Field of the invention The invention relates in a general manner to antennas, and in particular to 5 omnidirectional antennas. Prior art Marine platforms (for example surface boats) are generally equipped with immersed sonar antennas for detecting and/or pinpointing objects under the water. A sonar antenna comprises an assembly of stacked transducers ensuring the 10 emission of the acoustic signals and mounted on a support. The reception of the signals is performed by an assembly of receivers (for example hydrophones) arranged according to a chosen configuration with respect to the configuration of the assembly of the emission transducers. In existing embodiments, the antenna has a generally cylindrical or spherical 15 shape and comprises an assembly of elementary emission transducers (piezoelectric rings) superposed along the axis of the antenna, each transducer having a ring shape as described in application FR2 776 161. Such transducers can be of "Tonpilz" type and ensure both emission and 2 o reception. However, the diameter of the rings being related to the desired emission frequency, the lower the desired frequency, the larger the ring must be. Such antennas are therefore bulky and have a relatively significant weight. Moreover, transducers of "Tonpilz" type make it necessary to equip the active element (piezoelectric, magneto- or electro-strictive material) with bulky mechanical 25 components (rear seismic mass, pavilion and leaktight casing in particular). Such an antenna architecture is therefore unsuitable for the design of low-frequency antennas for surface vessels of low tonnage (in particular less than 1500 Tonnes in mass) or for submarines of low tonnage (in particular less than 6000 Tonnes in mass). 30 wo 2016/046377 2 PCT /EP20 15/072131 In another known approach, the omnidirectional sonar antenna comprises a vertical array of compact transducers of "flex-tensor" type operating in a reduced frequency band in active mode (1800-2300 Hz). This type of antenna is dedicated to emission alone. This architecture is sufficiently compact and exhibits a relatively 5 low weight. However, antennas of this type do not make it possible to obtain the frequency band width necessary for modern wide-band sonars. Another known architecture of omnidirectional sonar antenna comprises a vertical array of active emission rings, in which the interior of the rings is insulated from the 10 medium in which the antenna bathes (according to a technology called "Air Backed Ring" or ABR). This type of antenna is used in particular for helibornesonar applications, such as for example the ·solution described in patent application FR 1303023, and exhibits the advantage of offering greater compactness with low weight. However, these antennas are limited in terms of 15 frequency band on account of the mono-resonant behavior of the active rings used in ABR mode. In yet other embodiments, as described for example in patent EP1356450B1, the omnidirectional sonar antenna comprises a vertical array of compact and 20 wideband emission transducers, whose walls are in contact with a fluid in the liquid state (according to a technology called "Free-flooded Rings" or FFR). The presence of liquid improves the acoustic performance of the antenna. Reception is ensured by an assembly of omnidirectional hydrophones placed on a lightweight structure transparent to acoustic waves in the frequency band used. 25 This type of omnidirectional sonar antenna architecture is particularly suitable for the towed SONAR antennas of surface vessels and for certain hull SONARs for surface vessels. The antennas embodied with FFR rings addressing the medium frequency region can be relatively compact and wide-band. However, such 30 antennas exhibit limitations in terms of compactness and performance in respect of sound level and bandwidth which are due mainly: wo 2016/046377 3 PCT/EP2015/072131 to the presence around the active elements of metallic and/or elastomeric leaktightness devices; and to the regular spacing between the rings. 5 General definition of the invention The aim of the invention is in particular to alleviate the aforementioned drawbacks, by proposing an omnidirectional antenna intended to equip a sonar, the antenna being centered around a longitudinal axis and comprising an assembly of emission 10 rings stacked along the longitudinal axis, each emission ring being formed around the longitudinal axis. Advantageously, the emission rings are assembled in groups of rings, the antenna comprising at least two groups of rings and each group of rings comprising at least tW,o rings. The inter-ring spacings between the rings of one and the same group and the inter-group spacings between two successive 15 groups of rings are chosen so as to optimize the emission bandwidth and the sound level. In particular, the inter-ring spacings between the rings of one and the same group can be a function of the cavity frequency of the group of rings while the inter-group spacings between two successive groups of rings are a function of the frequency of operational use of the emission rings. 20 According to a characteristic, the rings can be made of piezoelectric material. In one embodiment, the sum of the inter-group spacing between two groups of rings (p), of the inter-ring spacing (d) between two of rings and of twice the height 25 (h) of a ring can be substantially equal to half the wavelength of the frequency of operational use of the emission rings (20). According to another characteristic, the inter-ring spacing between the rings of one and the same group can also be chosen as a function of the radial frequency of the group of rings. wo 2016/046377 4 PCT /EP2015/072131 According to another characteristic, the inter-ring spacing between two rings of one and the same group can in particular be chosen so as to position the cavity frequency of the group of rings below the radial frequency of said ring. 5 In particular, the cavity frequency of each ring can be coupled with the radial frequency of said ring. 10 According to another characteristic, the emission rings can be immersed directly in a dielectric fluid. The internal cavity of each emission ring can in particular be in contact with the dielectric fluid. In one embodiment, the antenna can be housed in a leaktight enclosure filled with 15 the dielectric fluid. The enclosure can also be over-pressurized or be placed in hydrostatic equilibrium with the exterior medium. 2 o According to another characteristic, the rings are fed group-wise in parallel. The inter-ring spacing between two rings can vary within one and the same group. The inter-group spac1ng between two groups of the antenna can vary for the 2 5 assembly of groups of the antenna. The proposed embodiments thus make it possible to reduce the mass and the volume of the acoustic emission antenna of the SONAR, as well as its complexity of embodiment, while optimizing the sound level and the bandwidth of emission 30 frequencies, thus making it possible to obtain optimal acoustic performance. Description of the figures wo 2016/046377 5 PCT/EP2015/072131 Other characteristics and advantages of the invention will become apparent with the aid of the description which follows and of the figures of the appended drawings in which: 5 - Figure 1 is a diagram representing an exemplary marine platform on which an omnidirectional antenna according to the various embodiments can be fixed; 10 - Figure 2 is a perspective view of an omnidirectional sonar antenna, according to one embodiment of the invention; · - Figure 3 is a persp~ctive view of an exemplary reception base; - Figure 4 represents ~n exemplary elementary ring structure; - Figure 5 represents another exemplary elementary ring structure; - Figure 6 represents yet another exemplary elementary ring structure; - Figure 7 is a diagram repre.se,nting the omnidirectional sonar antenna, according to one embodiment; 15 -Figure 8 represents a frequency response chart obtained with various exemplary embodiments of omnidirectional antenna; and 20 - Figure 9 represents a frequency response chart obtained with exemplary embodiments of omnidirectional antenna according to the invention comprising an assembly of stacked groups of rings. The drawings and the annexes to the description will be able not only to serve to better elucidate the description, but also to contribute to the definition of the invention, if appropriate. 25 Detailed description Figure 1 is a diagram representing an exemplary structure 1 on which may be mounted an omnidirectional antenna 100, according to certain embodiments. 30 The omnidirectional antenna 100 is intended to be immersed at least partially in the water (for example at sea) to detect objects under the water by emission of sound waves. It can be mounted on any fixed or mobile structure 1, such as for '·.. -~ wo 2016/046377 6 PCT/EP2015/072131 example under a floating or anchored marine platform or a surface vessel as illustrated in Figure 1. Figure 2 illustrates the arrangement of the various elements of the antenna according to certain embodiments. 5 The omnidirectional antenna 100 comprises an emission base 2 comprising an assembly of elementary transducers 200 stacked along an axis 10 (hereinafter called the "longitudinal axis of the antenna"), the transducers being configured to emit sound waves. The-antenna 100 can in particular be fixed on the bottbm of the structure 1. The emission transducers 200 can cooperate with a recepti'on base 3 10 comprising an assembly of omnidirectional receivers for receiving the signals. In particular, the emission base (forming an emission antenna) consisting of the elementary transduce,rs. ,:ZOO .. can be distinct from the reception base (formino a reception antenna). In one embodiment, the omnidirectional antenna 100 can be a sonar antenna 15 intended to equip an active sonar. The subsequent description will be given with reference to an antenna 100 of sonar antenna type by way of nonlimiting example. In such an embodiment, the receivers of the emission base are hydrophones. The omnidirectional antenna 100 can have a generally cylindrical shape so as to be omnidirectional in terms of bearing. The elevational directivity depends on its 20 extension along its axis of revolution 10. The elementary transducers 200 comprise an assembly of emission rings 20, each ring being centered around an axis parallel to the axis 10 of the antenna 100. The emission rings 20 are superposed along the longitudinal axis of the antenna. In particular, the emission rings can be substantially identical and centered around 25 the longitudinal axis of the antenna 100. The diameter D of each ring 20 is suitable for the emission frequency. According to one aspect of the invention, the rings 20 are assembled in groups, each group constituting an elementary transducer 200 (in the subsequent wo 2016/046377 . 7 PCT/EP2015/072131 description, the groups of rings will thus be designated by the reference 200). The groups of rings 200 are spaced apart by a chosen pitch (the pitch will also be called the "intergroup spacing" hereinafter) in the direction of stacking, defined by the axis 10. 5 According to another characteristic, each group 200 (elementary emission transducer) comprises a chosen number of rings. In one embodiment, the various groups of rings 200 comprise the same number of rings and are spaced apart by one and the same distance (i.e. the intergroup spacing is identical between the various groups). 10 In the embodiment of Figure 2, the emission base 2 comprises three mutually spaced pairs of rings, according to the same chosen intergroup spacing (denoted .... , ,, .: .... :· - .... ·~p"),,,8nd each group. of rings '200 comprises a . .pair..oLr.ings: ,_,,,,._,_,_,, "·'"" " The groups of rings 200 are held in position by a holding structure. The antenna 1 00 can be linked up via cables or connectors to electronic 15 equipment disposed for example on the structure 1 and configured to feed electrical power to the antenna 100 and to ensure the exchange of data with the antenna 100. In particular, each emission ring 20 can be controlled separately by means of a power amplifier so as to produce a downward elevational emission lobe, for example by acoustic decoupling. As a variant, each group of rings 200 20 can be fed separately, using parallel feed. Such a configuration of the rings 20 makes it possible to optimize the emission bandwidth of the antenna and the sound level. In certain embodiments, the reception base 3 can be placed coaxially with the 25 emission base. According to another characteristic, securing tie rods 202 can be used to fasten the rings of one and the same group together or of the whole antenna, as illustrated in Figure 2. The tie rods 202 may be for example metallic tie rods. wo 2016/046377 8 PCT/EP2015/072131 As a supplement, inter-group clamping blocks 204 can be placed in the gaps separating two successive groups of rings. The clamping blocks 204 can form part of the assemblage and can take for example the form of plastic blocks through which the tie rods 202 pass. The tie rods 202 can comprise metallic tie rods 5 passing through the plastic blocks which serve as blocks. The assembly of elements of the emission base 2 can be clamped between the components 205 (annulus) which allow mechanical solidity of the emission antenna independently of all of the surrounding structure. One of the annuli 205 can form the interface with the support structure 1 represented in Figure-1. · 10 In the embodiments where the rings are of substantially identical dimensions and centered around the longitudinal axis of the antenna 10, they can be superposed · one above another so that the inter-group clamping blocks 204 be opposite one 'another in the direction defined by the longitudiriaf'axis''ib". <> •.... '· ' .,., < ~. The antenna 100 can furthermore comprise a profiled annulus 205 whose 15 diameter is at least equal to the diameter of the rings placed at each end of the stack to hold the assembly of rings and facilitate installation of the emission antenna 100. Figure 3 illustrates an example of positioning of the reception base 3. In the example of Figure 3, the receivers 31 are hydrophones fixed on the mechanical 20 holding structure 33 of the emission base 2. The holding structure 33 can be in particular transparent to acoustic waves in the frequency band used. The assembly of receivers 31 can form part of the emission antenna's mechanical holding structure. The receivers 31 of the reception antenna 3 can for example be hydrophones distributed around the emission antenna 100 and with no physical 25 link with the emission antenna 100. In particular, the receivers 31 forming the reception antenna can be disposed substantially column-wise or quincuncially on the holding structure 33 surrounding the emission antenna, along the longitudinal axis 10. wo 2016/046377 9 PCT/EP2015/072131 As represented in Figure 3, the hydrophones 31 can comprise. an assembly of elementary hydrophones distributed around the emission antenna 1 00 on supports 32 and with no physical link with the emission antenna 100. In the embodiment of Figure 3, the elementary hydrophones are arranged as three coaxial annuli 5 represented schematically by the dashed curves 311, 312 and 313 and centered around the axis 10. The annuli 311, 312 and 313 are spaced a chosen distance apart, along the axis 10. The emission antenna 100 can be arranged inside the holding structure 33 and held by the latter. 1 o The emission rings 20 can be active rings made of piezoelectric material (for example active rings of piezoelectric ceramic). Each ring 20 can for example ' . - -. . , , . '"' ·" .. · ••. · .· . . .~nmprise . an assembly . '"lfc.d:\A\)ments placed inside ·an .annulus o,f in~uiRting . ··- -_,,. ... .~ '• .- ' ' - . . .. · .. ·. . -·. - . ., .. . substance (made for example of glass fiber/resin wound directly on the ceramics) as represented in Figure 4 or in the form of a composite ring forming a shrink ring 15 as represented in Figure 5. Such segments 201 can be separated from one another by metallic components in the form of wedges 202 that can be moved toward the center of the ring by means of a device, thus making it possible to part the segments and to impose a mechanical prestress in the ceramic ring. The segments can be overlaid against a shrink fitting annulus (or assembled by 20 gluing). In particular, each ring can be a ring prestressed by a jig formed of an assembly of piezoelectric segments grouped to form substantially identical sectors. As a variant, each ring can be produced as a single ceramic component (monolithic shape) as illustrated in Figure 6. 25 In certain embodiments, the emission antenna and/or the internal cavity of the emission rings 20 can bathe in a non-ionic dielectric fluid 207, such as for example oil. In particular, the emission antenna 100 can be placed in a leaktight enclosure 208 30 which can be over-pressurized and which can contain the non-ionic dielectric fluid 5 10 wo 2016/046377 10 PCT/EP2015/072131 207. Thus, it is not necessary to use an electrical insulation and/or leaktightness device around the emission rings 20 (such as for example a shrouding, an overmolding around the rings or mechanical components for electrical insulation and leaktightness of the rings). The elimination of leaktight sealing by visco-elastic substance makes it possible to minimize the losses through heating of these substances and thus to discernibly increase the electro-acoustic efficiency. The conventional efficiency of about 50% obtained with conventional emission antennas can be increased to about 75%. As a supplement, it may be useful to provide a fine layer of varnish on the rings mainly to protect the rings during their manipulation or their transport in the phase of assembling the omnidirectional antenna 1QQ. . '·'" __ . ,.• .. "· .. .- ... : ,,. ·''"'' 15 By eliminating all the losses induced by the presence of the materials usually used to achieve the leaktightness and electrical insulation functions, the electro-acoustic efficiency of each emission ring 20, and therefore the "sound level to overall volume" ratio and the "sound level to mass" ratio of the emission antenna 100, are optimized. 20 The dielectric fluid 207 in which the emission rings 20 bathe can furthermore have a heat sink function for draining the heat generated by the active rings during emission. Indeed, it behaves as a heat-carrying fluid which cools the ceramic rings by natural convection in particular, thus making it possible to optimize the sound 2 5 level emitted and the duration of use at full load. In the embodiments where the rings 20 bathe in the fluid 207, each ring 20 constitutes a vibrating ring in a surrounding fluid and therefore exhibits at least two resonant frequencies acoustically coupled to the fluid: 3 o - a radial mode, obtained on the basis of alternations of extension/compression of the constituent material of the ring, in which the deformation of the ring 5 wo 2016/046377 11 PCT/EP2015/072131 corresponds to such alternations of radial extension/compression around the rest position of the ring; - a cavity mode, obtained by setting the fluid contained inside the volume defined by the ring and depending, to first order, on the height of the ring into resonance. The cavity mode can be activated by feeding each group of rings in parallel. In the embodiment where each emission ring 20 is made of piezo-electric substance, the energy necess-ary for radial resonance can be provided by the 10 alternating electrical excitation injected on the ceramic. The energy used to set the cavity mode into resonance can likewise be induced by the radial mode of the ring. In certain embodiments, the cavity mode and. the radi<:~l. mode are. coupled to obtain a significant operating frequency band so that each ring 20 can operate in 15 wideband. In particular, for each ring 20, the cavity frequency is chosen to be less than the radial frequency, thus allowing optimal operation. Figure 7 is a diagram showing in greater detail the arrangement of the emission rings 20. As shown in Figure 7, the groups of rings 200 are a distance p apart, this 20 constituting the "inter-group spacing". Figure 7 shows more precisely 4 groups of rings 200, each group comprising 2 rings. According to another characteristic of the invention, the inter-group spacing p between the various groups 200 of rings is chosen so as to optimize the operation of the antenna. 25 According to another characteristic, the inter-ring spacing, denoted "d', between the rings of one and the same group (for example pair) is chosen so as to control the cavity frequency of the group of rings 200. In particular, the inter-ring spacing, denoted "d', between the rings of one and the same group (for example pair) is chosen as a function of the cavity frequency of the group of rings 200 and/or of the 30 radial frequency of the ring group. wo 2016/046377 12 PCT /EP20 15/072131 In particular, in the embodiments where the rings 20 of one and the same group 200 are placed in one and the same fluid region and where the inter-ring distance d is large · compared with the wavelength of the emitted acoustic waves (representing the ratio between the speed of sound in the fluid of the region 5 considered and the frequency of use of the antenna), the cavity frequency and the radial frequency of the ring group 200 are substantially identical to those obtained for a lone ring. In the embodiments where the spacing d is small compared with the wavelength of the emitted acoustic waves, the cavity frequency of the group of rings may drop in frequency down to the limit case where.d=O. In particular; in the 10 embodiment where d=O, the 'cavity frequency of the pair may be half that· of the lone ring. The omnidirectional antenna 100 can in particular be configured so that, whatever - · ' ' .... · the inter-ring spacing· do the· radial frequency of the elementary rings remains unchanged. 15 The optimization of the inter-ring spacing d for a given antenna thus makes it possible to vary the cavity frequency of the antenna and to optimize it for a given operation. The inter-ring distance d between the elementary rings thus makes it possible for the cavity frequency of the antenna to be best positioned with respect to the needs 2 o of the antenna 100. The inter-group spacing p between two groups of the antenna can advantageously be chosen so as to optimize the acoustic efficiency of the emission base 2. In particular, the inter-group spacing p can be chosen as a function of the frequency of operational use of the emission base. In one embodiment, the inter-group 25 spacing p can be chosen equal to half the wavelength of the frequency of operational use of the emission base 2. The inter-group spacing p can thus be optimized either from an acoustic point of view (bandwidth and sensitivity to emission) or from a more general point of view, including the emission chain, so as to have the maximum of active power in the antenna over the largest possible 30 frequency band. wo 2016/046377 13 PCT /EP20 15/072131 The groups of rings separated by the inter-group distance d can be fed with an appropriate phase shift to obtain an antenna mode making it possible to emit with a steering of the main lobe along the axis of revolution of the antenna. In the embodiments where the antenna 100 is submerged in a fluid and comprises 5 . a fluid in the internal cavity of each emission ring 20, the presence of fluid makes it possible to use the rings in FFR mode ("Free-flooded Rings" technology) and therefore to obtain wide-band operation. In the FFR mode, the internal walls of the emission rings 20 are in contact with a fluid in the liquid state. In such an FFR mode, when the minimum inter-ring distance "d" between rings of 10 one and the same group is chosen so as to optimize acoustic operation according to the cavity mode of the ring, the electro-acoustic efficiency obtained is much . .. . . ·- . . , are.ater.thcm .that obtainPd,).~cC:JIIily frceqt~ency of the grouf.l of rP.lgs (200) while· th-e intergroup spacings between two successive groups of rings are a function of the 15 frequency of operational use of the emission rings (20). 2. The omnidirectional antenna (1 00) as claimed in claim 1, characterized in that the rings (20) are made of piezoelectric material. 20 3. The omnidirectional antenna (100) as claimed in one of the preceding claims, characterized in that the sum of the inter-group spacing between two groups of rings p, of the inter-ring spacing d between two of rings and of twice the height of a ring is substantially equal to half the wavelength of the frequency of operational use of the emission rings (20). 25 30 4. The omnidirectional antenna (100) as claimed in one of the preceding claims, characterized in that the inter-ring spacing between the rings of one and the same group is furthermore chosen as a function of the radial frequency of the group of rings. 5. The omnidirectional antenna (1 00) as claimed in claim 4, characterized in that the inter-ring spacing between two rings of one and the same group is chosen so wo 2016/046377 19 PCT/EP2015/072131 as to position the cavity frequency of the group of rings below the radial frequency of said ring. 6. The omnidirectional antenna (100) as claimed· in one of claims 4 and 5, 5 characterized in that the cavity frequency of each ring is coupled with the radial frequency of said ring. 7. The omnidirectional antenna (100) as claimed in one of the preceding claims, characterized in that the emission rings (20) are immersed directly in a dielectric 10 fluid. 8. The omnidirectional antenna (100) as claimed in claim 7, characterized in that . the internal cavity of each emission ring (20) ·is in contact with said dielectric fluid. 15 9. The omnidirectional antenna (100) as claimed in one of claims 7 and 8, characterized in that the antenna (1 00) is housed in a leaktight enclosure (208) filled with said dielectric fluid. 10. The omnidirectional antenna (100) as claimed in claim 9, characterized in that 20 the enclosure (208) is over-pressurized. 25 11. The omnidirectional antenna (1 00) as claimed in one of claims 9 and 10, characterized in that the enclosure (208) is placed in hydrostatic equilibrium with the exterior medium. 12. The omnidirectional antenna (100) as claimed in one of the preceding claims, characterized in that the rings (20) are fed group-wise in parallel. 13. The omnidirectional antenna (100) as claimed in one of the preceding claims, 3 o characterized in that the inter-ring spacing between two rings (20) varies within one and the same group. WO 2016/046377 20 PCT!EP2015/072131 14. The omnidirectional antenna (100) as claimed in one of the preceding claims, characterized in that the intercgroup spacing between two groups of the antenna varies for the assembly of groups of the antenna (200).