EP4278412A1 - Radome and antenna system with elevation compensation function - Google Patents

Radome and antenna system with elevation compensation function

Info

Publication number
EP4278412A1
EP4278412A1 EP22702160.7A EP22702160A EP4278412A1 EP 4278412 A1 EP4278412 A1 EP 4278412A1 EP 22702160 A EP22702160 A EP 22702160A EP 4278412 A1 EP4278412 A1 EP 4278412A1
Authority
EP
European Patent Office
Prior art keywords
radome
antenna
diffracting structure
support surface
elevation angle
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP22702160.7A
Other languages
German (de)
French (fr)
Inventor
Brigitte Loiseaux
Thi-Quynh-Van HOANG
Bernard DEMOTES-MAINARD
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Thales SA
Original Assignee
Thales SA
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Thales SA filed Critical Thales SA
Publication of EP4278412A1 publication Critical patent/EP4278412A1/en
Pending legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q1/00—Details of, or arrangements associated with, antennas
    • H01Q1/42—Housings not intimately mechanically associated with radiating elements, e.g. radome
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q1/00—Details of, or arrangements associated with, antennas
    • H01Q1/27—Adaptation for use in or on movable bodies
    • H01Q1/28—Adaptation for use in or on aircraft, missiles, satellites, or balloons
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q15/00—Devices for reflection, refraction, diffraction or polarisation of waves radiated from an antenna, e.g. quasi-optical devices
    • H01Q15/02—Refracting or diffracting devices, e.g. lens, prism
    • H01Q15/08—Refracting or diffracting devices, e.g. lens, prism formed of solid dielectric material

Definitions

  • the invention relates to a radome having a function of compensation for the elevation of incident electromagnetic waves; it also relates to an antenna system comprising such a radome. It falls within the field of antennas for telecommunications, in particular space, and more particularly antennas for space telecommunications intended to be deployed on mobile platforms, terrestrial, such as trains or buses, or airborne (in English, "satcom on the move >>, i.e. “satellite telecommunications in motion”).
  • the antennas used are generally of the planar type for reasons of size and aerodynamics.
  • Steerable parabolic antennas are sometimes placed at the level of the empennage of business jets, but these solutions are unsatisfactory for large-scale commercial operation given the increase in consumption caused by the protrusion formed by the antenna and its radome.
  • their installation often requires a specific and costly certification step.
  • adjustable planar antennas - mechanically or electronically - have a gain which decreases with elevation (defined as being the angle between a direction and the horizon). This leads to a reduction in the information rate, or even a break in the link, from or to a satellite low on the horizon (typically 30° or less).
  • WO 2010144170 discloses the use of negative refractive index metamaterials. However, this technology is not mature enough for industrial use.
  • Yet another approach consists in combining a metasurface radome with the use of an active antenna where the emission law includes angular precompensation for the formation of the radiation pattern complementary to the function of the radome (Alice Benini et al , "Phase-Gradient Meta-Dome for Increasing Grating-Lobe-Free Scan Range in Phased Arrays", IEEE Transactions On Antennas And Propagation, Vol. 66, No. 8, August 2018, 3973; WO 2019/165684).
  • This solution is very complex and expensive.
  • this object is achieved thanks to a radome equipped, in its inner peripheral part, with a diffractive structure which can be locally assimilated to a diffraction grating intended to operate in the Bragg regime in transmission.
  • This diffractive structure introduces a deflection of electromagnetic waves at grazing incidence, which increases their angle of elevation, without significantly affecting the propagation of waves whose direction of propagation is closer to the normal to the antenna.
  • the diffractive structure is carried by a structure which can be either integrated into the peripheral part of the shell of the radome, or located close to but physically separated from the latter. In both cases, it can advantageously be made of polymer or composite materials by three-dimensional (3D) printing, in particular by deposition of molten yarn.
  • An object of the invention is therefore a radome comprising a shell of dielectric material having an outer face and an inner face, the inner face defining, with a support surface, a volume intended to contain an antenna, characterized in that that it has, on or close to at least one peripheral region of its internal face, a diffracting structure which can be locally assimilated to a transmission diffraction grating operating in a spectral range in the microwave domain, the diffracting structure being configured so that an incident electromagnetic wave (OEI), at at least one wavelength X o of said spectral range in the microwave range propagating at an elevation angle 0 O of between 5° and 30° relative to the surface of support, satisfies the Bragg condition, and that a wave diffracted (OED) by the structure propagates with an elevation angle (0') greater than that of said incident electromagnetic wave; and so that the diffraction efficiency is less than 50% for electromagnetic waves propagating at an elevation angle greater than or equal to 40°.
  • OEI incident electromagnetic wave
  • the diffracting structure may include alternating layers of at least two dielectric materials with different dielectric permittivities, inclined with respect to said support surface.
  • the diffracting structure can be assimilated locally to a thick transmission grating in said spectral range of the microwave domain.
  • the diffracting structure can be configured so that an incident electromagnetic wave, at at least one wavelength X o of said spectral range in the microwave range, propagates at an elevation angle 0o of between 10° and 20°, with respect to the support surface, satisfies the Bragg condition, and that a wave diffracted by the structure propagates with an elevation angle ⁇ ′ greater than that of said incident electromagnetic wave.
  • the diffracting structure can be configured so that the diffraction efficiency is less than 20%, for electromagnetic waves propagating at an elevation angle greater than or equal to 40°, and preferably greater than or equal to 30° .
  • the inclination of the diffracting structure can be dimensioned so that an incident electromagnetic wave at the wavelength X o and having an elevation angle 0 O is deflected by an angle between 20 ° and 40°. This dimensioning can in particular consist in choosing the thicknesses and the inclination of the layers, or more generally the modulus and the direction of the wave vector of the grating (in turn a function of the spatial variation of the refractive index).
  • the diffracting structure can be physically separated from the shell made of dielectric material.
  • the diffracting structure can be made in one piece with the shell made of dielectric material.
  • the radome can then be manufactured by additive manufacturing.
  • Another object of the invention is an antenna system comprising such a radome and a depointable antenna located inside the volume delimited by the support surface and the internal face of the shell of the radome, the antenna being adapted to transmit or receive electromagnetic waves in a spectral range in the microwave domain, the diffraction grating in transmission being adapted to deflect a said electromagnetic wave whose elevation angle with respect to the support surface is less than one predetermined threshold by increasing its elevation angle. This deflection compensates in whole or in part for the decrease in antenna gain with elevation angle.
  • a structure “near" the internal face of the shell is meant a structure whose maximum distance from said internal face is much less - typically by at least a factor of 10 - than the diameter of said shell, or more generally at its greatest lateral dimension (ie in a plane parallel to the support surface).
  • diffractive structure locally comparable to a diffraction grating is meant a structure having a spatial variation of its dielectric permittivity which can either be periodic along at least one dimension over all or part of its extent, or deviate of a perfect periodicity by an amplitude or period modulation.
  • the period modulations in particular, must be relatively slow and/or weak, with for example fluctuations between periods successive not exceeding 10%.
  • a “quasi-periodic” variation By way of example, it may be a “chirped” network.
  • FIG.1 a sectional view of an antenna system comprising a radome according to a first embodiment of the invention
  • FIG. 2 a sectional view of an antenna system comprising a radome according to a second embodiment of the invention
  • FIG. 3 a detail view of the diffractive structure equipping the radome of [Fig. 1] illustrating its operation;
  • FIG. 4 a sectional view of an antenna system comprising a radome according to a third embodiment of the invention.
  • the RDM radome of Figure 1 comprises a CD shell of dielectric material - typically a polymer.
  • the shell typically has a rounded periphery, for example in the shape of a circular crown, and a flat top with a smooth transition between the two regions.
  • the thickness of the shell is represented constant for the sake of simplicity. However, in reality, it is usually variable, calculated in such a way as to optimize the radiation. It will then be necessary to take this into account when designing the diffractive structure.
  • the shell is delimited by an outer face FE and an inner face Fl, and rests on a support surface PS, generally planar, which carries a planar antenna A which can operate in transmission and/or in reception (in the following, we will consider the case of an antenna operating in reception, but the generalization does not pose any difficulty, by virtue of the law of inverse return).
  • a support surface PS generally planar, which carries a planar antenna A which can operate in transmission and/or in reception (in the following, we will consider the case of an antenna operating in reception, but the generalization does not pose any difficulty, by virtue of the law of inverse return).
  • the part of the hull located vertically to the antenna is at least approximately flat, so as not to distort the radiation pattern.
  • the internal face Fl of the shell and the support surface PS delimit a totally or partially closed volume V.
  • the antenna A is for example of the planar type - such as a "patch”, “leaky wave” or “slot array” antenna - and advantageously depointable, that is to say having a diagram of steerable antenna, especially in elevation.
  • the misalignment can be obtained by various means other than a mechanical orientation of the antenna, for example a phase control in the case of an array antenna or a system of Risley prisms.
  • the antenna A is designed to operate in the microwave range (between 1 GHz and 300 GHz, that is to say wavelengths between about 30 cm and about 1 mm).
  • the radome of FIG. 1 differs from a radome of the prior art by the presence, inside the volume V, of a diffracting structure SD, locally comparable to a diffraction grating, located in a peripheral part, that is to say external, in a radial direction measured parallel to the support surface, of this volume and close to the internal face F1 of the shell.
  • the structure SD is frustoconical and, seen in section, it is in the form of two planar elements, inclined with respect to the support surface PS and to the antenna A. It is physically separated from the CD shell although it is close to, or may even touch, its inner face.
  • the SD structure can be produced by additive manufacturing techniques (“3D printing”), in particular by deposition of fused wire (FDM, that is to say “Fused Deposition Modeling”), by a material such as acrylonitrile butadiene styrene (ABS), cyclic olefin copolymers (CGC), Polyetheretherketone (PEEK) and polyetherketoneketone (PEKK), the latter two being high performance polymers with particularly high thermal stability .
  • FDM fused wire
  • CGC cyclic olefin copolymers
  • PEEK Polyetheretherketone
  • PEKK polyetherketoneketone
  • the SD structure could be manufactured by assembling dielectric materials having different permissivities. For example, it is possible to produce a stack of layers of such materials, then cut it in a direction inclined with respect to the planes of the layers. However, this approach is much more complex to implement.
  • the SD structure is integrated into the CD shell and forms part of its inner face.
  • the SD structure can be "printed" on the internal face of a pre-existing shell, or the shell-diffracting structure assembly can be entirely produced by additive manufacturing.
  • Such a “monolithic” embodiment avoids the need for an assembly step, but requires compatibility between the materials used for the shell and for the diffractive structure.
  • An embodiment in two parts, as in FIG. 1, also has advantages, especially for aeronautical applications: the diffracting structure is isolated from the mechanical deformations of the shell; moreover, the latter does not have to undergo a new certification process.
  • the SD structure exhibits a periodic or quasi-periodic variation in its dielectric permittivity. It is therefore similar, in a first approximation, to a thick “holographic” diffraction grating.
  • it comprises an alternation of bands formed by layers CM1, CM2 of at least two materials having different dielectric permittivities and, preferably, substantially real (not introducing significant losses).
  • CM1, CM2 of at least two materials having different dielectric permittivities and, preferably, substantially real (not introducing significant losses).
  • the case of a periodic grating will be considered here, in which the layers of the same type all have the same thickness, which is constant over the entire extent of each layer.
  • Thick means here that the thickness "e" of the network satisfies the condition and preferably Q>10 where X o is the wavelength in vacuum of the electromagnetic radiation to be diffracted A is the spatial period, or not, of the grating and n the average refractive index of the material constituting it.
  • X o is the wavelength in vacuum of the electromagnetic radiation to be diffracted
  • A is the spatial period, or not, of the grating
  • n the average refractive index of the material constituting it.
  • - A is the spatial period, or pitch, of the grating, that is to say the sum of the thicknesses of the layers CM1 and CM2 (in the case of a grating with variable pitch, the pitch A is defined locally);
  • - K is the wave vector of the grating, the orientation of which is perpendicular to the layers and the modulus is 2/A;
  • an incident electromagnetic wave OEI coming from outside the radome, forms an angle 0 (elevation angle) with the plane of the support surface PS (or a plane tangent to this surface) and an angle a with the direction normal n.
  • the normal n therefore forms an angle cc+0 with the surface PS.
  • the diffracted electromagnetic wave OED resulting from the interaction of OEI with the grating, forms an angle 0' with the plane of the support surface PS (or a plane tangent to this surface).
  • the incident electromagnetic wave OEI is optimally diffracted when its wavelength X o and its elevation angle satisfy, exactly or approximately, the Bragg condition:
  • n mean is the average refractive index of the structure (remember that, for non-magnetic materials, the refractive index is the square root of the
  • the diffraction efficiency defined as the ratio between the intensity of a diffracted electromagnetic wave and that of the corresponding incident electromagnetic wave, depends on the difference An between the refractive indices of the layers CM1, CM2; the thickness "e" of the network,
  • An is the difference between the refractive indices of the layers CM1, CM2 and "e" the grating thickness.
  • the formalism of Kogelnik also makes it possible to determine an angular range of acceptance A0 - defined as the angular range, centered around the angle 0 O - in which the diffraction efficiency at the wavelength X o is greater than or equal to half of its maximum value.
  • This equation makes it possible to calculate 0' as a function of 0, , A and X.
  • the diffracting structure is dimensioned such that the incident electromagnetic waves having an elevation angle 0 lower than a critical value 0 C are diffracted towards higher angles of incidence 0′, while the waves having higher elevation angle are not effectively diffracted.
  • the angle 0 C corresponds to an elevation below which the gain of the antenna is considered insufficient - for example less than 20% of the maximum gain corresponding to normal incidence.
  • the dimensioning consists in choosing the materials of the layers CM1, CM2 (and therefore the parameters n mean and An), their possibly variable thicknesses (and therefore the parameter A in the case of a periodic structure) as well as the corner (
  • the thickness e is chosen, for a fixed value An, to maximize the diffraction efficiency.
  • the waves at grazing incidence for which the antenna gain would be very low, are diffracted so as to reach the antenna with higher elevations; the waves whose elevation is already satisfactory are only very slightly affected by the structure.
  • the optimum thickness e is about 3.5 mm if only the first order of diffraction is considered. If the network is optimized for operation at order 2, we find a thickness of 10.5 mm, which may be easier to manufacture. More generally, the designer may have to choose operation at an order greater than 1 for technological reasons or sensitivity to polarization.
  • the diffracting structure is sized so that the Bragg condition is satisfied for an angle ⁇ o of less than 30° and generally between 5° and 30° and preferably between 10° and 20°.
  • the deflection induced by the diffracting structure should preferably be between 20° and 40°.
  • the diffraction efficiency is generally desirable for the diffraction efficiency to be less than 50%, and preferably less than 20%, for electromagnetic waves propagating at an elevation angle greater than or equal to 40°, and preferably greater than or equal to at 30°.
  • the diffracting structure can be dimensioned using the Kogelnik formalism, by means of numerical optimization algorithms, genetic algorithms, etc.
  • the diffractive structure can be produced by using a single structured material at a sub-wavelength scale to induce a variation in its refractive index.
  • the structuring can consist, for example, of holes or pillars.
  • the [Fig. 5A] shows a detail of a diffracting structure having two layers CS1, CS2 of the same material (for example a polymer), one of which - CS1 - is massive and has a dielectric permittivity ECSI and a refractive index n CSi while the other has a network of cavities with a square section of side less than X 0 /2 and therefore has a refractive index intermediate between n CSi and the refractive index of air.
  • CS1, CS2 of the same material
  • ECSI dielectric permittivity
  • n CSi refractive index n CSi
  • the other has a network of cavities with a square section of side less than X 0 /2 and therefore has a refractive index intermediate between n
  • a more gradual variation in the refractive index is obtained by inserting between CS1 and CS2 a third layer CS3 having smaller cavities than those of CS2, and therefore an effective refractive index closer to n C if-
  • the [ Fig. 5C] and [Fig. 5D] illustrate structures based on the same principle but in which the structuring of the layers CS2' and CS3' consists of pillars with a square base.
  • the shape of the cavities or pillars can be arbitrary, for example circular, hexagonal etc.
  • This embodiment lends itself particularly well to additive manufacturing and makes it possible to avoid the complexity associated with the use of several materials (multiple deposition heads, compatibility between the materials, etc.). However, other manufacturing techniques such as machining and casting are also possible.
  • the SD structure does not have to be frustoconical; it can for example match the rounded shape of the internal face of the shell.
  • the diffraction grating may have a variable pitch, in particular at its ends, and more particularly when approaching its upper end (furthest from the surface PS) to limit the secondary diffraction lobes.
  • the diffracting structure SD covers the entire internal face of the shell and has a variable pitch, so that its diffraction efficiency at the wavelength Xo decreases progressively, down to s cancel at normal angle of attack. The advantage is to avoid discontinuities having undesirable effects on the radiation pattern of the antenna-radome assembly.
  • the network can comprise more than two types of distinct layers. This is even generally preferable in order to offer the designer more degrees of freedom to optimize the performance of the assembly.
  • the variation of the refractive index can also be progressive, for example sinusoidal. This is generally desirable, again to avoid discontinuities, and can be achieved in additive manufacturing by melting at the interface between two layers.
  • the invention applies to any type of antenna, not necessarily planar, and to radomes which may have different shapes and may or may not contain elements, in particular conductors, for example frequency-selective surfaces, liable to affect the propagation electromagnetic waves.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Astronomy & Astrophysics (AREA)
  • Aviation & Aerospace Engineering (AREA)
  • General Physics & Mathematics (AREA)
  • Remote Sensing (AREA)
  • Details Of Aerials (AREA)
  • Aerials With Secondary Devices (AREA)

Abstract

Radome (RDM) comprising a shell (CD) made of dielectric material having an outer face (FE) and an inner face (FI), the inner face defining, together with a supporting surface (PS), a volume (V) that is intended to contain an antenna (A), characterized in that it has, in or close to at least one peripheral region of its inner face, a diffracting structure (SD, SD') locally akin to a transmission diffraction grating. Antenna system comprising such a radome containing an antenna (A), for example an aimable antenna.

Description

DESCRIPTION DESCRIPTION
Titre de l’invention : Radôme et système d’antenne avec fonction de compensation d’élévation Title of the invention: Radome and antenna system with elevation compensation function
[0001] L’invention porte sur un radôme présentant une fonction de compensation de l’élévation des ondes électromagnétiques incidentes ; elle porte également sur un système d’antenne comprenant un tel radôme. Elle relève du domaine des antennes pour télécommunications, notamment spatiales, et plus particulièrement des antennes de télécommunications spatiales destinées à être déployées sur des plateformes mobiles, terrestres, tels que des trains ou des autobus, ou aéroportées (en anglais, « satcom on the move >>, c’est-à-dire « télécommunications satellitaires en mouvement »). The invention relates to a radome having a function of compensation for the elevation of incident electromagnetic waves; it also relates to an antenna system comprising such a radome. It falls within the field of antennas for telecommunications, in particular space, and more particularly antennas for space telecommunications intended to be deployed on mobile platforms, terrestrial, such as trains or buses, or airborne (in English, "satcom on the move >>, i.e. “satellite telecommunications in motion”).
[0002] Dans ce type d’application, et notamment dans le cas de plateformes aéroportées, les antennes utilisées sont généralement de type planaire pour des raisons d’encombrement et d’aérodynamique. Des antennes paraboliques orientables sont parfois placées au niveau de l’empennage des avions d’affaires, mais ces solutions sont insatisfaisantes pour une exploitation commerciale à grande diffusion compte-tenu de l’accroissement de consommation entraînée par la protubérance formée par l’antenne et son radôme. En outre, leur installation nécessite souvent une étape de certification spécifique et coûteuse. [0002] In this type of application, and in particular in the case of airborne platforms, the antennas used are generally of the planar type for reasons of size and aerodynamics. Steerable parabolic antennas are sometimes placed at the level of the empennage of business jets, but these solutions are unsatisfactory for large-scale commercial operation given the increase in consumption caused by the protrusion formed by the antenna and its radome. In addition, their installation often requires a specific and costly certification step.
[0003] D’autre part, les antennes planaires dépointables - de manière mécanique ou électronique - présentent un gain qui décroit avec l’élévation (définie comme étant l’angle entre une direction et l’horizon). Cela conduit à une réduction du débit d’informations, voire à une rupture de la liaison, depuis ou vers un satellite bas sur l’horizon (typiquement 30° ou moins). [0003] On the other hand, adjustable planar antennas - mechanically or electronically - have a gain which decreases with elevation (defined as being the angle between a direction and the horizon). This leads to a reduction in the information rate, or even a break in the link, from or to a satellite low on the horizon (typically 30° or less).
[0004] Diverses solutions ont été proposées pour accroître la plage angulaire de fonctionnement de ces antennes. Par exemple WO 2010144170 divulgue l’utilisation de métamétériaux à indice de réfraction négatif. Cette technologie n’est cependant pas suffisamment mûre pour une utilisation industrielle. Various solutions have been proposed to increase the angular operating range of these antennas. For example WO 2010144170 discloses the use of negative refractive index metamaterials. However, this technology is not mature enough for industrial use.
[0005] L’article de E. Gandini et al. « A Low-Profile Low-Cross Polarization Dielectric Dome Antenna for Wide-Scanning Applications >> 13th European Conference on Antennas and Propagation (EuCAP 2019), ainsi que le document WO 2019/067474, proposent l’utilisation d’un radôme à épaisseur variable, se comportant comme une lentille divergente. L’inconvénient de cette approche est que le radôme déforme le lobe de rayonnement de l’antenne même pour des angles d’élévation importants. [0005] The article by E. Gandini et al. "A Low-Profile Low-Cross Polarization Dielectric Dome Antenna for Wide-Scanning Applications >> 13th European Conference on Antennas and Propagation (EuCAP 2019), as well as the document WO 2019/067474, propose the use of a radome with thickness variable, behaving like a diverging lens. The disadvantage of this approach is that the radome distorts the radiation lobe of the antenna even for large elevation angles.
[0006] Encore une autre approche consiste à combiner un radôme à métasurface avec l’utilisation d’une antenne active où la loi d’émission comporte une précompensation angulaire de formation du diagramme de rayonnement complémentaire à la fonction du radôme (Alice Benini et al, « Phase-Gradient Meta- Dome for Increasing Grating-Lobe-Free Scan Range in Phased Arrays », IEEE T ransactions On Antennas And Propagation, Vol. 66, No. 8, August 2018, 3973 ; WO 2019/165684). Cette solution est très complexe et coûteuse. [0006] Yet another approach consists in combining a metasurface radome with the use of an active antenna where the emission law includes angular precompensation for the formation of the radiation pattern complementary to the function of the radome (Alice Benini et al , "Phase-Gradient Meta-Dome for Increasing Grating-Lobe-Free Scan Range in Phased Arrays", IEEE Transactions On Antennas And Propagation, Vol. 66, No. 8, August 2018, 3973; WO 2019/165684). This solution is very complex and expensive.
[0007] Il existe donc un besoin pour une solution simple et économique permettant d’accroître la plage angulaire de fonctionnement d’une antenne planaire, et plus particulièrement d’augmenter son gain pour des faibles angles d’élévation. L’invention vise à apporter une telle solution. [0007]There is therefore a need for a simple and economical solution making it possible to increase the angular operating range of a planar antenna, and more particularly to increase its gain for low elevation angles. The invention aims to provide such a solution.
[0008] Conformément à l’invention ce but est atteint grâce à un radôme équipé, dans sa partie périphérique intérieure, d’une structure diffractive assimilable localement à un réseau de diffraction destiné à fonctionner en régime de Bragg en transmission. Cette structure diffractive introduit une déflexion des ondes électromagnétiques en incidence rasante, qui augmente leur angle d’élévation, sans affecter sensiblement la propagation des ondes dont la direction de propagation est plus proche de la normale à l’antenne. La structure diffractive est portée par une structure qui peut être soit intégrée à la partie périphérique de la coque du radôme, soit située à proximité mais physiquement séparée de cette dernière. Dans les deux cas, elle peut avantageusement être réalisée en matériaux polymères ou composites par impression tridimensionnelle (3D), notamment par dépôt de fil fondu. In accordance with the invention, this object is achieved thanks to a radome equipped, in its inner peripheral part, with a diffractive structure which can be locally assimilated to a diffraction grating intended to operate in the Bragg regime in transmission. This diffractive structure introduces a deflection of electromagnetic waves at grazing incidence, which increases their angle of elevation, without significantly affecting the propagation of waves whose direction of propagation is closer to the normal to the antenna. The diffractive structure is carried by a structure which can be either integrated into the peripheral part of the shell of the radome, or located close to but physically separated from the latter. In both cases, it can advantageously be made of polymer or composite materials by three-dimensional (3D) printing, in particular by deposition of molten yarn.
[0009] L’utilisation de réseaux de diffractions pour introduire une déflexion des ondes électromagnétiques émises par ou dirigées vers une antenne est connue, par exemple, de AU 2018 311 770, US 2007/002305 et US 2,638,588. Cependant, il n’est pas connu d’utiliser des structures de type réseau de diffraction agencés à la périphérie d’un radome pour défléchir les ondes électromagnétiques présentant un faible angle d’élévation sans perturber, ou en perturbant de manière marginale, la propagation des ondes électromagnétiques présentant un angle d’élévation plus important. [0010] Un objet de l’invention est donc un radôme comprenant une coque en matériau diélectrique présentant une face externe et une face interne, la face interne définissant, avec une surface de support, un volume destiné à contenir une antenne, caractérisé en ce qu’il présente, sur ou à proximité d’au moins une région périphérique de sa face interne, une structure diffractante assimilable localement à un réseau de diffraction en transmission opérant dans une plage spectrale dans le domaine hyperfréquence, la structure diffractante étant configurée pour qu’une onde électromagnétique incidente (OEI), à au moins une longueur d’onde Xo de ladite plage spectrale dans le domaine hyperfréquence se propageant selon un angle d’élévation 0O compris entre 5° et 30°par rapport à la surface de support, satisfasse la condition de Bragg, et qu’une onde diffractée (OED) par la structure se propage avec un angle d’élévation (0’) supérieur à celui de ladite onde électromagnétique incidente ; et pour que l’efficacité de diffraction soit inférieure à 50% pour les ondes électromagnétiques se propageant selon un angle d’élévation supérieur ou égal à 40°. [0009] The use of diffraction gratings to introduce a deflection of the electromagnetic waves emitted by or directed towards an antenna is known, for example, from AU 2018 311 770, US 2007/002305 and US 2,638,588. However, it is not known to use structures of the diffraction grating type arranged at the periphery of a radome to deflect the electromagnetic waves having a low angle of elevation without disturbing, or by disturbing in a marginal way, the propagation electromagnetic waves with a higher angle of elevation. An object of the invention is therefore a radome comprising a shell of dielectric material having an outer face and an inner face, the inner face defining, with a support surface, a volume intended to contain an antenna, characterized in that that it has, on or close to at least one peripheral region of its internal face, a diffracting structure which can be locally assimilated to a transmission diffraction grating operating in a spectral range in the microwave domain, the diffracting structure being configured so that an incident electromagnetic wave (OEI), at at least one wavelength X o of said spectral range in the microwave range propagating at an elevation angle 0 O of between 5° and 30° relative to the surface of support, satisfies the Bragg condition, and that a wave diffracted (OED) by the structure propagates with an elevation angle (0') greater than that of said incident electromagnetic wave; and so that the diffraction efficiency is less than 50% for electromagnetic waves propagating at an elevation angle greater than or equal to 40°.
[0011] Selon des modes de réalisation particuliers d’un tel radôme : [0011] According to particular embodiments of such a radome:
[0012] - La structure diffractante peut comporter une alternance de couches d’au moins deux matériaux diélectriques de permittivités diélectriques différentes, inclinées par rapport à ladite surface de support. [0012] The diffracting structure may include alternating layers of at least two dielectric materials with different dielectric permittivities, inclined with respect to said support surface.
[0013] - La structure diffractante peut être assimilable localement à un réseau de transmission épais dans ladite plage spectrale du domaine hyperfréquence. [0013] The diffracting structure can be assimilated locally to a thick transmission grating in said spectral range of the microwave domain.
[0014] - La structure diffractante peut être configurée pour qu’une onde électromagnétique incidente, à au moins une longueur d’onde Xo de ladite plage spectrale dans le domaine hyperfréquence se propageant selon un angle d’élévation 0o compris entre 10° et 20°, par rapport à la surface de support, satisfasse la condition de Bragg, et qu’une onde diffractée par la structure se propage avec un angle d’élévation 0’ supérieur à celui de ladite onde électromagnétique incidente. [0014] The diffracting structure can be configured so that an incident electromagnetic wave, at at least one wavelength X o of said spectral range in the microwave range, propagates at an elevation angle 0o of between 10° and 20°, with respect to the support surface, satisfies the Bragg condition, and that a wave diffracted by the structure propagates with an elevation angle θ′ greater than that of said incident electromagnetic wave.
[0015] - La structure diffractante peut être configurée pour que l’efficacité de diffraction soit inférieure à 20%, pour les ondes électromagnétiques se propageant selon un angle d’élévation supérieur ou égal à 40°, et préférentiellement supérieur ou égal à 30°. [0016] - L’inclinaison de la structure diffractante peut être dimensionnée de telle sorte qu’une onde électromagnétique incidente à la longueur d’onde Xo et présentant un angle d’élévation 0O soit défléchie d’un angle compris entre 20° et 40°. Ce dimensionnement peut notamment consister à choisir les épaisseurs et l’inclinaison des couches, ou plus généralement le module et la direction du vecteur d’onde du réseau (à son tour fonction de la variation spatiale de l’indice de réfraction). [0015] The diffracting structure can be configured so that the diffraction efficiency is less than 20%, for electromagnetic waves propagating at an elevation angle greater than or equal to 40°, and preferably greater than or equal to 30° . - The inclination of the diffracting structure can be dimensioned so that an incident electromagnetic wave at the wavelength X o and having an elevation angle 0 O is deflected by an angle between 20 ° and 40°. This dimensioning can in particular consist in choosing the thicknesses and the inclination of the layers, or more generally the modulus and the direction of the wave vector of the grating (in turn a function of the spatial variation of the refractive index).
[0017] - La structure diffractante peut être physiquement séparée de la coque en matériau diélectrique. [0017] The diffracting structure can be physically separated from the shell made of dielectric material.
[0018] - Inversement, la structure diffractante peut être réalisée d’une seule pièce avec la coque en matériau diélectrique. Le radôme peut alors être fabriqué par fabrication additive. - Conversely, the diffracting structure can be made in one piece with the shell made of dielectric material. The radome can then be manufactured by additive manufacturing.
[0019] Un autre objet de l’invention est un système d’antenne comprenant un tel radôme et une antenne dépointable située à l’intérieur du volume délimité par la surface de support et la face interne de la coque du radôme, l’antenne étant adaptée pour émettre ou recevoir des ondes électromagnétiques dans une plage spectrale dans le domaine hyperfréquence, le réseau de diffraction en transmission étant adapté pour défléchir une dite onde électromagnétique dont l’angle d’élévation par rapport à la surface de support est inférieur à un seuil prédéterminé en augmentant son angle d’élévation. Cette déflexion permet de compenser en tout ou en partie la diminution du gain de l’antenne avec l’angle d’élévation. Another object of the invention is an antenna system comprising such a radome and a depointable antenna located inside the volume delimited by the support surface and the internal face of the shell of the radome, the antenna being adapted to transmit or receive electromagnetic waves in a spectral range in the microwave domain, the diffraction grating in transmission being adapted to deflect a said electromagnetic wave whose elevation angle with respect to the support surface is less than one predetermined threshold by increasing its elevation angle. This deflection compensates in whole or in part for the decrease in antenna gain with elevation angle.
[0020] Par une structure « à proximité >> de la face interne de la coque on entend une structure dont la distance maximale à ladite face interne est très inférieure - typiquement d’au moins un facteur 10 - au diamètre de ladite coque, ou plus généralement à sa plus grande dimension latérale (c’est à dire dans un plan parallèle à la surface de support). [0020] By a structure "near" the internal face of the shell is meant a structure whose maximum distance from said internal face is much less - typically by at least a factor of 10 - than the diameter of said shell, or more generally at its greatest lateral dimension (ie in a plane parallel to the support surface).
[0021 ] Par « structure diffractive assimilable localement à un réseau de diffraction >> on entend une structure présentant une variation spatiale de sa permittivité diélectrique qui peut soit être périodique selon au moins une dimension sur toute ou partie de son étendue, soit s’écarter d’une périodicité parfaite par une modulation d’amplitude ou de période. Les modulations de période, en particulier, doivent être relativement lentes et/ou faibles, avec par exemple des fluctuations entre périodes successives n’excédant pas 10%. On peut parler dans ce cas d’une variation « quasi-périodique ». A titre d’exemple il peut s’agir d’un réseau « chirpé >>. [0021] By "diffractive structure locally comparable to a diffraction grating" is meant a structure having a spatial variation of its dielectric permittivity which can either be periodic along at least one dimension over all or part of its extent, or deviate of a perfect periodicity by an amplitude or period modulation. The period modulations, in particular, must be relatively slow and/or weak, with for example fluctuations between periods successive not exceeding 10%. In this case, we can speak of a “quasi-periodic” variation. By way of example, it may be a “chirped” network.
[0022] D’autres caractéristiques, détails et avantages de l’invention ressortiront à la lecture de la description faite en référence aux dessins annexés donnés à titre d’exemple et qui représentent, respectivement : Other characteristics, details and advantages of the invention will become apparent on reading the description given with reference to the appended drawings given by way of example and which represent, respectively:
[0023] [Fig.1 ], une vue en coupe d’un système d’antenne comprenant un radôme selon un premier mode de réalisation de l’invention ; [0023] [Fig.1], a sectional view of an antenna system comprising a radome according to a first embodiment of the invention;
[0024] [Fig. 2] une vue en coupe d’un système d’antenne comprenant un radôme selon un second mode de réalisation de l’invention ; [0024] [Fig. 2] a sectional view of an antenna system comprising a radome according to a second embodiment of the invention;
[0025] [Fig. 3] une vue de détail de la structure diffractive équipant le radôme de la [Fig. 1] illustrant son fonctionnement ; et [0025] [Fig. 3] a detail view of the diffractive structure equipping the radome of [Fig. 1] illustrating its operation; and
[0026] [Fig. 4] une vue en coupe d’un système d’antenne comprenant un radôme selon un troisième mode de réalisation de l’invention. [0026] [Fig. 4] a sectional view of an antenna system comprising a radome according to a third embodiment of the invention.
[0027] [Fig. 5A], [Fig. 5B], [Fig. 5C] et [Fig. 5D], des exemples de structures diffractives selon diverses variants d’un quatrième mode de réalisation de l’invention. [0027] [Fig. 5A], [Fig. 5B], [Fig. 5C] and [Fig. 5D], examples of diffractive structures according to various variants of a fourth embodiment of the invention.
[0028] Le radôme RDM de la figure 1 comprend une coque CD en matériau diélectrique - typiquement un polymère. Dans les applications de télécommunications spatiales sur plateformes mobiles, la coque présente typiquement une périphérie arrondie, par exemple en forme de couronne circulaire, et un sommet plat avec une transition douce entre les deux régions. Dans le mode de réalisation de la figure, l’épaisseur de la coque est représentée constante dans un souci de simplicité. Cependant, en réalité, elle est généralement variable, calculée de manière à optimiser le rayonnement. Il faudra alors en tenir compte lors de la conception de la structure diffractive. La coque est délimitée par une face externe FE et une face interne Fl, et repose sur une surface de support PS, généralement plane, qui porte une antenne planaire A pouvant fonctionner en émission et/ou en réception (dans la suite, on considérera le cas d’une antenne fonctionnant en réception, mais la généralisation ne pose aucune difficulté, en vertu de la loi du retour inverse). Généralement, la partie de la coque située à la verticale de l’antenne est au moins approximativement plane, de manière à ne pas déformer le diagramme de rayonnement. La face interne Fl de la coque et la surface de support PS délimitent un volume totalement ou partiellement fermé V. [0029] L’antenne A est par exemple de type planaire - telle qu’une antenne « patch », « leaky wave >> ou « slot array >> - et avantageusement dépointable, c’est-à-dire présentant un diagramme d’antenne orientable, notamment en élévation. Le dépointage peut être obtenu par différents moyens autres qu’une orientation mécanique de l’antenne, par exemple un contrôle de phase dans le cas d’une antenne réseau ou un système de prismes de Risley. Typiquement, l’antenne A est conçue pour opérer dans le domaine des hyperfréquences (entre 1 GHz et 300 GHz, c’est-à-dire des longueurs d’onde comprise entre environ 30 cm et environ 1 mm). The RDM radome of Figure 1 comprises a CD shell of dielectric material - typically a polymer. In space telecommunications applications on mobile platforms, the shell typically has a rounded periphery, for example in the shape of a circular crown, and a flat top with a smooth transition between the two regions. In the embodiment of the figure, the thickness of the shell is represented constant for the sake of simplicity. However, in reality, it is usually variable, calculated in such a way as to optimize the radiation. It will then be necessary to take this into account when designing the diffractive structure. The shell is delimited by an outer face FE and an inner face Fl, and rests on a support surface PS, generally planar, which carries a planar antenna A which can operate in transmission and/or in reception (in the following, we will consider the case of an antenna operating in reception, but the generalization does not pose any difficulty, by virtue of the law of inverse return). Generally, the part of the hull located vertically to the antenna is at least approximately flat, so as not to distort the radiation pattern. The internal face Fl of the shell and the support surface PS delimit a totally or partially closed volume V. The antenna A is for example of the planar type - such as a "patch", "leaky wave" or "slot array" antenna - and advantageously depointable, that is to say having a diagram of steerable antenna, especially in elevation. The misalignment can be obtained by various means other than a mechanical orientation of the antenna, for example a phase control in the case of an array antenna or a system of Risley prisms. Typically, the antenna A is designed to operate in the microwave range (between 1 GHz and 300 GHz, that is to say wavelengths between about 30 cm and about 1 mm).
[0030] Le radôme de la figure 1 se différencie d’un radôme de l’art antérieur par la présence, à l’intérieur du volume V, d’une structure diffractante SD, localement assimilable à un réseau de diffraction, située dans une partie périphérique, c’est à dire externe, dans une direction radiale mesurée parallèlement à la surface de support, de ce volume et à proximité de la face interne Fl de la coque. Dans le mode de réalisation de la figure 1 , la structure SD est tronconique et, vue en coupe, elle se présente sous la forme de deux éléments planaires, inclinés par rapport à la surface de support PS et à l’antenne A. Elle est physiquement séparée de la coque CD bien qu’elle soit proche, ou puisse même toucher, sa face intérieure. The radome of FIG. 1 differs from a radome of the prior art by the presence, inside the volume V, of a diffracting structure SD, locally comparable to a diffraction grating, located in a peripheral part, that is to say external, in a radial direction measured parallel to the support surface, of this volume and close to the internal face F1 of the shell. In the embodiment of FIG. 1, the structure SD is frustoconical and, seen in section, it is in the form of two planar elements, inclined with respect to the support surface PS and to the antenna A. It is physically separated from the CD shell although it is close to, or may even touch, its inner face.
[0031] Avantageusement, la structure SD peut être réalisée par des techniques de fabrication additive (« impression 3D >>), notamment par dépôt de fil fondu (FDM, c’est-à-dire « Fused Deposition Modeling >>), en un matériau tel que l’acrylonitrile butadiène styrène (ABS), les copolymères d’oléfines cycliques (CGC), le Polyétheréthercétone (PEEK) et le polyéthercétonecétone (PEKK), ces deux derniers étant des polymères à hautes performances présentant une stabilité thermique particulièrement élevée. Ces matériaux peuvent être chargés pour en moduler les propriétés diélectriques. Advantageously, the SD structure can be produced by additive manufacturing techniques (“3D printing”), in particular by deposition of fused wire (FDM, that is to say “Fused Deposition Modeling”), by a material such as acrylonitrile butadiene styrene (ABS), cyclic olefin copolymers (CGC), Polyetheretherketone (PEEK) and polyetherketoneketone (PEKK), the latter two being high performance polymers with particularly high thermal stability . These materials can be charged to modulate their dielectric properties.
[0032] En variante, la structure SD pourrait être fabriquée par assemblage de matériaux diélectriques ayant des permettivités différentes. Par exemple, il est possible de réaliser un empilement de couches de tels matériaux, puis le découper dans une direction inclinée par rapport aux plans des couches. Cette approche est néanmoins beaucoup plus complexe à mettre en oeuvre. Alternatively, the SD structure could be manufactured by assembling dielectric materials having different permissivities. For example, it is possible to produce a stack of layers of such materials, then cut it in a direction inclined with respect to the planes of the layers. However, this approach is much more complex to implement.
[0033] Dans le mode de réalisation de la figure 2, la structure SD est intégrée à la coque CD et forme une partie de sa face interne. Par exemple, la structure SD peut être « imprimée >> sur la face interne d’une coque préexistante, ou l’ensemble coque - structure diffractante peut être entièrement réalisé par fabrication additive. Une telle réalisation « monolithique >> évite la nécessité d’une étape d’assemblage, mais nécessite une compatibilité entre les matériaux utilisés pour la coque et pour la structure diffractive. Une réalisation en deux parties, comme dans la figure 1 , présente aussi des avantages surtout pour des applications aéronautiques : la structure diffractante est isolée des déformations mécaniques de la coque ; en outre cette dernière ne doit pas subir un nouveau processus de certification. In the embodiment of Figure 2, the SD structure is integrated into the CD shell and forms part of its inner face. For example, the SD structure can be "printed" on the internal face of a pre-existing shell, or the shell-diffracting structure assembly can be entirely produced by additive manufacturing. Such a “monolithic” embodiment avoids the need for an assembly step, but requires compatibility between the materials used for the shell and for the diffractive structure. An embodiment in two parts, as in FIG. 1, also has advantages, especially for aeronautical applications: the diffracting structure is isolated from the mechanical deformations of the shell; moreover, the latter does not have to undergo a new certification process.
[0034] Comme mentionné plus haut, la structure SD présente une variation périodique ou quasi-périodique de sa permittivité diélectrique. Elle s’apparente donc, en première approximation, à un réseau de diffraction « holographique >> épais. Dans sa forme la plus simple, elle comprend une alternance de bandes formées par des couches CM1 , CM2 d’au moins deux matériaux ayant des permittivités diélectriques différentes et, de préférence, sensiblement réelles (n’introduisant pas de pertes significatives). Dans un souci de simplicité on considérera ici le cas d’un réseau périodique, dans lequel les couches d’un même type ont toutes la même épaisseur, qui est constante sur toute l’étendue de chaque couche. « Epais >> signifie ici que l’épaisseur « e >> du réseau satisfait la condition et de préférence Q > 10 où Xo est la longueur d’onde dans le vide du rayonnement électromagnétique devant être diffracté A est la période spatiale, ou pas, du réseau et n l’indice de réfraction moyen du matériau le constituant. La théorie de ces réseaux est décrite en détail dans l’article de H. Kogelnik « Coupled wave analysis of thick hologram gratings >> Bell Scientific Technical Journal 48, 29089 (1969). La généralisation au cas d’une structure à pas variable ne pose pas de difficulté de principe. As mentioned above, the SD structure exhibits a periodic or quasi-periodic variation in its dielectric permittivity. It is therefore similar, in a first approximation, to a thick “holographic” diffraction grating. In its simplest form, it comprises an alternation of bands formed by layers CM1, CM2 of at least two materials having different dielectric permittivities and, preferably, substantially real (not introducing significant losses). For the sake of simplicity, the case of a periodic grating will be considered here, in which the layers of the same type all have the same thickness, which is constant over the entire extent of each layer. "Thick" means here that the thickness "e" of the network satisfies the condition and preferably Q>10 where X o is the wavelength in vacuum of the electromagnetic radiation to be diffracted A is the spatial period, or not, of the grating and n the average refractive index of the material constituting it. The theory of these gratings is described in detail in the article by H. Kogelnik "Coupled wave analysis of thick hologram gratings" Bell Scientific Technical Journal 48, 29089 (1969). The generalization to the case of a structure with variable pitch does not pose any difficulty in principle.
[0035] Sur la figure 3 : In Figure 3:
- n représente la normale à la structure diffractive SD en un point du plan de la figure ; - n represents the normal to the diffractive structure SD at a point of the plane of the figure;
- A est la période spatiale, ou pas, du réseau, c’est-à-dire la somme des épaisseurs des couches CM1 et CM2 (dans le cas d’un réseau à pas variable, le pas A est défini localement) ; - K est le vecteur d’onde du réseau, dont l’orientation est perpendiculaire aux couches et le module vaut 2 / A ; - A is the spatial period, or pitch, of the grating, that is to say the sum of the thicknesses of the layers CM1 and CM2 (in the case of a grating with variable pitch, the pitch A is defined locally); - K is the wave vector of the grating, the orientation of which is perpendicular to the layers and the modulus is 2/A;
- <|) est l’angle formé par le vecteur K et le plan de la surface de support PS (ou un plan tangent à cette surface) ; - <|) is the angle formed by the vector K and the plane of the support surface PS (or a plane tangent to this surface);
- une onde électromagnétique incidente OEI, provenant de l’extérieur du radôme, forme un angle 0 (angle d’élévation) avec le plan de la surface de support PS (ou un plan tangent à cette surface) et un angle a avec la direction normale n. La normale n forme donc un angle cc+0 avec la surface PS. - an incident electromagnetic wave OEI, coming from outside the radome, forms an angle 0 (elevation angle) with the plane of the support surface PS (or a plane tangent to this surface) and an angle a with the direction normal n. The normal n therefore forms an angle cc+0 with the surface PS.
- L’onde électromagnétique diffractée OED, résultant de l’interaction de OEI avec le réseau, forme un angle 0’ avec le plan de la surface de support PS (ou un plan tangent à cette surface). - The diffracted electromagnetic wave OED, resulting from the interaction of OEI with the grating, forms an angle 0' with the plane of the support surface PS (or a plane tangent to this surface).
[0036] L’onde électromagnétique incidente OEI est diffractée de manière optimale lorsque sa longueur d’onde Xo et son angle d’élévation satisfont, de manière exacte ou approchée, la condition de Bragg : The incident electromagnetic wave OEI is optimally diffracted when its wavelength X o and its elevation angle satisfy, exactly or approximately, the Bragg condition:
[0038] où nmoy est l’indice de réfraction moyen de la structure (on rappelle que, pour des matériaux non magnétiques, l’indice de réfraction est la racine carrée de la [0038] where n mean is the average refractive index of the structure (remember that, for non-magnetic materials, the refractive index is the square root of the
. . ,, , permittivité diélectrique relative) et I angle d elevation mesure a I intérieur de la structure considérée comme un milieu homogène d’indice de réfraction nmoy- Lorsque la condition de Bragg (1 ) est satisfaite, l’efficacité de diffraction, définie comme le rapport entre l’intensité d’une onde électromagnétique diffractée et celle de l’onde électromagnétique incidente correspondante, dépend de la différence An entre les indices de réfraction des couches CM1 , CM2 ; de l’épaisseur « e >> du réseau, de. . ,, , relative dielectric permittivity) and I angle of elevation measured inside the structure considered as a homogeneous medium of refractive index n mean - When the Bragg condition (1) is satisfied, the diffraction efficiency, defined as the ratio between the intensity of a diffracted electromagnetic wave and that of the corresponding incident electromagnetic wave, depends on the difference An between the refractive indices of the layers CM1, CM2; the thickness "e" of the network,
. ' ÏTLOy . .. . la longueur d onde Xo et de I angle d incidence UQ . Dans le cas particulier =it/2, elle est proportionnelle à : . ' ÏTLOy . .. . the wavelength X o and the angle of incidence U Q . In the particular case =it/2, it is proportional to:
[0040] où An est la différence entre les indices de réfraction des couches CM1 , CM2 et « e >> l’épaisseur de réseau. Le formalisme de Kogelnik permet aussi de déterminer une plage angulaire d’acceptance A0 - définie comme la plage angulaire, centrée autour de l’angle 0O - dans laquelle l’efficacité de diffraction à la longueur d’onde Xo est supérieure ou égale à la moitié de sa valeur maximale. [0040] where An is the difference between the refractive indices of the layers CM1, CM2 and "e" the grating thickness. The formalism of Kogelnik also makes it possible to determine an angular range of acceptance A0 - defined as the angular range, centered around the angle 0 O - in which the diffraction efficiency at the wavelength X o is greater than or equal to half of its maximum value.
[0041 ] Le vecteur d’onde p de l’onde électromagnétique incidente et celui, a, de l’onde diffractée, sont liés par la relation suivante : The wave vector p of the incident electromagnetic wave and that, a, of the diffracted wave, are linked by the following relationship:
[0042] o = p - K (3) [0042] o = p - K (3)
[0043] Cette équation permet de calculer 0’ en fonction de 0, , A et X. This equation makes it possible to calculate 0' as a function of 0, , A and X.
[0044] Typiquement, la structure diffractante est dimensionnée de telle sorte que les ondes électromagnétiques incidentes présentant un angle d’élévation 0 inférieure à une valeur critique 0C sont diffractées vers des angles d’incidence 0’ plus élevés, tandis que les ondes présentant un angle d’élévation plus important ne sont pas diffractées efficacement. A son tour l’angle 0C correspond à une élévation en deçà de laquelle le gain de l’antenne est considéré insuffisant - par exemple inférieur à 20% du gain maximal correspondant à une incidence normale. [0044] Typically, the diffracting structure is dimensioned such that the incident electromagnetic waves having an elevation angle 0 lower than a critical value 0 C are diffracted towards higher angles of incidence 0′, while the waves having higher elevation angle are not effectively diffracted. In turn, the angle 0 C corresponds to an elevation below which the gain of the antenna is considered insufficient - for example less than 20% of the maximum gain corresponding to normal incidence.
[0045] Le dimensionnement consiste à choisir les matériaux des couches CM1 , CM2 (et donc les paramètres nmoy et An), leurs épaisseurs, éventuellement variables (et donc le paramètre A dans le cas d’une structure périodique) ainsi que l’angle (|). L’épaisseur e est choisie, pour une valeur An fixée, pour maximiser l’efficacité de diffraction. The dimensioning consists in choosing the materials of the layers CM1, CM2 (and therefore the parameters n mean and An), their possibly variable thicknesses (and therefore the parameter A in the case of a periodic structure) as well as the corner (|). The thickness e is chosen, for a fixed value An, to maximize the diffraction efficiency.
[0046] On peut considérer par exemple le cas d’un réseau conçu pour fonctionner à une longueur d’onde Xo de 1 cm (~30 GHz), présentant une inclinaison cc=10° par rapport au plan PS et réalisé en des matériaux présentant des indices de réfraction de 1 ,63 et 2,83. En choisissant A ~ 5 mm et cp ~ 64° l’on obtient un réseau de diffraction dans lequel la condition de Bragg est satisfaite pour 0o=1 O°. Une onde incidente avec une élévation de 10° (angle de Bragg) est diffractée à un angle 0’=35°. Par contre, des ondes présentant un angle d’élévation supérieur ou égal à 30° sont peu diffractées (efficacité de diffraction inférieure ou égale à 15%). Le but de l’invention est donc atteint : les ondes en incidence rasante, pour lesquelles le gain d’antenne serait très faible, sont diffractées de manière à parvenir sur l’antenne avec des élévations plus importantes ; les ondes dont l’élévation est déjà satisfaisante ne sont que très peu affectées par la structure. [0047] L’épaisseur e optimale vaut environ 3,5 mm si on ne considère que l’ordre 1 de diffraction. Si le réseau est optimisé pour un fonctionnement à l’ordre 2, on trouve une épaisseur de 10,5 mm, ce qui peut être plus facile à fabriquer. Plus généralement, le concepteur peut être amené à choisir un fonctionnement à un ordre supérieur à 1 pour des raisons technologiques ou de sensibilité à la polarisation. We can consider for example the case of a network designed to operate at a wavelength X o of 1 cm (~30 GHz), having an inclination cc=10° with respect to the plane PS and produced in materials having refractive indices of 1.63 and 2.83. By choosing A ~ 5 mm and cp ~ 64°, a diffraction grating is obtained in which the Bragg condition is satisfied for 0 o =1 O°. An incident wave with an elevation of 10° (Bragg angle) is diffracted at an angle 0'=35°. On the other hand, waves having an elevation angle greater than or equal to 30° are hardly diffracted (diffraction efficiency less than or equal to 15%). The object of the invention is therefore achieved: the waves at grazing incidence, for which the antenna gain would be very low, are diffracted so as to reach the antenna with higher elevations; the waves whose elevation is already satisfactory are only very slightly affected by the structure. The optimum thickness e is about 3.5 mm if only the first order of diffraction is considered. If the network is optimized for operation at order 2, we find a thickness of 10.5 mm, which may be easier to manufacture. More generally, the designer may have to choose operation at an order greater than 1 for technological reasons or sensitivity to polarization.
[0048] D’une manière générale, la structure diffractante est dimensionnée pour que la condition de Bragg soit satisfaite pour un angle 0o inférieur à 30° et généralement compris entre 5° et 30° et préférentiellement entre 10° et 20°. Pour les ondes à la longueur d’onde Xo incidentes à l’angle de Bragg 00, la déflexion induite par la structure diffractante doit préférentiellement être comprise entre 20° et 40°. In general, the diffracting structure is sized so that the Bragg condition is satisfied for an angle θo of less than 30° and generally between 5° and 30° and preferably between 10° and 20°. For the waves at the wavelength X o incident at the Bragg angle 0 0 , the deflection induced by the diffracting structure should preferably be between 20° and 40°.
[0049] Il est généralement souhaitable que l’efficacité de diffraction soit inférieure à 50%, et préférentiellement inférieure à 20%, pour les ondes électromagnétiques se propageant selon un angle d’élévation supérieur ou égal à 40°, et préférentiellement supérieur ou égal à 30°. It is generally desirable for the diffraction efficiency to be less than 50%, and preferably less than 20%, for electromagnetic waves propagating at an elevation angle greater than or equal to 40°, and preferably greater than or equal to at 30°.
[0050] La structure diffractante peut être dimensionnée en utilisant le formalisme de Kogelnik, au moyen d’algorithmes numériques d’optimisation, des algorithmes génétiques, etc. The diffracting structure can be dimensioned using the Kogelnik formalism, by means of numerical optimization algorithms, genetic algorithms, etc.
[0051] En variante, la structure diffractive peut être réalisée en utilisant un seul matériau structuré à une échelle sous-longueur d’onde pour induire une variation de son indice de réfraction. La structuration peut être constituée, par exemple, de trous ou de piliers. La [Fig. 5A], par exemple, montre un détail d’une structure diffractante présentant deux couches CS1 , CS2 d’un même matériau (par exemple un polymère) dont une - CS1 - est massive et présente une permettivité diélectrique ECSI et un indice de réfraction nCSi tandis que l’autre présente un réseau de cavités à section carrée de côté inférieur à X0/2 et présente de ce fait un indice de réfraction intermédiaire entre nCSi et l’indice de réfraction de l’air. Dans le cas de la [Fig. 5B] une variation plus progressive de l’indice de réfraction est obtenue en intercalant entre CS1 et CS2 une troisième couche CS3 présentant des cavités plus petites que celles de CS2, et donc un indice de réfraction effectif plus proche de nCsi- Les [Fig. 5C] et [Fig. 5D] illustrent des structures basées sur le même principe mais dans lesquelles la structuration des couches CS2’ et CS3’ est constituée de piliers à base carrée. Bien évidemment, la forme des cavités ou piliers peut être quelconque, par exemple circulaire, hexagonale etc. Ce mode de réalisation se prête particulièrement bien à la fabrication additive et permet d’éviter la complexité associée à l’utilisation de plusieurs matériaux (têtes de dépôts multiples, compatibilité entre les matériaux...). Cependant, d’autres techniques de fabrication telles que l’usinage et le moulage sont également envisageables. As a variant, the diffractive structure can be produced by using a single structured material at a sub-wavelength scale to induce a variation in its refractive index. The structuring can consist, for example, of holes or pillars. The [Fig. 5A], for example, shows a detail of a diffracting structure having two layers CS1, CS2 of the same material (for example a polymer), one of which - CS1 - is massive and has a dielectric permittivity ECSI and a refractive index n CSi while the other has a network of cavities with a square section of side less than X 0 /2 and therefore has a refractive index intermediate between n CSi and the refractive index of air. In the case of [Fig. 5B] a more gradual variation in the refractive index is obtained by inserting between CS1 and CS2 a third layer CS3 having smaller cavities than those of CS2, and therefore an effective refractive index closer to n C if- The [ Fig. 5C] and [Fig. 5D] illustrate structures based on the same principle but in which the structuring of the layers CS2' and CS3' consists of pillars with a square base. Of course, the shape of the cavities or pillars can be arbitrary, for example circular, hexagonal etc. This embodiment lends itself particularly well to additive manufacturing and makes it possible to avoid the complexity associated with the use of several materials (multiple deposition heads, compatibility between the materials, etc.). However, other manufacturing techniques such as machining and casting are also possible.
[0052] L’invention a été décrite en référence à des modes de réalisation particuliers, mais de nombreuses variantes sont possibles. Par exemple : The invention has been described with reference to particular embodiments, but many variants are possible. For instance :
La structure SD ne doit pas nécessairement être tronconique ; elle peut par exemple épouser la forme arrondie de la face interne de la coque. The SD structure does not have to be frustoconical; it can for example match the rounded shape of the internal face of the shell.
Comme mentionné plus haut, le réseau de diffraction peut présenter un pas variable, notamment à ses extrémités, et plus particulièrement en se rapprochant de son extrémité supérieure (la plus éloignée de la surface PS) pour limiter les lobes secondaires de diffraction. Dans le mode de réalisation de la [Fig. 4], par exemple, la structure diffractante SD” couvre l’intégralité de la face interne de la coque et présente un pas variable, de telle sorte que son efficacité de diffraction à la longueur d’onde Xo diminue progressivement, jusqu’à s’annuler en incidence normale. L’avantage est d’éviter des discontinuités ayant des effets indésirables sur le diagramme de rayonnement de l’ensemble antenne-radome. As mentioned above, the diffraction grating may have a variable pitch, in particular at its ends, and more particularly when approaching its upper end (furthest from the surface PS) to limit the secondary diffraction lobes. In the embodiment of [FIG. 4], for example, the diffracting structure SD” covers the entire internal face of the shell and has a variable pitch, so that its diffraction efficiency at the wavelength Xo decreases progressively, down to s cancel at normal angle of attack. The advantage is to avoid discontinuities having undesirable effects on the radiation pattern of the antenna-radome assembly.
Le réseau peut comporter plus de deux types de couches distinctes. Cela est même généralement préférable afin d’offrir au concepteur davantage de degrés de liberté pour optimiser les performances de l’ensemble. La variation de l’indice de réfraction peut également être progressive, par exemple sinusoïdale. Cela est généralement désirable, encore une fois pour éviter des discontinuités, et peut être obtenu en fabrication additive par fusion à l’interface entre deux couches. The network can comprise more than two types of distinct layers. This is even generally preferable in order to offer the designer more degrees of freedom to optimize the performance of the assembly. The variation of the refractive index can also be progressive, for example sinusoidal. This is generally desirable, again to avoid discontinuities, and can be achieved in additive manufacturing by melting at the interface between two layers.
L’invention s’applique à tout type d’antenne, non nécessairement planaire, et à des radomes pouvant présenter des formes différentes et contenant ou pas des éléments, notamment conducteurs, par exemple des surfaces sélectives en fréquence, susceptibles d’affecter la propagation des ondes électromagnétiques. The invention applies to any type of antenna, not necessarily planar, and to radomes which may have different shapes and may or may not contain elements, in particular conductors, for example frequency-selective surfaces, liable to affect the propagation electromagnetic waves.

Claims

REVENDICATIONS Radôme (RDM) comprenant une coque (CD) en matériau diélectrique présentant une face externe (FE) et une face interne (Fl), la face interne définissant, avec une surface de support (PS), un volume (V) destiné à contenir une antenne (A), caractérisé en ce qu’il présente, sur ou à proximité d’au moins une région périphérique de sa face interne, une structure diffractante (SD, SD’, SD”) assimilable localement à un réseau de diffraction en transmission opérant dans une plage spectrale dans le domaine hyperfréquence, la structure diffractante étant configurée pour qu’une onde électromagnétique incidente (CEI), à au moins une longueur d’onde Xo de ladite plage spectrale dans le domaine hyperfréquence se propageant selon un angle d’élévation 0O compris entre 5° et 30° par rapport à la surface de support, satisfasse la condition de Bragg, et qu’une onde diffractée (OED) par la structure se propage avec un angle d’élévation (0’) supérieur à celui de ladite onde électromagnétique incidente ; et pour que l’efficacité de diffraction soit inférieure à 50% pour les ondes électromagnétiques se propageant selon un angle d’élévation supérieur ou égal à 40°. Radôme selon la revendication 1 dans lequel la structure diffractante comporte une alternance de couches (CM1 , CM2) d’au moins deux matériaux diélectriques de permittivités diélectriques différentes, inclinées par rapport à ladite surface de support. Radôme selon la revendication 1 dans lequel la structure diffractante comporte une alternance de couches (CS1 , CS2, CS3, CS2’, CS3’) dont au moins certaines présentent une structuration à une échelle inférieure à une longueur d’onde de ladite plage spectrale modifiant leur permittivité diélectrique effective Radôme selon l’une des revendications précédentes dans lequel la structure diffractante est assimilable localement à un réseau de transmission épais dans ladite plage spectrale du domaine hyperfréquence. Radôme selon l’une des revendications précédentes dans lequel la structure diffractante est configurée pour qu’une onde électromagnétique incidente (OEI), à au moins une longueur d’onde Xo de ladite plage spectrale dans le domaine hyperfréquence se propageant selon un angle d’élévation 0O compris entre 10° et 20°, par rapport à la surface de support, satisfasse la condition de Bragg, et qu’une onde diffractée (OED) par la structure se propage avec un angle d’élévation (0’) supérieur à celui de ladite onde électromagnétique incidente. Radôme selon la revendication 5 dans lequel la structure diffractante est configurée pour que l’efficacité de diffraction soit inférieure à 20%, pour les ondes électromagnétiques se propageant selon un angle d’élévation supérieur ou égal à 40°, et préférentiellement supérieur ou égal à 30°. Radôme selon l’une des revendications 5 ou 6 dans lequel l’inclinaison de la structure diffractante est dimensionnée de telle sorte qu’une onde électromagnétique incidente à la longueur d’onde Xo et présentant un angle d’élévation 0O soit défléchie d’un angle compris entre 20° et 40°. Radôme selon l’une des revendications précédentes dans lequel la structure diffractante (SD) est physiquement séparée de la coque en matériau diélectrique. Radôme selon l’une des revendications 1 à 7 dans lequel la structure diffractante (SD’, SD”) est réalisée d’une seule pièce avec la coque en matériau diélectrique. Radôme selon la revendication 9, fabriqué par fabrication additive. Système d’antenne comprenant un radôme (RDM) selon l’une des revendications précédentes et une antenne (A) dépointable située à l’intérieur du volume (V) délimité par la surface de support (PS) et la face interne (Fl) de la coque (CD) du radôme, l’antenne étant adaptée pour émettre ou recevoir des ondes électromagnétiques dans une plage spectrale dans le domaine hyperfréquence, le réseau de diffraction en transmission étant adapté pour défléchir une dite onde électromagnétique dont l’angle d’élévation par rapport à la surface de support est inférieur à un seuil prédéterminé (0) en augmentant son angle d’élévation. Radome (RDM) comprising a shell (CD) of dielectric material having an outer face (FE) and an inner face (Fl), the inner face defining, with a support surface (PS), a volume (V) intended to contain an antenna (A), characterized in that it has, on or close to at least one peripheral region of its internal face, a diffracting structure (SD, SD', SD”) which can be locally assimilated to a diffraction grating in transmission operating in a spectral range in the microwave domain, the diffracting structure being configured so that an incident electromagnetic wave (IEC), at at least one wavelength X o of said spectral range in the microwave domain propagates according to a elevation angle 0 O between 5° and 30° with respect to the support surface, satisfies the Bragg condition, and that a wave diffracted (OED) by the structure propagates with an elevation angle (0' ) greater than that of said electromagnetic wave incident ; and so that the diffraction efficiency is less than 50% for electromagnetic waves propagating at an elevation angle greater than or equal to 40°. Radome according to Claim 1, in which the diffracting structure comprises alternating layers (CM1, CM2) of at least two dielectric materials of different dielectric permittivities, inclined with respect to the said support surface. Radome according to Claim 1, in which the diffracting structure comprises an alternation of layers (CS1, CS2, CS3, CS2', CS3') of which at least some have a structuring on a scale smaller than a wavelength of the said spectral range modifying their effective dielectric permittivity Radome according to one of the preceding claims, in which the diffracting structure is locally comparable to a thick transmission network in the said spectral range of the microwave domain. Radome according to one of the preceding claims, in which the diffracting structure is configured so that an incident electromagnetic wave (OEI), at at least one wavelength X o of the said spectral range in the microwave range propagating at an angle d elevation 0 O between 10° and 20°, with respect to the support surface, satisfies the Bragg condition, and that a wave diffracted (OED) by the structure propagates with an elevation angle (0') greater than that of said incident electromagnetic wave. Radome according to Claim 5, in which the diffracting structure is configured so that the diffraction efficiency is less than 20%, for the electromagnetic waves propagating at an elevation angle greater than or equal to 40°, and preferably greater than or equal to 30°. Radome according to one of Claims 5 or 6, in which the inclination of the diffracting structure is dimensioned such that an incident electromagnetic wave at the wavelength X o and having an elevation angle 0 O is deflected d an angle between 20° and 40°. Radome according to one of the preceding claims, in which the diffracting structure (SD) is physically separated from the shell made of dielectric material. Radome according to one of Claims 1 to 7, in which the diffracting structure (SD', SD”) is produced in one piece with the shell of dielectric material. Radome according to claim 9, manufactured by additive manufacturing. Antenna system comprising a radome (RDM) according to one of the preceding claims and a depointable antenna (A) situated inside the volume (V) delimited by the support surface (PS) and the internal face (Fl) of the shell (CD) of the radome, the antenna being adapted to transmit or receive electromagnetic waves in a spectral range in the microwave range, the transmission diffraction grating being adapted to deflect a said electromagnetic wave whose angle of elevation relative to the support surface is lower than a predetermined threshold (0) by increasing its elevation angle.
EP22702160.7A 2021-01-14 2022-01-13 Radome and antenna system with elevation compensation function Pending EP4278412A1 (en)

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