Note: Descriptions are shown in the official language in which they were submitted.
I
OMNIDIRECTIONAL ANTENNA
Field of the invention
The invention relates in a general manner to antennas, and in particular to
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 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
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
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 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).
Date recue/ date received 2022-02-18
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2
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
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
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 heliborne-
sonar 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
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
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.
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
antennas exhibit limitations in terms of compactness and performance in
respect
of sound level and bandwidth which are due mainly:
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- to the presence around the active elements of metallic and/or elastomeric
leaktightness devices; and
- to the regular spacing between the rings.
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
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 two rings. The inter-ring spacings between the rings
of
one and the same group and the inter-group spacings between two successive
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.
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
(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.
4
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.
In particular, the cavity frequency of each ring can be coupled with the
radial frequency of said
ring.
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 the dielectric
fluid.
The enclosure can also be over-pressurized or be placed in hydrostatic
equilibrium with the
exterior medium.
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 spacing between two groups of the antenna can vary for the
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 frequencies, thus
making it possible to
obtain optimal acoustic performance.
According to another aspect, there is provided an omnidirectional antenna
intended to equip a
sonar, the antenna being centered around a longitudinal axis and comprising an
assembly of
emission rings stacked along said longitudinal axis, said emission rings being
directly immersed
in a dielectric fluid, each emission ring being formed around said
longitudinal axis, wherein each
Date Recue/Date Received 2022-09-07
4a
ring constitutes a vibrating ring in said dielectric fluid and comprises an
internal cavity including
an internal fluid, each ring presenting at least two resonance frequencies
acoustically coupled to
said fluid, said resonance frequencies comprising a cavity frequency
corresponding to a cavity
mode and a radial frequency corresponding to a radial mode, wherein 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 two rings, and wherein the inter-ring spacings
between the rings of
one and the same group are 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, wherein the inter-ring spacing between
the rings of one and
.. the same group of rings is furthermore chosen as a function of the radial
frequency of the group
of rings.
Description of the figures
Date Recue/Date Received 2022-09-07
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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;
- Figure 2 is a perspective view of an omnidirectional sonar antenna,
according to
one embodiment of the invention;
- Figure 3 is a perspective view of an exemplary reception base;
- Figure 4 represents an 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 representing the omnidirectional sonar antenna,
according
to one embodiment;
- Figure 8 represents a frequency response chart obtained with various
exemplary
embodiments of omnidirectional antenna; and
- 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.
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.
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
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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.
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 bottom of
the
structure 1. The emission transducers 200 can cooperate with a reception base
3
comprising an assembly of omnidirectional receivers for receiving the signals.
In
particular, the emission base (forming an emission antenna) consisting of the
elementary transducers 200 can be distinct from the reception base (forming a
reception antenna).
In one embodiment, the omnidirectional antenna 100 can be a sonar antenna
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
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
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
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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.
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).
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"), and each group of rings 200 comprises a pair of rings.
The groups of rings 200 are held in position by a holding structure.
The antenna 100 can be linked up via cables or connectors to electronic
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
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
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.
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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
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.
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 longitudinal axis 10.
The antenna 100 can furthermore comprise a profiled annulus 205 whose
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
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
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.
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As represented in Figure 3, the hydrophones 31 can comprise an assembly of
elementary hydrophones distributed around the emission antenna 100 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
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.
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
comprise an assembly of segments placed inside an annulus of insulating
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
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
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.
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
which can be over-pressurized and which can contain the non-ionic dielectric
fluid
CA 02962492 2017-03-24
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).
5
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 100.
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.
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
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:
- 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
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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 necessary for radial resonance can be provided by the
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 radial mode are coupled to
obtain a significant operating frequency band so that each ring 20 can operate
in
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
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.
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
radial frequency of the ring group.
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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
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=0. In particular,
in the
embodiment where d=0, 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 d, the radial frequency of the elementary rings remains
unchanged.
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
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
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
frequency band.
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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
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
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
greater than that obtained with conventional omnidirectional emission
antennas.
The dielectric fluid in which the emission antenna 100 bathes and/or which is
in
contact with the internal cavity of each ring (in the FFR mode) can have
similar
acoustic characteristics to water (in particular, density, speed of sound,
acoustic
impedance), such as for example a specific mineral oil.
The dielectric fluid can also have optimized thermal characteristics in
relation to
the cooling of the active rings by natural convection.
In the embodiments where the emission antenna is placed in an enclosure 208
filled with the dielectric fluid, the enclosure 208 is an acoustically
transparent
enclosure, such as for example made of composite material of fiber, resin
(glass,
carbon,...), or rubber or polyurethane elastomer.
Such an enclosure 208 can be in particular over-pressurized to push back the
limits in terms of cavitation of the emission antenna 100.
The enclosure 208 can furthermore be configured to be in hydrostatic
equilibrium
with the exterior medium, and this may be of particular interest in
applications
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onboard variable-immersion vehicles (such as for example submarines, towed
bodies, drones, etc..
As a supplement or as a variant, the enclosure 208 can be partially clad with
acoustic material (for example anechoic or by masking) so as to optimize the
radiation pattern of the emission antenna and/or the signal response of the
antenna and/or the noise of the associated reception base (3).
The omnidirectional antenna 100 according to the various embodiments exhibits
optimized compactness with respect to the conventional solutions. Indeed, the
various embodiments make it possible to address the low part of the frequency
band through a fluid mode which has limited dependency with respect to the
physical structure of the antenna (for a given physical dimension, the
frequency
band is widened toward the low frequencies).
The various embodiments of the invention thus facilitate installation of the
acoustic
antenna on a marine platform such as a surface vessel, in particular of low
tonnage and shallow draft, or on a submarine, for which the volume available
as
superstructures is very constrained.
The omnidirectional antenna according to the various embodiments can also be
used in any type of sonar application, such as for example in applications of
airborne sonar type or fixed or mobile maritime surveillance devices.
Figure 8 shows the frequency response chart obtained with various exemplary
embodiments of omnidirectional antenna.
In the chart of Figure 8, the horizontal axis corresponds to the frequency
axis (in
Hz) and the vertical axis corresponds to the sensitivity to emission
(sensitivity as
voltage Sv in dB pPaN). The curves are characterized by two maxima
corresponding respectively to the cavity mode and to the radial mode:
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In the chart of Figure 8:
- Curve Cl corresponds to the frequency response obtained with a conventional
antenna of "Free Flooded" type. The first maximum is observed at the frequency
F,
corresponding to operation in cavity mode and to the resonance wavelength of
the
5 cavity Ac, while the second maximum is observed at the frequency F
corresponds
to the operation in radial mode and to the wavelength 4.
- Curve C2 corresponds to the frequency response obtained with an exemplary
embodiment of omnidirectional antenna according to the prior art comprising a
pair
of glued rings: the maxima are attained for a frequency L2 and Fr.
10 - Curve C3 corresponds to the frequency response obtained with an exemplary
embodiment of omnidirectional antenna according to the invention comprising a
group of rings, the rings being spaced apart by a distance d = ¨ h, with h
designating the height of each ring for rings of the same height (represented
in
Figure 7) or d = _ P.12 _ t1.2.2 if the two adjacent rings of one and the same
group
15 have
different heights h1 and h2. The subsequent description will be given with
reference to rings of the same height h by way of nonlimiting example. In the
exemplary embodiment corresponding to curve C3, the maxima are attained for a
frequency Fe', the frequency F; being able to take all values between F- Ic-2
and Fc as a
function of the distance d.
The inventors have thus established that a spacing d between the rings (inter-
ring
=
spacing) of one and the same group that is very small and dependent on the
=
resonance wavelength of the cavity A,. (for example d = 5%¨ h as represented
on
curve C3) makes it possible to optimize the response in a wider frequency band
than in the conventional embodiments. Thus, the inter-ring spacing d can
advantageously be chosen such that:
d = f(A) ¨ h, where f is a function of A.
For example, the function f can be chosen equal to f(A) = with a lying
a
between 5 and 6.
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It should be noted that this criterion relating to the spacing d between the
rings can
alternatively be formulated in the form of a criterion relating to the height
h of the
rings, given that h = d, or of a criterion relating to the distance L
(represented in Figure 7) between two successive groups of rings, with L = d +
2h+ p (d designating the inter-ring distance, p the inter-group distance and h
the
height of the rings) as illustrated by the chart of Figure 7.
Figure 9 represents the frequency response obtained with an exemplary
embodiment of omnidirectional antenna according to the invention comprising an
assembly of stacked groups of rings 200, the rings of one and the same group
of
rings being spaced a distance d = _ h apart. The various curves represented in
Figure 9 (C4, C5 and C6) correspond to a distance L between two successive
groups of rings taken equal to half the wavelength of the frequency of
operational
use of the emission rings (), the distance L being defined by L = d +2h+ p (d
designating the inter-ring distance, p the inter-group distance and h the
height of
the rings), for various values of frequencies.
More precisely:
- curve C4 corresponds to L = at a frequency F;
- curve C5 corresponds to L = at a frequency Fr;
=
- curve C6 corresponds to L = at a frequency ¨Fc+2Fr.
Figure 9 thus shows that the frequency band obtained with certain embodiments
of
the invention is wider than for a conventional antenna and exhibits a sound
level
equalized over the whole frequency band. The inventors have established that
such a result is related to the choice of the inter-group distance p and inter-
ring
distance d. In particular, the distances p and d can be chosen so as to
optimize
the sound level as a function of needs.
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The various embodiments make it possible to optimize the sound level and the
bandwidth of the emission frequencies. The acoustic performance of the
emission
antenna is advantageously optimized so as to cover the entirety of the
environment conditions and propagation conditions, whether in deep water or
shallow water conditions, potentially strongly reverberating.
Although not limited to such applications, the proposed embodiments have
particular advantages in the field of low- and medium-frequency SONAR systems
allowing the detection/classification of submarines.
The invention is not limited to the embodiments described hereinabove by way
of
nonlimiting example. It encompasses all the variant embodiments that could be
envisaged by the person skilled in the art. In particular, the invention is
not limited
to a particular arrangement of the receivers 31 forming the reception antenna
3,
nor to a particular architecture for embodying the emission rings 20. Nor is
the
invention limited to a spacing d between rings of one and the same group
(inter-
ring spacing) that is constant within one and the same group. For example, the
inter-ring spacing d can be variable within one and the same group so as to
best
match the cavity modes of each group to its position in the antenna. Likewise,
nor
is the invention limited to an inter-group spacing p that is constant between
two
successive groups. A variable inter-group spacing may be for example chosen as
a function of the required performance, of the position of the group with
respect to
= the axis of the antenna, etc. Moreover, the invention is not limited to
rings 20 of
identical dimensions within one and the same group 20. For example, For
.. example, the rings 20 of one and the same group 200 can have a different
height.
More generally, the configuration of the various groups 200 can differ from
one
group to another.
