AU2013200057A1 - A wide band compact antenna with dual linear polarization - Google Patents

Abstract

A wide band compact antenna with dual linear polarization An antenna (2) for emitting/receiving electromagnetic waves, of the type comprising a reflective plane (12), an absorptive surface (10) and two orthogonal dipoles (18) each comprising two radiating elements (4). The radiating elements (4) are substantially planar and each have a general triangular shape. Fig. 1 22 24 3224 22 1s 1 4 24 16 32 - '0 3

Description

AUSTRALIA Patents Act COMPLETE SPECIFICATION (ORIGINAL) Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: Name of Applicant: Thales and Universite de Rennes 1 Actual Inventor(s): Olivier Clauzier, Franck Colombel, Mohammed Himdi, Cyrille Le Meins Address for Service and Correspondence: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: A WIDE BAND COMPACT ANTENNA WITH DUAL LINEAR POLARIZATION Our Ref: 960471 POF Code: 10160/522852, 522854 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): 6-01 so-I- 2 A wide band compact antenna with dual linear polarization This application claims priority from French Application No. 11 04120 filed on 27 December 2011, the contents of which are to be taken as incorporated herein by this 5 reference. The invention relates to a compact antenna. More particularly, the invention relates to an antenna for emitting/receiving electromagnetic waves, of the type comprising: 10 - two orthogonal dipoles, each dipole comprising two radiating elements, - a reflective plane, and - an absorptive surface. The invention is located in the field of antennas and of antenna systems dedicated to applications for receiving and emitting electromagnetic waves in a very wide frequency 15 band. For example, the compact antenna operates 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 telecommunications, and are notably intended to be carried away on a vehicle, whether ?0 this be a land, airborne or naval vehicle. Consequently, these antennas are subject to a certain number of specific technical constraints. Thus, these antennas for example have to have visual discreetness or a low RES, for Radar Equivalent Surface, have high radio-electric performances such as a low SWR 25 for Stationary Wave Ratio, a high gain, etc., while having small dimensions. In certain cases, they also have to be adapted for receiving, emitting and/or discriminating electromagnetic waves regardless of their polarization. Further, they should have unidirectional radio-electric coverage. Finally, they should not degrade aerodynamics or roadway clearance of a vehicle 30 to which they are integrated and should have radio-electric performances, independent of the latter. Antennas meeting some of these constraints are known from the state of the art, for example in communications applications. Thus, US 7,692,603 B1 describes an antenna for which the radiating elements are 35 spiral. Such an antenna has small dimensions suitable for minimizing electromagnetic couplings and interferences with neighboring objects.

3 Further, there exist directional antennas of the monopole or dipole type with linear polarization, mainly with vertical linear polarization allowing coverage of the band comprised between 30 MHz and 500 MHz. However, these solutions do not give entire satisfaction. 5 Indeed, most antennas of the state of the art only have part of the features described above. Thus, most of the antennas known to one skilled in the art have a size which is incompatible with the criteria: o of visual discreetness, 0 o and/or reduction of RES, o and/or optimization of the aerodynamics of the vehicle to which they are integrated, o and/or observance of the road clearance of said vehicle. Further, the radio-electric performances of omnidirectional antennas of the dipole 5 type are substantially degraded when they are integrated to vehicles having metal portions located in the proximity of said antennas. Moreover, omnidirectional antennas of the monopole type require a sufficiently large ground plane as compared with the wavelength in order to obtain optimum radio-electric performances, the latter further being dependent on the surface area of the !0 vehicle on which they are laid out. These antennas thus have radio-electric performances impacted by the surface on which they are laid out. Further, they do not give the possibility of treating the multi-polarization aspect of electromagnetic waves. Finally, although these antennas have reduced dimensions as compared with dipole antennas, the latter remain sometimes incompatible with visual discreetness, RES, aerodynamics and/or road 25 clearance criteria. Also, spiral antennas have a limitation of the low frequency of use located around 200 MHz with regard to the size targeted by the object of the present application. Further, these antennas are mostly with right or left circular polarization and do not give the possibility of treating the multi-polarization aspect of electromagnetic waves from a single 30 antenna. Further, these antennas generally have gain and SWR values as well as a unidirectional nature of the radiation, which are insufficient at low frequencies. The latter type of antenna finally has radio-electric performances which are particularly degraded at low frequencies, i.e. below about 200 MHz, in the case when they have to be placed in a metal cavity intended to improve visual discreetness, RES and/or 35 aerodynamics of the vehicle to which they are integrated. The performance degradation level varies according to the diameter of this cavity.

4 The object of the invention is therefore to obtain an antenna with which meeting of these criteria may be improved. For this purpose, the invention relates to an antenna of the aforementioned type characterized in that the radiating elements are substantially planar and each have a 5 general triangular shape. According to other aspects of the invention, the wide band compact antenna comprises one or more of the following features, taken alone or according to any technically possible combination(s): - the radiating elements are all substantially comprised in a same plane; 10 - each radiating element has a slightly rounded free edge, the dipoles being substantially included in a circle, the free edge of each radiating element belonging to said circle; - each radiating element comprises an apex opposite to its rounded free edge, said apex of each radiating element being substantially oriented towards the center of said 15 circle; - each radiating element is laid out between the two radiating elements of the other dipole, two successive radiating elements being connected through a coiled crown; - the coiled crown comprises connection coils, each connection coil being connected to two successive radiating elements; 20 - a connection coil has two ends each connected via a resistor to one of the radiating elements to which the connection coil is connected; - the coiled crown comprises coiled crown portions, each coiled crown portion connecting two successive radiating elements and having several adjacent connection coils, a resistor being positioned between two adjacent connection coils; 25 - it is entirely comprised in a cylinder with a diameter substantially equal to 350 mm and with a height substantially equal to 150 mm; - it is suitable for emitting/receiving electromagnetic waves, the frequencies of which are comprised in the whole range of frequencies 30 MHz - 500 MHz, and advantageously in the whole range of frequencies 30 MHz - 800 MHz; 30 - it is capable of emitting and receiving electromagnetic waves having a polarization from among any 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. 35 Further, the invention relates to an land, airborne or naval vehicle of the type including: 5 - a planar surface and/or a cavity arranged in the vehicle, - an antenna as described above laid out on the planar surface and/or in the cavity. According to other aspects of the invention, the vehicle may comprise the following 5 feature: - the planar surface and/or the cavity are made from 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 illustrates a top view of a first embodiment of an antenna according to the 0 invention, - Fig. 2 illustrates a view of a portion of a connection coil of the antenna of Fig. 1, - Fig. 3 illustrates a top view of the absorptive surface in an antenna according to the invention, - Fig. 4 illustrates a side view of the first embodiment of an antenna according to 15 the invention, - Fig. 5 is a diagram illustrating the stationary wave ratio versus frequency for an antenna according to the first embodiment of the invention, - Fig. 6 is a diagram illustrating the gain of an antenna according to the first embodiment of the invention versus frequency, !0 - Fig. 7 illustrates radiation diagrams along the azimuthal plane of an antenna according to the first embodiment of the invention for frequencies with values of 30 MHz, 50 MHz, 100 MHz, 300 MHz and 500 MHz, - Fig. 8 illustrates a top view of a second embodiment of an antenna according to the invention, 25 - Fig. 9 is a diagram illustrating the stationary wave ratio versus frequency in an antenna according to the first and second embodiments of the invention, - Fig. 10 is a diagram illustrating the gain versus frequency of an antenna according to the first and second embodiments of the invention, - Fig. 11 illustrates a radiation diagram along the azimuthal plane of an antenna 30 according to the second embodiment for frequencies with values of 30 MHz, 50 MHz, 100 MHz, 300 MHz and 500 MHz and - Fig. 12 is a side view of the antenna of Fig. 4 positioned in a cavity. The antenna according to the invention 2 is intended to be attached on a surface of a mobile vehicle, for example of an element or structure of said vehicle. 35 Advantageously, the antenna 2 is intended to be attached in a metal cavity made in the skin of said vehicle and provided for this purpose.

6 The antenna 2 is intended for emitting and receiving electromagnetic waves, the frequencies of which are comprised in the whole range of frequencies 30 MHz - 500 MHz. Alternatively, it is intended for emitting and receiving electromagnetic waves, the frequencies of which are comprised in the whole range of frequencies 30 MHz - 800 MHz. 5 With reference to Fig. 1 and to Fig. 4, which illustrate a top view and a side view respectively of an antenna 2 in a first embodiment of the invention, the antenna 2 comprises a plurality of radiating elements 4 and a loaded coiled crown 6 (i.e. it comprises resistors as this will be seen subsequently) associated with the radiating elements 4 of the antenna 2. Further, it comprises means 8 for matching impedance and powering the 10 radiating elements 4, an absorptive surface 10, as well as a reflective plane 12. The radiating elements 4 are capable of receiving and emitting electromagnetic waves. For this purpose, the radiating elements 4 are made from an electrically conducting material. 15 In the embodiment of Fig. 1, the radiating elements 4 are made in printed technology known to one skilled in the art. With reference to Fig. 1, the antenna according to the invention 2 comprises four radiating elements 4. These elements 4 are planar, coplanar and each have a general triangular shape. 20 By (( general triangular shape ) is meant the shape of a triangle, a triangle for which one or several sides have slight roundedness, or further a triangle for which one or more apices are rounded, (( chipped a or g blunt )). In the example illustrated in Fig. 1, each radiating element 4 has three apices. Further, each radiating element 4 comprises a slightly rounded free edge 14 and an apex 25 16 opposite to said rounded edge 14. These radiating elements 4 are substantially comprised in a same plane P. Further, all substantially have the same dimensions. In the example of Fig. 1, the radiating elements 4 moreover have an aperture angle of about 40* allowing optimization of the impedance and gain performances of the 30 antenna 2 over the covered band width, while minimizing the size of the antenna, as this will be seen subsequently. The radiating elements 4 are distributed in two dipoles 18 each including two radiating elements 4. Each dipole 18 is capable of emitting or receiving electromagnetic waves having a 35 vertical linear polarization for one of them and horizontal linear polarization for the other. Emission and reception of waves of any polarization (linear polarization or circular 7 polarization or elliptical polarization) are then obtained by combining both the linear polarizations in an analog way by for example adding a coupling function or by digital processing, this being known to one skilled in the art. For this purpose, the dipoles 18 are orthogonal. Both radiating elements 4 of the 5 dipole 18 suitable for a given linear polarization are laid out between both radiating elements 4 of the other dipole 18, two successive radiating elements 4 being connected through the loaded coiled crown 6, as this will be seen subsequently. Both dipoles 18 are substantially included in a circle K of center 0, the rounded free edge 14 of each radiating element 4 belonging to this circle. Further, the opposite 0 apex 16 of each radiating element 4 is oriented towards the center 0 of the circle K. The dipoles 18 are thus symmetrical with respect to the center 0 of the circle K. The diameter of the circle K is equal to a fraction of the length of an electromagnetic wave, i.e. the diameter is equal to , wherein A is the wavelength and n is a strictly positive number. 5 For an ideal antenna with a small band width centered around a wavelength A , n is typically selected to be equal to 2. The dimensioning of the dipoles is then determined as a general rule by the ratio 2 independently of the resulting size. Now, the constraints on the size and bandwidth which the antenna 2 according to .0 the invention meets, are expressed by a substantial deviation with this scenario. Thus, in the relevant embodiment, the diameter of the circle K is taken to be substantially equal to 330 mm, n therefore being comprised approximately between 30 and 1.8, for electromagnetic waves with a frequency ranging from 30 MHz to 500 MHz respectively. 25 In certain embodiments, the variation range of n is adjustable according to the frequency band and to the sought radio-electric performances. In a known way, an antenna for which the radiating elements have small dimensions as compared with the length of the electromagnetic wave which they are intended to capture and/or emit have degraded radio-electric properties at the frequencies 30 corresponding to the wavelength. Thus, the coiled crown 6 is able to impart an inductance behavior to the antenna 2, with which it is possible to improve the gain values of the antenna 2 at low frequencies, particularly at frequencies below 300 MHz.

8 Further, the coiled crown 6 is able to optimize impedance matching and the gain of the antenna 2 at low frequencies. For this purpose, the coiled crown 6 comprises a plurality of connection coils 20 arranged between the radiating elements 4. 5 In the embodiment of Fig. 1, the coiled crown 6 thus comprises four connection coils 20 each connected on either side to two adjacent radiating elements 4. Each connection coil 20 is arranged between two radiating elements 4 so as to describe the arc of the circle K circumscribed to the dipoles 18 comprised between both radiating elements 4 to which it is connected. 10 As illustrated in Fig. 1, each coil 20 comprises a plurality of turns with constant diameter and pitch. Each of its turns comprises a point having a maximum distance to the point 0. The connection coils 20 are then positioned so that the circle K circumscribed to the dipoles 18 is substantially circumscribed to the whole of these points for a given connection coil 20, and this for each of the connection coils 20 which the coiled crown 6 15 comprises. The coiled crown 6 further comprises resistors 22 able to optimize impedance matching of the antenna 2. For this purpose, a resistor 22 is inserted between each end 24, which a connection coil 20 has, and the radiating element 4 to which the latter is connected. ?0 In the example of Fig. 1, the value of each resistor 22 is for example substantially equal to 300 Ohms. The value of the resistor 22 is adjustable depending on the frequency band and on the sought radio-electric performances. In a known way and with reference to Fig. 2, which illustrates a view of a connection coil 20, each connection coil 20 generates significant resonance around a 25 frequency value fo , this frequency fo a so-called (( resonance frequency of the connection coil 20 a being defined by: 29.85.( ) N.D fo being in MHz, H being the length of the connection coil 20 (in m), D the diameter of the connection coil 20 (in m) and N the number of wound wire turns of the connection coil 30 20. In the embodiment of Figs. 1 and 2, the diameter D of a connection coil 20 is equal to 20 mm, its pitch (the distance between two successive turns of the connection coil 20) 9 is taken to be equal to 5 mm, and the number of turns N of each connection coil 20 being taken equal to 26, which gives a length H of about 150 mm. The relationship (1) then gives a resonance frequency of the connection coil 20 of about 90 MHz. 5 This resonance then causes degradation of the SWR performances of the antenna 2 for the electromagnetic waves with a frequency close to the resonance frequency of the connection coils 20. It is conceivable that it is of interest to shift this resonance frequency by changing the dimensions of the connection coils 20 used, as this will be seen subsequently in a 10 second embodiment of the invention. In a known way to one skilled in the art, the absorptive surface 10 is capable of optimizing the impedance matching level, of increasing the directivity of the antenna 2 and therefore improving the gain of the antenna 2 at low frequencies. Further, this absorptive surface 10 gives the possibility of reducing the vertical size 15 of the antenna 2 as well as the impact of the surface onto which the antenna is attached, on the radio-electric performances of the antenna according to the invention 2. For this purpose, the absorptive surface 10 is made from a material of the ferrite type and is located in proximity to the radiating elements 4. The absorptive surface 10 thus absorbs a portion of the radiation emitted by the 0 antenna 2 in a direction opposite to its preferential radiation direction. With reference to Figs. 3 and 4, Fig. 3 illustrating a top view of the absorptive surface 10, in the relevant case, the latter has a generally octagonal shape and is comprised in a plane substantially parallel to the plane P of the radiating elements 4. The absorptive surface 10 is at a distance from the plane P of the radiating elements 4, 25 substantially equal to 15 mm. The absorptive surface 10 comprises a plurality of absorptive surface portions 26. In the example of Fig. 3, the absorptive surface 10 comprises nine absorptive surface portions 26 with a generally square shape and with a side length taken equal to 100 mm. 30 The absorptive surface 10 is thus comprised in a square of side 300 mm, the center C of which belongs to an axis A-A' perpendicular to the plane P and passing through 0. The four absorptive surface portions 26 located at the angles of this square have a bevel with a width about equal to 70 mm. With reference to Fig. 4, the absorptive surface 10 has a thickness of about 7 mm. 35 In a known way, the reflective plane 12 of the antenna according to the invention 2 is capable of providing a ground reference and of reflecting at high frequencies of the 10 band of the antenna 2, for example above about 350 MHz, a portion of the electromagnetic radiation emitted by the antenna 2 in a direction opposite to its preferential radiation direction - the latter being along the A-A' axis and, with reference to Fig. 4 in the direction of travel of the A-A' axis from its lower portion of the figure towards 5 the upper portion of the figure -, and thus increasing the radio-electric performances of the antenna 2 at high frequencies. Further, the reflective plane 12 contributes to minimizing the influence of the vehicle to which is integrated the antenna 2 on the radio-electric performances of the latter. 10 Thus, the reflective plane 12 is parallel to the plane P and distant from the latter by a distance equal to a fraction of the length of an electromagnetic wave, i.e. the distance is equal to , wherein A is the wavelength and m is a strictly positive number. For an ideal antenna with a small band width centered around a wavelength L , m is typically selected to be equal to 4. The distance from the reflective plane to the dipoles 15 is then determined by the / ratio, independently of the resulting size. Now, the constraints on size and on band width, met by the antenna 2 according to the invention, are expressed by a large deviation with this scenario. Thus, in the relevant embodiment, the distance from the reflective plane 12 to the plane P is substantially taken to be equal to 150 mm, m then being approximately !0 comprised between 67 and 4, for electromagnetic waves of frequency ranging from 30 MHz to 500 MHz, respectively. In certain embodiments, the variation range of m is adjustable according to the frequency band and to the sought radio-electric performances. The reflective plane 12 has a general circular shape with a central axis A-A' and 25 with a diameter of about 350 mm. The radiating elements 4, the reflective plane 12 and the absorptive surface 10 are parallel with each other and centered around the axis A-A'. With reference to Fig. 4, the antenna according to the invention 2 thus has dimensions such that it is substantially comprised in a cylinder of axis A-A', of diameter 30 350 mm and of height 150 mm. Further, the arrangement of the reflective plane 12 and of the absorptive surface 10 with respect to the dipoles 18 is able to minimize the interferences between the main radiation of the antenna 2 and the portion of the radiation in the direction opposite to the 11 preferential radiation direction of the antenna 2 which is reflected against the surfaces or neighboring objects and then interferes with the main radiation of the antenna 2. The combination of the reflective plane 12 and of the absorptive surface 10 is thus able to minimize the impact on the radio-electric performances of the surface or of the 5 cavity provided for receiving the antenna 2. More particularly, this combination gives the possibility of minimizing this impact when said surface or said cavity is made from a metal material. The impedance matching and powering means 8 for the antenna 2 are able to ensure impedance matching and powering of the dipoles 18 of the antenna 2 as well as 10 making the currents symmetrical in the radiating elements 4. For this purpose, the means 8 comprise two connectors 28, two impedance transformers 30, electric contacts 32 placed between the radiating elements 4 and the transformers 30. Further, these means 8 comprise electric contacts 34, 36 placed between the connectors 28 and the transformers 30, the reference electric contacts 36 15 being ground contacts. The connectors 28 are suitable for ensuring the electric interface between the antenna 2 and an emission and/or reception device (not shown) which is associated with it. In a known way, such connectors 28 are intended to be engaged with coaxial !0 cables (not shown) for example, and then have a core 38 and a ground 40 mating those of the coaxial cables to which they are connected. In the embodiment of Fig. 4, the core 38 of each connector 28 is connected to an asymmetrical route 44 which each impedance transformer 30 comprises, via an electric contact 34, and the ground 40 of each connector 28 is connected to a ground route 46 of 25 each transformer 30 via an electric contact 36, the ground 40 of each connector 28 being in electric continuity with the reflective plane 12 via an electric contact 35. Such electric contacts 34, 35, 36 are well known to one skilled in the art and will not be described here. In a known way, an impedance transformer 30 is adapted so as to maximize power 30 transfer between the dipoles 18 of the antenna 2 and the emission and/or reception device with which the antenna 2 is associated. With each dipole 18 is associated an impedance transformer 30. As illustrated in Fig. 4, each impedance transformer 30 comprises two symmetrical routes 42 each connected to one of the radiating elements 4 of the corresponding dipole 35 18 via an electric contact 32, as well as an asymmetrical route 44 and a ground route 46, as described above.

12 The reference electric contacts 32 are well known to one skilled in the art and will not be described here. With reference to Fig. 5, which is a curve illustrating the SWR of the antenna 2 according to a first embodiment, versus frequency, the antenna 2 has stationary wave 5 ratio values of less than or of the order of 3 for frequencies above 200 MHz, i.e. it has good impedance matching properties over a wide band of frequencies. Moreover, with reference to Fig. 6, which is a curve illustrating the gain of the antenna 2 according to this embodiment, versus frequency, the antenna 2 according to this embodiment of the invention has a gain substantially equal to -16 dBi at 100 MHz, 0 -6 dBi at 200 MHz and becomes positive beyond 310 MHz. The gain is further comprised between -38 dBi and -16 dBi between 30 MHz and 100 MHz. Fig. 7 provides radiation diagrams in the azimuthal plane of an antenna 2 according to this first embodiment of the invention for frequencies which have the value of 5 30, 50,100, 300 and 500 MHz. It is seen on these diagrams that such an antenna 2 has a main radiation lobe, i.e. radiation in its preferential direction, which is stable versus frequency as well as a good front/back ratio, and this even at frequencies of less than or equal to 100 MHz. Further, the reflective plane 12 and the absorptive surface 10 contribute to optimizing the !0 impedance, the directivity and therefore the gain in the band of use of the antenna 2, as described above. As described above and with reference to Fig. 12, which is a side view of an antenna 2 positioned in a cavity 43, the presence of the absorptive surface 10 and of the reflective plane 12 thus makes the antenna 2 able to be positioned both on a surface 41 25 and in a cavity 43 made in the skin 45 of a land, naval or airborne vehicle 47, so that the radiating elements 4 are substantially flush with the aperture of the cavity 43. Thus, the antenna 2 according to this embodiment of the invention: - has a small size relatively to the wavelength of use, - has a wide passband, 30 - is visually discreet, - has good radio-electric performances for frequencies comprised between 30 and 500 MHz with regard to the set constraints, - is able to process electromagnetic waves regardless of their polarization, 35 - has a radiation allowing quasi-unidirectional radio-electric coverage over a wide frequency band, 13 - reduces the dependency towards the surface of the vehicle on which it is laid out, and - is advantageously suitable for being installed in a metal cavity, as illustrated in Fig. 12. 5 It is thus suitable for simultaneously meeting many constraints which the antennas of the state of the art only meet partially. Alternatively, an antenna 2 according to a second embodiment is contemplated wherein the radio-electric performances are further improved. Indeed, as illustrated in Fig. 5, the value of the SWR is greater than 3 for 0 frequencies comprised between 30 and 200 MHz and greater than 5 for frequencies comprised between 50 and about 150 MHz. This value of the SWR results from the presence of connection coils 20 which degrade the impedance matching of the antenna 2 around their resonance frequency. This alternative of the invention is then advantageously used for improving the 15 value of the stationary wave ratio at low frequencies. As illustrated in Fig. 8, which is a top view of the antenna 2 according to the second embodiment, the coiled crown 6 comprises four coiled crown portions 48. Each coiled crown portion 48 is positioned between two adjacent radiating elements 4 and has four adjacent connection coils 50. !0 Between two adjacent coils 50 is inserted a resistor 52. The value of the latter is for example substantially equal to 300 Ohms. This value is adjustable depending on the frequency band and on the sought radio-electric performances. The number of turns of each connection coil 50 is reduced as compared with the first embodiment of the invention, as illustrated in Fig. 8. 25 With reference to formula (1), the number of turns of each connection coil 50 is for example reduced from 26 to 6. The connection coils 50 then have a resonance frequency around 300 MHz. This shift in the resonance frequency of the connection coils 50 towards high frequencies is advantageous, insofar that from this frequency of 300 MHz, the radiation of 30 the antenna 2 is ensured by the dipoles 18 exclusively. The degraded SWR performances at low frequencies of the antenna 2 according to the first embodiment of the invention because of the resonance of the connection coils, may thus be improved. With reference to Figs. 9 and 10, which are curves illustrating the SWR and the 35 gain of an antenna 2, respectively according to both embodiments over the frequency range, it is then seen that the dimensions of the connection coils 50 according to the 14 second embodiment of the antenna 2 of the invention will have the value of the SWR drop from more than 9 at 100 MHz in the first embodiment of the antenna 2 according to the invention to about 5.2 at about 100 MHz in the second embodiment of the antenna 2 according to the invention. 5 Further, the gain of the antenna 2 remains substantially identical versus frequency regardless of the embodiment of the antenna 2 of the invention, the value of the gain being substantially equal to -16 dBi at 100 MHz, -6 dBi at 200 MHz and becoming positive beyond 330 MHz according to the second embodiment of the antenna 2 according to the invention. 0 Fig. 11 provides radiation diagrams of the antenna 2 according to the second embodiment of the invention. It is seen that the radiation diagrams are substantially similar to the radiation diagrams of an antenna 2 according to the first embodiment, the antenna 2 according to the second embodiment of the invention thereby having radio-electric directivity properties 5 similar to those of the antenna 2 according to the first embodiment. Alternatively, (not shown), the radiating elements 4 of the dipoles 18 of the antenna 2 according to both embodiments of the invention are dimensioned so as to be able to emit and/or receive electromagnetic waves with a frequency comprised between 30 MHz and 800 MHz. !0 Beyond 800 MHz, the radio-electric performances of the antenna 2 are degraded because of the limitations related to the diameter and height dimensions of the antenna 2 towards the wavelengths at high frequencies and to the pass band of the impedance transformer 30 used. In another alternative (not shown), the antenna 2 according to the invention is 25 protected by a radome, the shape and the material of which are determined according to criteria known to one skilled in the art.

Claims (7)

1.- An antenna (2) for emitting/receiving electromagnetic waves, of the type comprising: 5 - two orthogonal dipoles (18), each dipole (18) comprising two radiating elements (4), - a reflective plane (12) and - an absorptive surface (10), characterized in that the radiating elements (4) are substantially planar and each 10 have a general triangular shape.

2.- The antenna (2) according to claim 1, characterized in that the radiating elements (4) are all substantially comprised in a same plane (P). 15 3.- The antenna (2) according to claim 1 or 2, characterized in that each radiating element (4) has a slightly rounded free edge (14), the dipoles (18) being substantially included in a circle (K), the free edge (14) of each radiating element (4) belonging to said circle (K). 20 4.-.The antenna (2) according to claim 3, characterized in that each radiating element (4) comprises an apex (16) opposite to its rounded free edge (14), said apex (16) of each radiating element (14) being substantially oriented towards the center (0) of said circle (K). 25 5.- The antenna (2) according to any of the preceding claims, characterized in that each radiating element (4) is laid out between both radiating elements (4) of the other dipole (18), two successive radiating elements (4) being connected through a coiled crown (6). 30 6.- The antenna (2) according to claim 5, characterized in that the coiled crown (6) comprises connection coils (20), each connection coil (20) being connected to two successive radiating elements (4).

7.- The antenna (2) according to claim 6, characterized in that a connection coil 35 (20) has two ends (24), each connected via a resistor (22) to one of the radiating elements (4) to which the connection coil (20) is connected. 16

8.- The antenna (2) according to claim 5, characterized in that the coiled crown (6) comprises coiled crown portions (48), each coiled crown portion (48) connecting two successive radiating elements (4) and having several adjacent connection coils (50), a resistor (52) being positioned between two adjacent connection coils (50). 5

9.- The antenna (2) according to any of the preceding claims, characterized in that it is entirely comprised in a cylinder with a diameter substantially equal to 350 mm and with a height substantially equal to 150 mm. 0 10.- The antenna (2) according to any of the preceding claims, characterized in that it is suitable for emitting/receiving electromagnetic waves, the frequencies of which are comprised in the whole range of frequencies 30 MHz - 500 MHz, and advantageously in the whole range of frequencies 30 MHz - 800 MHz. 5 11.- The antenna (2) according to any of the preceding claims, characterized in that it is able to emit and receive electromagnetic waves having a polarization from among any linear polarization, a circular polarization or an elliptical polarization, each dipole (18) being able to emit/receive electromagnetic waves having a horizontal linear polarization for one of the dipoles (18) and a vertical linear polarization for the other dipole (18), 0 respectively.

12- A land, airborne or naval vehicle (47) of the type including: - a planar surface (41) and/or a cavity (43) arranged in the vehicle (47), - an antenna (2) according to any of the preceding claims laid out on the 25 planar surface (41) and/or in the cavity (43).

13.- The vehicle (47) according to claim 12, characterized in that the planar surface (41) and/or the cavity (43) are made from a metal material.