The invention relates to a wideband compact antenna of very small thickness and
with dual orthogonal linear polarization operating in the V/UHF bands.
More particularly, the invention relates to an antenna for emitting/receiving
electromagnetic waves, of the type comprising:
- two dipoles orthogonal to each other, each dipole comprising two radiating
elements,
- a metal plate, and
- an absorptive structure.
The invention is located in the field of antennas and wideband compact antenna
systems. These systems are dedicated to applications for receiving and emitting
electromagnetic waves in a very wide band of frequencies. For example, the compact
^h antenna according to the invention is intended to operate in the VHF and UHF bands, i.e.
at frequencies comprised between 30 MHz and 3 GHz, and more particularly at
frequencies comprised between 30 MHz and 500 MHz.
Such antennas are used for various purposes, for example in the field of radio
communications and are notably intended to be integrated to a vehicle, whether this be a
land, airborne or naval vehicle.
Consequently, these antennas are subject to many constraints.
Thus, for example, they have to:
• have reduced size,
• have low visual discretion or low RES, for Radar Equivalent Surface,
• have high radio-electric performances such as low SWR, for Stationary
Wave Ratio, high gain, etc.,
_ • be suitable for emitting or receiving electromagnetic waves regardless of
^ P their polarization (linear polarization, circular polarization and elliptical
polarization), and
• have unidirectional radio-electric coverage.
Finally, they should be compliant with the roadway clearance of land vehicles and
not degrade the aerodynamics of airborne vehicles to which they are integrated and have
independent radio-electric performances with respect to the latter.
Further, these antennas should have a very small thickness so as to be either laid
out directly on one of the surfaces of a vehicle or in a cavity provided for this purpose in
said vehicle, for example so that they are flush with a surface which it comprises.
Thus, the document « A novel compact dual-linear Polarized UWB Antenna for
VHF/UHF applications » describes a wideband compact antenna of the aforementioned
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type. The radiating elements of the antenna are curved and have meanders so as to
increase the electric length of the antenna and to thereby optimize low frequency
radio-electric performances. Further, the metal plate of the antenna is positioned on a disc
made with a ferrite material. It is found at a distance from the radiating elements so that it
reflects the electromagnetic waves emitted or received by the high frequency antenna.
However, this solution does not give entire satisfaction.
Firstly, this antenna does not allow use from 30 MHz with an acceptable SWR.
Secondly, because of the curved shape of the radiating elements, the antenna
forms a large protuberance protruding from the vehicle when it is laid out on a surface of
the latter, or imposes overdimensioning of the cavity in which it is laid out, which in
particular proves to be a penalty for certain vehicles.
Thirdly, taking into account the design of the antenna, the low frequency
^ 1 radio-electric properties of this antenna have to vary according to the vehicle on which it is
laid out and the latter will be particularly impacted in the case when this antenna is
positioned in a metal cavity.
The object of the invention is therefore to solve these problems.
For thiswpurpose, the invention relates to an antenna of the aforementioned type, ;
characterized in that the radiating elements are all substantially planar, the two dipoles
being substantially comprised in a same plane, and in that the absorptive structure is
interposed between the metal plate and the dipoles and is laid out in contact with the
metal plate.
According to other aspects of the invention, the wideband compact antenna
comprises one or more of the following features, taken alone or according to all technically
possible combination(s):
- each radiating element has a general disc sector shape;
^ P - said plane is at a distance d from the absorptive structure comprised between
1 mm and 2 mm;
- it comprises an impedance matching circuit made in printed technology;
- the metal plate comprises a sole, the impedance matching circuit being laid out in
said sole;
- it also comprises a protective radome;
- the absorptive structure has a general cylindrical shape;
- the height of the absorptive structure is comprised between 20 mm and 21 mm,
and advantageously has the value of 20 mm, and its diameter is comprised between
330 mm and 334 mm, and advantageously has the value of 330 mm;
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- the dipoles and the absorptive structure are integrally comprised in a cylinder
with a diameter substantially equal to 330 mm and with a height substantial equal to
22 mm;
- the electromagnetic waves which it is capable of emitting and receiving have
frequencies comprised in the whole range of frequencies 30 MHz - 500 MHz, and
advantageously in the whole range of frequencies 30 MHz - 700 MHz;
- it is capable of emitting and receiving electromagnetic waves having any
polarization from among a linear polarization, a circular polarization or an elliptical
polarization, each dipole being capable of emitting/receiving electromagnetic waves
having a horizontal linear polarization for one of the dipoles and a vertical linear
polarization for the other dipole, respectively.
Further, the invention relates to a land, airborne or naval vehicle of the type
^ f t including:
- a planar surface and/or a cavity,
- an antenna as described above and laid out on said surface and/or in said
cavity.
According to other aspects of the invention, the vehicle cbmprises one or more of
the following features, taken alone or according to all technically possible combination(s):
- the planar surface and/or the cavity are made in a metal material.
The invention will be better understood by means of the description which follows,
only given as an example and made with reference to the appended drawings wherein:
- Fig. 1 is a perspective view of a wideband compact antenna according to a first
embodiment of the invention;
- Fig. 2 is a sectional view of the antenna of Fig. 1 along the plane II;
- Fig. 3 is a curve illustrating the Stationary Wave Ratio of one of the two dipoles of
( ^ a wideband compact antenna according to the invention versus the frequency in MHz;
- Fig. 4 is a curve illustrating the insulation between the two dipoles of a wideband
compact antenna according to the invention versus the frequency in MHz;
- Fig. 5 is a curve illustrating the gain of one of the two dipoles of a wideband
compact antenna according to the invention versus the frequency in MHz;
- Fig. 6 illustrates radiation diagrams along the azimuthal plane of one of the two
dipoles of a wideband compact antenna according to the invention for frequencies having
the values of 30 MHz, 50 MHz, 100 MHz, 300 MHz and 500 MHz respectively;
- Fig. 7 is a side view of a wideband compact antenna according to a second
embodiment of the invention;
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- Fig. 8 is a side view of a wideband compact antenna according to the invention
comprising a protective radome; and
- Fig. 9 is a schematic illustration of the antenna of Fig. 8 laid out in a cavity made
in a vehicle.
In all the following, the expressions of «lower» and « upper» are used with
reference to the figures and in a non-limiting way.
The antenna according to the invention is intended to emit and receive
electromagnetic waves, the frequencies of which are preferentially comprised in the whole
range of frequencies 30 MHz - 500 MHz. Advantageously, it is intended to emit and
receive electromagnetic waves, the frequencies of which are comprised in the whole
range of frequencies 30 MHz - 700 MHz.
With reference to Figs. 1 and 2, the antenna 2 comprises radiating elements 4, an
^ ^ absorptive structure 6 and a metal plate 8. Further, it comprises means 10 for impedance
matching and for powering the radiating elements.
The radiating elements 4 are capable of emitting and receiving electromagnetic
waves.
For this purpose, the radiating elements 4 are made in an electrically conducting
material.
In the example of Fig. 1, the radiating elements 4 are made in printed technology
known to one skilled in the art.
The antenna 2 thus comprises four substantially planar radiating elements 4 with a
general triangular shape, and more specifically each with the shape of a disc sector. Each
radiating element 4 thus has a rounded edge 12 and an apex 14 opposite to said rounded
edge 12. Each radiating element 4 has an aperture angle a at its apex 14, the value of
which is substantially 45°.
c With this value of the aperture angle a, it is possible to optimize the impedance
and gain performances of the antenna 2 over the covered bandwidth, while minimizing its
size.
The radiating elements 4 are substantially included in a circle C of centre O, the
rounded edge 12 of each radiating element substantially belonging to said circle C.
Further, the apices 14 opposite to these rounded edges all substantially point towards the
point O.
The radiating elements 4 are all substantially comprised in a same plane P and
substantially have the same dimensions.
The radiating elements 4 are distributed in two dipoles 16A, 16B each comprising
two diametrically opposite radiating elements 4. Each dipole 16A, 16B is symmetrical
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relatively to said point O and has an axis of symmetry 17A, 17B comprised in the plane P,
passing through O and coinciding with the bisecting line of the angle at the apex 14 of
each of its radiating elements 4.
Each of the two dipoles 16A, 16B is capable of emitting and receiving
electromagnetic waves having vertical linear polarization for one of them and a horizontal
linear polarization for the other. Emission and reception of electromagnetic waves having
any polarization (linear polarization, circular polarization or elliptical polarization) are then
obtained by combining both linear polarizations either in an analog way for example by
adding a coupling function, or by digital processing, this being known to one skilled in the
art.
For this purpose both dipoles 16A, 16B are orthogonal, i.e. their axes of symmetry
17A, 17B are orthogonal. Further, each radiating element 4 of a dipole 16A adapted for
4 K emitting/receiving waves of given linear polarization is then laid out between both radiating
elements 4 of the dipole 16B adapted for emitting/receiving electromagnetic waves of the
complementary linear polarization, as illustrated in Fig. 1.
With reference to Figs. 1 and 2, the preferential radiation direction of the antenna 2
corresponds to an axis A-A' perpendicular to the plane P of the radiating elements 4 and
passing through the point O.
Always with reference to Fig. 1, the dipoles 16A, 16B are substantially included in
the circle C.
The diameter of the circle C is equal to a fraction of the length of an
electromagnetic wave, i.e. the diameter is equal to y , wherein k is the wavelength and
n is a strictly positive number.
For an ideal antenna with a small bandwidth centered around a wavelength X , n is
^ p typically selected to be equal to 2.
The dimensioning of the dipoles of this antenna is then, as a general rule,
determined by the ratio /I/ , / independently of the resulting size.
Now, the constraints on size and bandwidth which the antenna 2 is intended to
meet, are expressed by a large deviation with this scenario.
In the relevant embodiment, the diameter of the circle C is taken to be substantially
equal to 330 mm, n then being comprised approximately between 30 and 1.8 respectively
for electromagnetic waves with a frequency ranging from 30 MHz to 500 MHz.
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The geometry of the dipoles 16A, 16B notably have the effect of minimizing the
volume which they occupy, while having capability of emitting and receiving
electromagnetic waves of any polarization from a single antenna 2.
The absorptive structure 6 is capable of improving the impedance matching level
of the antenna 2 and of increasing its directivity by absorbing a portion of the back
radiation from the dipoles 16A, 16B of the antenna 2, i.e. radiation emitted in the direction
opposite to its preferential radiation direction. Therefore it is capable of optimizing the gain
of the antenna, particularly for low frequencies of its frequency band, for example at
frequencies comprised between 30 MHz and 200 MHz. Further, it is capable of minimizing
bulkiness as regards diameter and thickness of the antenna 2.
For this purpose, the absorptive structure 6 is interposed between the radiating
elements 4 and the metal plate 8. It is then both located in proximity to the radiating
4 ^ elements 4 and in contact with the metal plate 8. Further, it comprises an assembly of tiles
made from a ferrite type material known to one skilled in the art.
The absorptive structure 6 has a general cylindrical shape with an axis A-A' and
with a diameter substantially equal to the diameter of the circumscribed circle C to the
dipoles 16A, 16B, and more particularly comprised between 330 mm and 334 mm.
In the example of Figs. 1 and 2, it has a diameter substantially equal to 330 mm.
Moreover, the absorptive structure 6 has a height substantially comprised between
20 mm and 21 mm, and advantageously substantially equal to 20 mm.
This value corresponds to a good compromise between the radio-electric
performances of SWR and of gain between low and high frequencies, the resulting size of
the antenna 2, and the absorption properties related to the complex permittivity and
complex permeability characteristics of the material of the absorptive structure 6.
The arrangement of the absorptive structure 6 in proximity to the radiating
^ p elements 4 and in contact with the metal plate 8 gives the possibility of significantly
reducing the influence of the vehicle, to which the antenna 2 is integrated, on the
radio-electric performances at low frequencies, notably in the case when the antenna 2 is
laid out in a metal cavity.
The absorptive structure 6 is delimited vertically by a substantially planar upper
surface 18 and a lower surface 21 both parallel to the plane P. Said plane P is then
located at a distance d from said upper surface 18 comprised between 1 mm and 2 mm.
Further, the lower surface 21 is positioned in contact with the metal plate 8.
This low value of the distance d has the effect of self-matching the antenna 2 via
the matching and supply means 10, and therefore of generating a reduction in the value of
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the Stationary Wave Ratio of the antenna 2 in particular at low frequencies of its
frequency bands, for example at frequencies comprised between 30 MHz and 200 MHz.
The circle C and the absorptive structure 6 both have the same axis of revolution
A-A'. In the embodiment of Fig. 1, the dipoles 16A, 16B and said absorptive structure 6
are thus comprised in a cylinder of axis A-A' with a diameter substantially equal to
330 mm and a height substantially equal to 22 mm.
When the antenna 2 is laid out in a cavity, this notably gives the possibility of
minimizing the dimensions of said cavity, as this will be seen subsequently.
The absorptive structure 6 is adapted for letting through the impedance and supply
means 10. For this purpose, the absorptive structure 6 delimits a passage orifice 19 for
letting through impedance matching and supply means 10 as this will be seen
subsequently. This orifice has a general cylindrical shape of axis A-A' and with a small
J ^ diameter as compared with the diameter of the absorptive structure 6.
The metal plate 8 provides the functions of a ground plane as well as of a
mechanical and electrical interface between the antenna 2 and the structure on which the
antenna 2 is intended to be integrated.
The metal plate 8 is capable of providing a ground reference to the different
members of the antenna 2 and is capable of optimizing the directivity of the antenna 2 by
contributing to the reduction of the back radiation of the latter.
Further, the metal plate 8 is capable of being laid out in contact with a planar
surface of a vehicle, to which the antenna 2 is intended to be integrated.
For this purpose, the metal plate 8 is made from an electrically conducting material
known to one skilled in the art.
Further, it has a general discoidal shape of axis A-A' and is laid out in contact with
the absorptive structure 6.
^ P In the example of Figs. 1 and 2, the metal plate 8 has a diameter of about 350 mm
and thereby delimits a protrusion 20 of a general ring shape extending radially relatively to
the absorptive structure 6 and having a width Lsubstantially equal to 10 mm.
The metal plate 8 has an upper surface 22 as well as a lower surface 23.
The upper surface 22 is substantially planar and laid out in contact with the lower
surface 21 of the absorptive structure 6. It is further parallel to the plane P of the radiating
elements 4. Said surface 22 is then at a distance from said plane P equal to a fraction of
the length of an electromagnetic wave, i.e. the distance is equal to y , wherein k is the
wavelength and m is a strictly positive number.
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For an ideal antenna with a small bandwidth centered around a wavelength X , m
is typically selected to be equal to 4 upon considering that the space between the radiated
elements and the reflective plane of the ideal antenna is filled with air and therefore with
permittivity and permeability equal to 1. The distance from the metal plate to the dipoles is
then determined by the ratio /. independently of the resulting size.
Now, the constraints on size and on bandwidth, which the antenna 2 according to
the invention meet, are expressed by a large deviation with this scenario.
Thus, in the embodiment of Figs. 1 and 2, the distance from the metal plate 8 to
the plane P is substantially taken to be equal to 22 mm, m then being approximately
comprised between 450 and 27 for electromagnetic waves with a frequency ranging from
30 MHz to 500 MHz respectively.
^Qj The impedance matching and powering means 10 are capable of ensuring
impedance matching and powering of the dipoles 16A, 16B of the antenna 2 as well as
symmetrizing the currents flowing in the radiating elements 4.
For this purpose, these means 10 comprise two connectors 24, two impedance
; transformers 26 and electric contacts 28 connecting the radiating elements 4 to the
transformers 26. Further, these means 10 comprise electric contacts 30, 32 connecting
the connectors 24 and the impedance transformers 26, the reference electric contacts 32
being ground contacts.
The connectors 24 are adapted so as to ensure the electric interface between the
antenna 2 and an emissions and/or reception device (not shown) which is associated with
it. The connectors 24 are laid out through the metal plate 8 facing the passage orifice 19
of the absorptive structure 6.
In a known way, such connectors 24 are intended to be engaged with coaxial
^ P cables (not shown), and then have a core 34 and ground 36 mating those of the coaxial
cables to which they are connected.
In the embodiment of Fig. 2, the core 34 of each connector 24 is connected to an
asymmetrical route 40 which each impedance transformer 26 comprises, via an electric
contact 30 located in the passage orifice 19. The ground 36 of each connector 24 is
connected to a ground route 42 of each transformer 26 via an electric contact 32 also
located in the passage orifice 19. The ground 36 of each connector 24 is in electric
continuity with the metal plate 8 via an electric contact 31 laid out in contact with the lower
surface 23 of the metal plate 8.
Such electric contacts 30, 31, 32 are well known to one skilled in the art and will
not be described here.
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In a known way, an impedance transformer 26 is adapted in order to maximize
power transfer between the dipoles 16A, 16B of the antenna 2 and the emission and/or
reception device with which the antenna 2 is associated.
With each dipole 16A, 16B, is associated an impedance transformer 26.
As illustrated in Fig. 2, each impedance transformer 26 is laid out in the passage
orifice 19 and comprises two symmetrical routes 38 each connected to one of the
radiating elements 4 of the corresponding dipole 16 via an electric contact 28 as well as
an asymmetrical route 40 and a ground route 42, as described above.
The electric contacts 28 are also laid out in the passage orifice 19. They are well
known to one skilled in the art and will not be described here.
In the antenna 2 according to the invention, the geometry, the dimensions, the
properties and the relative positioning of the radiating elements 4, of the absorptive
j p structure 6 and of the metal plate 8 give the possibility of:
(i) minimizing the bulkiness of the antenna 2, notably resulting in a highly reduced
thickness. The antenna 2 has very small dimensions as compared with the wavelengths of
the electromagnetic waves which it is able to emit and receive. Further, the flatness of the
radiating elements 4 and the geometries and the dimensions of the absorptive structure 6
and of the metalplate 8 allow the volume occupied by the antenna 2 to be minimized,
(ii) maximizing the reduction of the back radiation of the antenna 2. This is
obtained by absorbing the back radiation from the dipoles 16A, 16B by means of the
absorptive structure 6. By applying the absorptive structure 6 onto the plate 8, it is further
possible to attenuate the currents on the metal plate 8 generated by the back radiation of
the dipoles 16A, 16B and which, if they were not attenuated, would re-radiate and would
interfere with the radiation in the preferential direction of the antenna 2.
The positioning of the absorptive structure 6 in proximity to the radiating elements
^ p 4 of the dipoles 16A, 16B gives the possibility of reducing the influence of neighboring
objects on the radio-electric performances. The antenna 2 thus has optimized
radio-electric performances (impedance, SWR, radiation, directivity and gain) and
maximized impedance towards its environment.
These combined features (i) and (ii) make the antenna 2 suitable for minimizing
the volume of the protruding protuberance which it forms relatively to the vehicle when it is
integrated to a surface of the latter, and for minimizing the dimensions of a cavity intended
to receive the antenna 2, said cavity being for example made from a metal material.
With reference to Figs. 1 and 2, during operation of the antenna 2, the radiating
elements 4 are powered by the emission/reception device associated with the antenna via
the impedance matching and powering means 10.
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The dipoles 16A, 16B emit and receive electromagnetic waves having any
polarization from among a linear, circular or elliptical polarization and having frequencies
comprised in the frequency band of the antenna 2.
These waves are then emitted and received preferentially along the emission
direction A-A' of the antenna 2.
With reference to Fig. 3, the SWR of the antenna 2 is less than 2.35 for 1 for a
rated impedance of 50 Ohms over the frequency range 30 MHz - 500 MHz, i.e. it has a
very good impedance match over its frequency band.
With reference to Fig. 4, which is a representative curve of the decoupling between
both dipoles 16A, 16B versus frequency, it is seen that the insulation between the two
dipoles 16A, 16B is greater than 30 dB on the frequency band of the antenna 2.
With reference to Fig. 5, it is seen that the gain obtained on one of the dipoles is
4 Q greater than -8 dBi over the frequency range 200 MHz - 500 MHz, greater than -5 dBi
over the frequency range 230 MHz - 470 MHz. Further, this gain has the value of -35 dBi
at 30 MHz, -17 dBi at 100 MHz, and -12 dBi at 150 MHz.
Finally, with regard to Fig. 6, the antenna 2 has quasi-unidirectional radio-electric
coverage over its frequency band. ~
With reference to Fig. 7, in a second embodiment of the invention, the impedance
matching and supply means 10 are integrated to an impedance matching circuit 44,
except for the electric contacts 28 connecting the dipoles 16A, 16B to said impedance
matching circuit 44 and the connectors 24.
This circuit 44 is made in printed technology, known to one skilled in the art, and is
then positioned in a sole 46 which the metai plate 8 comprises.
In practice, the sole 46 comprises a cavity 461 dedicated for this purpose and
which is accessible via a removable metal cover 462.
^ P The metal plate 8 then delimits four passage orifices 48 of cylindrical shape,
located facing the passage orifice 19 of the absorptive structure 6.
The passage orifices 48 are spaced apart angularly by 90° from each other and
are each intended for letting through an electric contact 28.
The electric contacts 28 are then positioned in the passage orifices 19, 48 so as to
connect the dipoles 16A, 16B and the circuit 44.
The sole 46 has a general cylindrical shape with an axis A-A' and a diameter of
less than or equal to the diameter of the metal plate 8 and comprises a lower surface 50.
The connectors 24 are then attached to the impedance matching circuit 44 in the
sole 46 so as to protrude from the lower surface 50, as illustrated in Fig. 7.
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The electric contacts 31 ensure electric continuity between the ground 36 of each
connector 24 and the removable metal cover 462.
With reference to Fig. 8, the antenna 2 also comprises a radome 52 capable of
protecting said antenna 2 and of allowing the passage of electromagnetic radiations
emitted and received by the antenna2.
For this purpose, the radome 52 has a general cylindrical shape and is made from
a material known to one skilled in the art, of the epoxy glass, polyamide or further PEEK
type, etc.,
The radome 52 is delimited radially by a side wall 54 having a thickness of less
than or equal to the width I of the protrusion 20 and vertically by a transverse wall 56 of
discoidal shape.
The radome 52 is thus able to be attached on the protrusion 20 in a protective
J f e position illustrated in Fig. 8 and in which its axis coincides with the axis A-A'.
The radome 52 delimits a cylindrical cavity 58 with dimensions mating the
dimensions of the cylinder in which are comprised the dipoles 16A, 16B and the
absorptive structure 6.
In practice, the dimensions of the cavity 58 increased by distances e and s'
ranging from the order of 1 millimeter to a few millimeters, and corresponding to a gap
respectively existing between the dipoles 16A, 16B and the transverse wall 56 and
between the absorptive structure 6 and the side wall 54 of the radome 52 in the protective
position of the latter.
In the example of Fig. 8, this cavity 58 therefore has a diameter of about
330+2.f' mm and a height of about 22+ s mm.
Still in the example of Fig. 8 which illustrates the antenna 2 according to the
second embodiment, the side wall 54 is attached on the protrusion 20 by attachment
^ ^ means (not shown) so that the dipoles 16A, 16B and the absorptive structure 6 are
entirely comprised in the cavity 58, as illustrated in Fig. 8.
As illustrated in Fig. 8, the antenna 2 provided with its radome 52 is comprised in a
cylinder of axis A-A' and with a diameter substantially equal to 350 mm.
With reference to Fig. 9, the antenna 2 provided with its radome 52 is able to be
laid out on a planar surface 60 of a cylindrical cavity 62 made for this purpose in a vehicle
64, the metal plate 8 being in contact with said surface 60.
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In the example of Fig. 9, which illustrates an antenna 2 according to the second
embodiment of the invention, it is then the lower surface 50 of the sole 46 of the metal
plate 8 which is in contact with the planar surface 60.
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The cylindrical cavity 62 has a diameter substantially equal to the diameter of the
antenna 2 and a height substantially equal to the height of the radome 52, to which is
added the height of the metal plate 8.
Preferably, said surface 60 and said cylindrical cavity 62 are made from a metal
material.
In the surface 60 an aperture 66 is made for connecting via the connectors 24, the
impedance matching and supply means 10 to the emission/reception device (not shown)
of the antenna 2 which is associated with it.
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lAVe Claim:
1.- An antenna (2) for emitting/receiving electromagnetic waves, of the type
comprising:
- two dipoles (16A, 16B) orthogonal to each other, each dipole (16A, 16B)
comprising two radiating elements (4),
- a metal plate (8), and
- an absorptive structure (6),
characterized in that the radiating elements (4) are all substantially planar, both
dipoles (16A, 16B) being substantially comprised in a same plane (P) and in that the
absorptive structure (6) is interposed between the metal plate (8) and the dipoles (16A,
JML 168) and is laid out in contact with the metal plate (8).
2.- The antenna (2) according to claim 1, characterized in that each radiating
element (4) has a general disc sector shape.
3.- The antenna (2) according to claim 1 or 2, characterized in that said plane (P)
is at a distance d from the absorptive structure (6), comprised between 1 mm and 2 mm.
4.- The antenna (2) according to any of the preceding claims, characterized in that
it comprises an impedance matching circuit (44) made in printed technology.
5.- The antenna (2) according to claim 4, characterized in that the metal plate (8)
comprises a sole (46), the impedance matching circuit (44) being laid out in said sole (46).
6.- The antenna (2) according to any of the preceding claims, characterized in that
it also comprises a protective radome (52).
7.- The antenna (2) according to any of the preceding claims, characterized in that
the absorptive structure (6) has a general cylindrical shape.
8.- The antenna according to claim 7, characterized in that the height of the
absorptive structure (6) is comprised between 20 mm and 21 mm, and advantageously
has the value of 20 mm, and its diameter is comprised between 330 mm and 334 mm,
and advantageously has the value of 330 mm.
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9.- The antenna (2) according to any of the preceding claims, characterized in that
the dipoles (16A, 16B) and the absorptive structure (6) are entirely comprised in a cylinder
with a diameter substantially equal to 330 mm and with a height substantially equal to
22 mm.
10.- The antenna (2) according to any of the preceding claims, characterized in
that the electromagnetic waves which it is capable of emitting and receiving, have
frequencies comprised in the whole range of frequencies 30 MHz - 500 MHz, and
advantageously in the whole range of frequencies 30 MHz - 700 MHz.
11.- The antenna (2) according to any of the preceding claims, characterized in
that it is able to emit and receive electromagnetic waves having any polarization from
among a linear polarization, a circular polarization or an elliptical polarization, each dipole
^ l * ' (16A, 16B) being capable of emitting/receiving electromagnetic waves having a horizontal
linear polarization for one of the dipoles (16B) and a vertical linear polarization for the
other dipole (16A) respectively.
12.- An land, airborne, or naval vehicle (64) of the type including:
- a planar surface (60) and/or a cavity (62),
- an antenna (2) according to any of the preceding claims and laid out on
said surface (60) and/or in said cavity (62).
13.- The vehicle (64) according to claim 12, characterized in that the planar
surface (60) and/or the cavity (62) are made from a metal material.