antenna 2 and is capable of optimizing the directivity of the antenna 2 by contributing to the reduction of the back radiation of the latter.

Further, the metal plate 8 is capable of being laid out in contact with a planar surface of a vehicle, to which the antenna 2 is intended to be integrated.

For this purpose, the metal plate 8 is made from an electrically conducting material known to one skilled in the art.

Further, it has a general discoidal shape of axis A-A' and is laid out in contact with the absorptive structure 6.

In the example of Figs. 1 and 2, the metal plate 8 has a diameter of about 350 mm and thereby delimits a protrusion 20 of a general ring shape extending radially relatively to the absorptive structure 6 and having a width ]_substantially equal to 10 mm.

The metal plate 8 has an upper surface 22 as well as a lower surface 23.

The upper surface 22 is substantially planar and laid out in contact with the lower surface 21 of the absorptive structure 6. It is further parallel to the plane P of the radiating elements 4. Said surface 22 is then at a distance from said plane P equal to a fraction of the length of an electromagnetic wave, i.e. the distance is equal to

wherein λ is the wavelength and m is a strictly positive number.

For an ideal antenna with a small bandwidth centered around a wavelength λ , m is typically selected to be equal to 4 upon considering that the space between the radiated elements and the reflective plane of the ideal antenna is filled with air and therefore with permittivity and permeability eaual to 1. The distance from the metal plate to the dipoles is then determined by the ratio

independently of the resulting size.

Now, the constraints on size and on bandwidth, which the antenna 2 according to the invention meet, are expressed by a large deviation with this scenario.

Thus, in the embodiment of Figs. 1 and 2, the distance from the metal plate 8 to the plane P is substantially taken to be equal to 22 mm, m then being approximately comprised between 450 and 27 for electromagnetic waves with a frequency ranging from 30 MHz to 500 MHz respectively.

The impedance matching and powering means 10 are capable of ensuring impedance matching and powering of the dipoles 16A, 16B of the antenna 2 as well as symmetrizing the currents flowing in the radiating elements 4.

For this purpose, these means 10 comprise two connectors 24, two impedance transformers 26 and electric contacts 28 connecting the radiating elements 4 to the transformers 26. Further, these means 10 comprise electric contacts 30, 32 connecting the connectors 24 and the impedance transformers 26, the reference electric contacts 32 being ground contacts.

The connectors 24 are adapted so as to ensure the electric interface between the antenna 2 and an emissions and/or reception device (not shown) which is associated with it. The connectors 24 are laid out through the metal plate 8 facing the passage orifice 19 of the absorptive structure 6.

In a known way, such connectors 24 are intended to be engaged with coaxial cables (not shown), and then have a core 34 and ground 36 mating those of the coaxial cables to which they'are connected.

In the embodiment of Fig. 2, the core 34 of each connector 24 is connected to an asymmetrical route 40 which each impedance transformer 26 comprises, via an electric contact 30 located in the passage orifice 19. The ground 36 of each connector 24 is connected to a ground route 42 of each transformer 26 via an electric contact 32 also located in the passage orifice 19. The ground 36 of each connector 24 is in electric continuity with the metal plate 8 via an electric contact 31 laid out in contact with the lower surface 23 of the metal plate 8.

Such electric contacts 30, 31, 32 are well known to one skilled in the art and will not be described here.

In a known way, an impedance transformer 26 is adapted in order to maximize power transfer between the dipoles 16A, 16B of the antenna 2 and the emission and/or reception device with which the antenna 2 is associated.

With each dipole 16A, 16B, is associated an impedance transformer 26.

As illustrated in Fig. 2, each impedance transformer 26 is laid out in the passage orifice 19 and comprises two symmetrical routes 38 each connected to one of the radiating elements 4 of the corresponding dipole 16 via an electric contact 28 as well as an asymmetrical route 40 and a ground route 42, as described above.

The electric contacts 28 are also laid out in the passage orifice 19. They are well known to one skilled in the art and will not be described here.

In the antenna 2 according to the invention, the geometry, the dimensions, the properties and the relative positioning of the radiating elements 4, of the absorptive structure 6 and of the metal plate 8 give the possibility of: (i) minimizing the bulkiness of the antenna 2, notably resulting in a highly reduced thickness. The antenna 2 has very small dimensions as compared with the wavelengths of the electromagnetic waves which it is able to emit and receive. Further, the flatness of the radiating elements 4 and the geometries and the dimensions of the absorptive structure 6 and of the metal plate 8 allow the volume occupied by the antenna 2 to be minimized, (ii) maximizing the reduction of the back radiation of the antenna 2. This is obtained by absorbing the back radiation from the dipoles 16A, 16B by means of the absorptive structure 6. By applying the absorptive structure 6 onto the plate 8, it is further possible to attenuate the currents on the metal plate 8 generated by the back radiation of the dipoles 16A, 16B and which, if they were not attenuated, would re-radiate and would interfere with the radiation in the preferential direction of the antenna 2.

The positioning of the absorptive structure 6 in proximity to the radiating elements 4 of the dipoles 16A, 16B gives the possibility of reducing the influence of neighboring objects on the radio-electric performances. The antenna 2 thus has optimized radio-electric performances (impedance, SWR, radiation, directivity and gain) and maximized impedance towards its environment.

These combined features (i) and (n) make the antenna 2 suitable for minimizing the volume of the protruding protuberance which it forms relatively to the vehicle when it is integrated to a surface of the latter, and for minimizing the dimensions of a cavity intended to receive the antenna 2, said cavity being for example made from a metal material.

With reference to Figs. 1 and 2, during operation of the antenna 2, the radiating elements 4 are powered by the emission/reception device associated with the antenna via the impedance matching and powering means 10.

The dipoles 16A, 16B emit and receive electromagnetic waves having any polarization from among a linear, circular or elliptical polarization and having frequencies comprised in the frequency band of the antenna 2.

These waves are then emitted and received preferentially along the emission direction A-A’ of the antenna 2.

With reference to Fig. 3, the SWR of the antenna 2 is less than 2.35 for 1 for a rated impedance of 50 Ohms over the frequency range 30 MHz - 500 MHz, i.e. it has a very good impedance match over its frequency band.

With reference to Fig. 4, which is a representative curve of the decoupling between both dipoles 16A, 16B versus frequency, it is seen that the insulation between the two dipoles 16A, 16B is greater than 30 dB on the frequency band of the antenna 2.

With reference to Fig. 5, it is seen that the gain obtained on one of the dipoles is greater than -8 dBi over the frequency range 200 MHz - 500 MHz, greater than -5 dBi over the frequency range 230 MHz - 470 MHz. Further, this gain has the value of -35 dBi at 30 MHz, -17 dBi at 100 MHz, and -12 dBi at 150 MHz.

Finally, with regard to Fig. 6, the antenna 2 has quasi-unidirectional radio-electric coverage over its frequency band.

With reference to Fig. 7, in a second embodiment of the invention, the impedance matching and supply means 10 are integrated to an impedance matching circuit 44, except for the electric contacts 28 connecting the dipoles 16A, 16B to said impedance matching circuit 44 and the connectors 24.

This circuit 44 is made in printed technology, known to one skilled in the art, and is then positioned in a sole 46 which the metal plate 8 comprises.

In practice, the sole 46 comprises a cavity 461 dedicated for this purpose and which is accessible via a removable metal cover 462.

The metal plate 8 then delimits four passage orifices 48 of cylindrical shape, located facing the passage orifice 19 of the absorptive structure 6.

The passage orifices 48 are spaced apart angularly by 90° from each other and are each intended for letting through an electric contact 28.

The electric contacts 28 are then positioned in the passage orifices 19, 48 so as to connect the dipoles 16A, 16B and the circuit 44.

The sole 46 has a general cylindrical shape with an axis A-A’ and a diameter of less than or equal to the diameter of the metal plate 8 and comprises a lower surface 50.

The connectors 24 are then attached to the impedance matching circuit 44 in the sole 46 so as to protrude from the lower surface 50, as illustrated in Fig. 7.

The electric contacts 31 ensure electric continuity between the ground 36 of each connector 24 and the removable metal cover 462.

With reference to Fig. 8, the antenna 2 also comprises a radome 52 capable of protecting said antenna 2 and of allowing the passage of electromagnetic radiations emitted and received by the antenna2.

For this purpose, the radome 52 has a general cylindrical shape and is made from a material known to one skilled in the art, of the epoxy glass, polyamide or further PEEK type, etc.,

The radome 52 is delimited radially by a side wall 54 having a thickness of less than or equal to the width I of the protrusion 20 and vertically by a transverse wall 56 of ' discoidal shape.

The radome 52 is thus able to be attached on the protrusion 20 in a protective position illustrated in Fig. 8 and in which its axis coincides with the axis A-A’.

The radome 52 delimits a cylindrical cavity 58 with dimensions mating the dimensions of the cylinder in which are comprised the dipoles 16A, 16B and the absorptive structure 6.

In practice, the dimensions of the cavity 58 increased by distances ε and s' ranging from the order of 1 millimeter to a few millimeters, and corresponding to a gap respectively existing between the dipoles 16A, 16B and the transverse wall 56 and between the absorptive structure 6 and the side wall 54 of the radome 52 in the protective position of the latter.

In the example of Fig. 8, this cavity 58 therefore has a diameter of about 330+ 2.ε' mm and a height of about 22+ ε mm.

Still in the example of Fig. 8 which illustrates the antenna 2 according to the second embodiment, the side wall 54 is attached on the protrusion 20 by attachment means (not shown) so that the dipoles 16A, 16B and the absorptive structure 6 are entirely comprised in the cavity 58, as illustrated in Fig. 8.

As illustrated in Fig. 8, the antenna 2 provided with its radome 52 is comprised in a cylinder of axis A-A' and with a diameter substantially equal to 350 mm.

With reference to Fig. 9, the antenna 2 provided with its radome 52 is able to be laid out on a planar surface 60 of a cylindrical cavity 62 made for this purpose in a vehicle 64, the metal plate 8 being in contact with said surface 60.

In the example of Fig. 9, which illustrates an antenna 2 according to the second embodiment of the invention, it is then the lower surface 50 of the sole 46 of the metal plate 8 which is in contact with the planar surface 60.

The cylindrical cavity 62 has a diameter substantially equal to the diameter of the antenna 2 and a height substantially equal to the height of the radome 52, to which is added the height of the metal plate 8.

Preferably, said surface 60 and said cylindrical cavity 62 are made from a metal material.

In the surface 60 an aperture 66 is made for connecting via the connectors 24, the impedance matching and supply means 10 to the emission/reception device (not shown) of the antenna 2 which is associated with it.

Where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

Claims (13)

1. - An antenna for emitting/receiving electromagnetic waves, of the type comprising: - two dipoles orthogonal to each other, each dipole comprising two radiating elements, - a metal plate, and - an absorptive structure, wherein the metal plate provides the functions of a ground plane as well as of a mechanical and electrical interface between the antenna and the structure on which the antenna is intended to be integrated in that the radiating elements are all substantially planar, both dipoles being substantially comprised in a same plane and in that the absorptive structure is interposed between the metal plate and the dipoles and is laid out in contact with the metal plate.

2. - The antenna according to claim 1, wherein each radiating element has a general disc sector shape.

3. - The antenna according to claim 1 or 2, wherein said plane is at a distance d from the absorptive structure, comprised between 1 mm and 2 mm.

4. - The antenna according to any one of the preceding claims, wherein it comprises an impedance matching circuit made in printed technology.

5. - The antenna according to claim 4, wherein the metal plate comprises a sole, the impedance matching circuit being laid out in said sole.

6. - The antenna according to any one of the preceding claims, wherein it also comprises a protective radome.

7. - The antenna according to any one of the preceding claims, wherein the absorptive structure has a general cylindrical shape.

8. - The antenna according to claim 7, wherein the height of the absorptive structure is comprised between 20 mm and 21 mm, and advantageously has the value of 20 mm, and its diameter is comprised between 330 mm and 334 mm, and advantageously has the value of 330 mm.

9. - The antenna according to any one of the preceding claims, wherein the dipoles and the absorptive structure are entirely comprised in a cylinder with a diameter substantially equal to 330 mm and with a height substantially equal to 22 mm.

10. - The antenna according to any one of the preceding claims, wherein the electromagnetic waves which it is capable of emitting and receiving, have frequencies comprised in the whole range of frequencies 30 MHz - 500 MHz, and advantageously in the whole range of frequencies 30 MHz - 700 MHz.

11. - The antenna according to any one of the preceding claims, wherein it is able to emit and receive electromagnetic waves having any polarization from among a linear polarization, a circular polarization or an elliptical polarization, each dipole being capable of emitting/receiving electromagnetic waves having a horizontal linear polarization for one of the dipoles and a vertical linear polarization for the other dipole respectively.

12. - An land, airborne, or naval vehicle of the type including: - a planar surface and/or a cavity, - an antenna according to any one of the preceding claims and laid out on said surface and/or in said cavity.

13. - The vehicle according to claim 12, wherein the planar surface and/or the cavity are made from a metal material.