IMPROVED HORN ANTENNA

Description

[0001] The invention relates to the field of broadband frequency horn antennas.

[0002] When the operating frequency is above 18GHz (i.e., above the Ku band), the dimensions of the horn are reduced, making such a solution compatible with the integration constraints on supporting structures.

[0003] Consequently, horn antennas are now used in signals intelligence (SIGINT) and electronic countermeasures applications. A horn antenna can be used alone as a high-gain antenna or as a component of a high-gain, directional beam array for transmitting or receiving systems. A horn antenna can also be used as a component of a planar array for amplitude direction finding. Finally, a horn antenna can be used as a component of a multiplanar array for phase-direction / interferometry applications.

[0004] As depicted on the figure 1, a horn antenna includes a waveguide whose flared end, constituting the horn of the antenna, crosses a ground plane so as to emerge above the latter in order to emit and / or receive a principal radiation A in the upper half-space located above the ground plane.

[0005] However, when a horn is positioned close to a ground plane (more commonly a metallic plane), creeping B waves are generated at the top face of the ground plane and propagate radially away from the horn to the surface of the ground plane.

[0006] These creeping waves can be amplified by physical and / or electromagnetic discontinuities, particularly at the edge of the ground plane. The creeping waves can then combine with the main radiation, resulting in an alteration of the radiation pattern of the evolved horn antenna in the far field. In particular, the radiation pattern exhibits pronounced oscillations of its main lobe (as shown in curves CE1 and CH1 of the graphs of the figure 5 This represents a serious alteration of the radiation pattern, which can lead to detection errors. It should be noted that the deterioration of the radiation pattern is more pronounced at higher operating frequencies.

[0007] Creeping waves are also amplified by the presence of a radome, which is necessary to cover the antenna horn and protect it from external interference. More specifically, depending on its thickness and the real part of the relative permittivity of the material it is made of, the radome accentuates creeping waves. It can also modify the period of these waves for a given frequency and / or angular range.

[0008] To mitigate these creeping waves and their negative effects, it is known to move the horn mouth away from the ground plane. However, this solution is insufficient, especially when aiming to create thin horn antennas. Therefore, the height the horn protrudes above the ground plane cannot be significantly increased. Furthermore, by moving the ground plane away from the horn mouth, the antenna loses efficiency because the energy radiated by the horn is no longer confined to the half-space in front of it.

[0009] Another solution is to create corrugations on the upper face of the ground plane in order to trap creeping waves.

[0010] These corrugations have a depth of λ / 4 and a pitch between two successive corrugations less than or equal to λ / 2 (λ being the wavelength associated with the center frequency of the operating frequency range of the horn antenna under consideration). These corrugations behave as resonators capable of absorbing surface waves.

[0011] While this solution has the advantage of allowing effective trapping of creeping waves at the center frequency, it has many disadvantages: the working frequency range is reduced, since the absorption effect is optimized for the center frequency and the efficiency of these resonators is reduced as soon as one deviates too much from this center frequency (relative bandwidth between 15% and 20%); strong manufacturing constraints, since the thickness of the ground plane must be greater than the depth of the corrugations, the number of corrugations must be sufficient to effectively absorb surface waves, the machining precision of the corrugations must be high to avoid affecting the antenna response at high working frequencies, and the thinning of the ground plane raises problems of mechanical resistance.

[0012] Another state-of-the-art solution involves integrating a frequency-selective surface (FSS) into the horn antenna's radome. An FSS is a layer created by arranging metallic elements in a periodic pattern.

[0013] For example, the radome is made of a dielectric material. It has on its lower face (facing the horn and the ground plane) a first FSS surface and a second FSS surface on its upper face.

[0014] However, the primary objective of such a solution is not to trap creeping waves, but rather to minimize the radar cross-section (RCS).

[0015] Furthermore, this solution presents the following disadvantages: a loss of radiation efficiency, since the FSS surfaces at the horn antenna absorb part of the radiated energy; a limited bandwidth, since the metallic elements constituting the FSS surfaces are sized to be effective at the center frequency and the resonant effect on which the operation of an FSS surface is based decreases sharply when moving away from the center frequency (relative bandwidth around 10%); a complexity in manufacturing such a radome; the need to protect this composite radome with an additional radome covering the second FSS surface exposed to damage; the need for a minimum distance between the radome and the horn mouth for the FSS surfaces to be effective, which increases the thickness of the horn antenna accordingly.

[0016] Finally, another prior art solution consists of creating a high impedance surface - HIS ("High Impedance Surface") around the mouth of the antenna horn on the upper face of the ground plane.

[0017] A HIS surface is composed of periodically arranged metallic elements connected to the ground plane by a metallized via.

[0018] Such a solution has the advantage of being compatible with conventional printed circuit board manufacturing technologies.

[0019] However, it has the disadvantage of once again leading to a limited working frequency band because of the sizing of the metallic elements which constrains the domain in which this surface actually allows to absorb creeping waves (electromagnetic band gap less than 20%).

[0020] Thus, none of the known solutions make it possible to create a compact horn antenna (i.e., with a reduced thickness), with an extended working frequency range, while limiting the effects of creeping waves on radiation performance.

[0021] Furthermore, we know, from document US 2017 / 273562, of an electromagnetic radiation probe for controlling biological tissue, which includes a means of transmission-reception housed in a cavity, which is covered with a material absorbing electromagnetic radiation.

[0022] We know, from document CN 102 810 743 B, the positioning of a metamaterial around the horn of an antenna to attenuate the creeping waves propagating on the surface of the horn.

[0023] The present invention therefore aims to provide a solution to all or part of these problems.

[0024] For this purpose the invention relates to a horn antenna according to the attached claims.

[0025] The invention and its advantages will be better understood upon reading the following detailed description of various embodiments of the invention, given solely by way of illustrative and non-limiting examples, the description being made with reference to the accompanying drawings on which: There figure 1 is a schematic axial cross-sectional representation of a horn antenna according to the prior art; The figure 2 is an exploded perspective representation of a preferred embodiment of a horn antenna according to the invention; The figure 3 is a schematic representation in longitudinal section along an axial plane of the horn antenna of the figure 2 ; There figure 4 represents different variations in the implementation of the resistive film of the antenna of the figure 2 ; and, The figure 5represents the gains of a prior art horn antenna and a horn antenna according to the invention in plane E and in plane H. Preferred method of implementation

[0026] By referring to figures 2 And 3 , a preferred embodiment of the horn antenna according to the invention will be presented.

[0027] The horn antenna 11 comprises, along a Z axis, called vertical, a ground plane 12 and a horn 13.

[0028] The ground plane 12, for example, has a rectangular parallelepiped external shape with a square base and a reduced thickness e. Alternatively, the ground plane is cylindrical in shape.

[0029] It includes a central recess 21 of cylindrical shape, of radius R0 and of depth p. This recess has a bottom 22 and a peripheral edge 23. The bottom 22 is the upper face of the ground plane where the creeping waves develop.

[0030] The ground plane 12 is equipped with a central opening 24 through which the horn 13 passes.

[0031] For example, horn 13 has a constant cross-section (i.e., along a plane orthogonal to the Z-axis), for example, rectangular in shape. Alternatively, the horn's cross-section can have other shapes, for example, be flared and / or circular.

[0032] The horn 13 is arranged so that its mouthpiece 31 is placed above the bottom 22 of the recess 21 of the ground plane 12, at a height h above the latter.

[0033] The horn antenna 11 includes a radome 14 which closes the recess 21 of the ground plane 12 and covers the mouth 31 of the horn 13.

[0034] Radome 14 is essentially in the shape of a disk with a radius R1, less than R0.

[0035] Its lower face 41, oriented towards the bottom 22 of the mass plane 12, is advantageously provided with a central housing 42, shaped so as to receive the mouthpiece 31 of the horn 13.

[0036] According to the invention, the horn antenna 11 incorporates at least one resistive film 15.

[0037] It is arranged so as to surround the horn 13 and to be received in the recess 21 of the ground plane 12.

[0038] In this embodiment, the resistive film is arranged in a transverse plane. It is inscribed in a disk of radius R1.

[0039] The resistive film 15 is thin. It has a thickness k, typically between 10 µm and 20 µm. Such a thickness allows integration without increasing the total thickness e of the horn antenna 11.

[0040] The resistive film 15 has a central opening 54, the cross-section of which preferably corresponds to that of the horn 13, so that the resistive film 15 is positioned as close as possible to the horn (along a transverse plane), to maximize its effectiveness in reducing creeping waves.

[0041] Different variations in the production of resistive film will be presented below in relation to the figures 4 .

[0042] In the implementation of figures 2 And 3 , the horn antenna 11 advantageously includes a support layer 16 suitable for supporting resistive film 15.

[0043] In this embodiment, layer 16 is arranged in a transverse plane. It is inscribed in a disk of radius R1.

[0044] Layer 16 has a thickness that allows the resistive film 15 to be positioned slightly back from the mouthpiece 31 of the horn 13 in the vertical direction. It also fills the recess 21 in the ground plane 12.

[0045] The material for layer 16 is preferably a dielectric or (magneto)dielectric material. It has a low dielectric constant, typically less than or equal to 2. It is a low-loss material. The choice of such a material contributes to the attenuation of creeping waves, particularly by not accentuating the propagation of creeping waves at the underside of the radome.

[0046] In this embodiment, the resistive film 15 is bonded to the layer 16. The adhesive film is referenced by the number 17 on the figure 3It preferentially has a thickness between 50 µm and 250 µm. The adhesive film is actually thicker than the resistive film. It was not shown on the figure 2 .

[0047] The radome 14 and the assembly consisting of the resistive film 15 and the layer 16 are held in position on the ground plane 12 by a series of screws, one of which is shown on the figure 3 and bears the reference 18. Holes possibly tapped are provided in the corner 11 antenna components to receive these screws. Other ways of implementing this

[0048] Other implementation methods are possible.

[0049] In particular, the radome, the central recess, the resistive film, and the support layer could have a rectangular parallelepiped shape.

[0050] The resistive film can, for example, be placed directly on the upper face of the ground plane 12. For example, the film is glued to this upper face.

[0051] The resistive film can, for example, be placed directly on the lower face 41 of the radome 14. For example, the film is glued to this lower face.

[0052] Other ways of fixing the resistive film onto a support layer, the ground plane or the radome are known to those skilled in the art, and fixing by gluing is just one particularly simple example to implement.

[0053] Several resistive films can, for example, be provided, arranged vertically one above the other. Two successive resistive films are advantageously separated by an intermediate layer similar to the support layer 16, but whose thickness is reduced so that the resulting laminate does not negatively impact the overall thickness of the horn antenna.

[0054] A layer, such as support layer 16, can be made by superimposing several elementary layers.

[0055] The horn antenna may optionally be non-planar. In this case, the resistive film(s) (and where applicable, each support layer) are shaped to follow the curvature of the ground plane.

[0056] Instead of gluing, other ways of fixing the resistive film (on the ground plane, the support layer and / or the radome) are conceivable. Variations in the production of resistive film

[0057] There figure 4 illustrates different variations in the production of resistive film.

[0058] In a first embodiment (a), the resistive film 115 is solid. It forms a continuous circular surface of radius R1 with a central opening 154 adapted to the outer contour of the horn. A resistive film is made of a single material having a single resistive value, for example between 100 and 10000 Ω / sq.

[0059] In order to have additional degrees of freedom to adapt the radio frequency performance of the horn antenna, the resistive film can have other configurations.

[0060] Thus, in a second variant (b), the resistive film 215 results from the combination of two materials having different resistive values. The first material forms a disk 221 with a central opening 254 and several concentric annular grooves, 222 and 223. The second material fills these grooves. The radius, depth, and / or thickness of a groove can be varied to adapt the antenna's properties. More than two materials with different resistive values ​​can be used. Instead of annular grooves, polygonal grooves can be used.

[0061] In a third variant (c), the resistive film 315 consists of several concentric rings, 321 to 329, the outer radius of one ring corresponding to the inner radius of the next. The material of each ring is chosen to create a radial resistive gradient with a minimum resistive value at the center and a maximum resistive value at the periphery. This limits the impact of a high resistive value near the radiating mouth of the horn, particularly on the flanks of the E-plane radiation pattern. Furthermore, it ensures optimal attenuation of creeping waves near the edge of the structure, thus limiting associated edge effects.

[0062] In a fourth embodiment (d), the resistive film 415 does not form a continuous surface, but a partial one. The resistive film does not completely cover the transverse plane around the mouthpiece of the horn. On the figure 4The resistive film 415, for example, is composed of several solid angular sectors, specifically two solid angular sectors 431 and 432 along the E-plane. These two solid angular sectors are therefore not contiguous. The flare of the angular sectors can be adjusted to fine-tune the properties of the horn antenna. More precisely, such a configuration has the advantage of not degrading the radiation efficiency of the horn antenna (at the radiating mouth) while ensuring efficient trapping of creeping waves over a wide frequency band. The effect of these waves on the undulations of the E-plane radiation pattern is thus minimized.

[0063] In a fifth embodiment (e), the resistive film 515 does not form a continuous surface, but a partial one. The resistive film does not completely cover the transverse plane around the mouthpiece of the horn. On the figure 4The resistive film 515, for example, is composed of several perforated angular sectors, specifically two perforated angular sectors 531 and 532 along plane E and two perforated angular sectors 533 and 534 along plane H, the perforated angular sectors not being contiguous. A perforated angular sector is, for example, made up of one or more concentric ring arcs. The flare of the angular sectors can be adjusted to fine-tune the properties of the horn antenna, as can the geometry of the ring arcs (spacing, thickness, material used, etc.).

[0064] Finally, in a sixth embodiment (f), the resistive film 615 results from combining embodiments (d) and (e) with two solid angular sectors 631 and 632 along plane E and two perforated angular sectors 633 and 634 along plane H. In this embodiment, the addition of resistive ring arcs joining the continuous angular sectors enhances the trapping of creeping waves and reduces the edge effects of the structure, particularly along plane H. The width of the resistive addition is preferably chosen to be less than or equal to λ / 4 (where λ is the wavelength chosen, usually, at the central operating frequency). This sixth embodiment is preferable because it offers the advantages of the fifth embodiment (e) while retaining the benefits of the solid solution in plane E (fourth embodiment (d)). This is the embodiment chosen for the Figures 1 And 2 .

[0065] It should be noted that resistive films can be optimized by electromagnetic simulation.

[0066] A resistive film can be advantageously produced using a conventional screen printing process. Alternatively, it can be produced using an equivalent process: aerosol printing, 3D printing, etc.

[0067] The resistive film, for example, is made with a carbon-enriched polymer ink, a material suitable for screen printing. Alternatively, the resistive film is a carbon-enriched thermoplastic, for example an ESD (Electrostatic Discharge) thermoplastic, a material suitable for 3D printing. Results

[0068] There figure 5highlights the positive impact of including a solid thin resistive film on the gain of a horn antenna compared to a prior art horn antenna. On graph (a) on the left of the figure 5 the gain is evaluated in plane E and on graph (b) on the right of the figure 5 The gain is evaluated in the H plane. The gains are given here for a frequency of 25 GHz.

[0069] In general, with the invention, we observe in particular: stabilization of the frequency dependence of the gain in the radio axis; at a given frequency, a clear reduction of the gain ripples in the main lobe of the radiation pattern; stabilization of the angular opening of the main lobe of the radiation pattern; and control of the rise of the secondary lobes of the radiation pattern.

Claims

1. A horn antenna (11) including: - a ground plane (12), delimiting an upper half-space; and, - a horn (13), forming one end of a waveguide, the horn crossing through the ground plane so that a mouth (31) of the horn is arranged at a predefined height above the ground plane in the upper half-space, characterized in that the horn antenna includes at least one resistive film (15), arranged around the horn (13), parallel to an upper face of the ground plane (12), the resistive film having an electrical resistance suitable for limiting creeping waves.

2. The horn antenna according to claim 1, wherein the resistive film is supported by the ground plane.

3. The horn antenna (11) according to claim 1, further including at least one support layer (16) suitable for supporting the resistive film (15), the support layer being arranged around the horn (13), parallel to the upper face of the ground plane (12).

4. The horn antenna (11) according to claim 3, wherein the support layer (16) is made of a material apt to attenuate creeping waves.

5. The horn antenna according to claim 3 or claim 4, wherein the support layer (16) results from the assembly of a plurality of elementary layers.

6. The horn antenna according to any of claims 3 to 5, including a first resistive film supported by an upper surface of the support layer and a second resistive film supported by a lower surface of the support layer.

7. The horn antenna according to any of claims 3 to 6, including a radome (14) covering the mouth of the horn (13).

8. The horn antenna according to claim 7, wherein the resistive film is supported by a lower face of the radome, oriented towards the ground plane.

9. The horn antenna (11) according to claim 7 or claim 8, wherein the radome (14) has an outer contour and wherein the resistive film (15) extends to the vicinity of the outer contour of the radome.

10. The horn antenna (11) according to claim 9, wherein the ground plane (12) has a raised peripheral edge for receiving the radome (14), the peripheral edge of the ground plane coinciding with the outer contour of the radome.

11. The horn antenna according to any of claims 1 to 10, wherein the resistive film (15) includes one or a plurality of solid angular sectors (631, 632).

12. The horn antenna according to any of claims 1 to 11, wherein the resistive film includes one or a plurality of concentric ring arcs (633, 634).

13. The horn antenna according to any of claims 1 to 12, wherein the resistive film (315) includes at least two concentric rings with different electrical resistances (321, 322).

14. The horn antenna (11) according to any of claims 1 to 13, wherein the resistive film (15) is made of a material compatible with manufacturing by screen printing.

15. The horn antenna according to any of claims 1 to 14, suitable for operating over a wide range of working frequencies above the X-band.