SYSTEM FOR REDUCING THE REFLECTIVITY OF AN ELECTROMAGNETIC WAVE INCIDENT ON A SURFACE AND DEVICE IMPLEMENTING THIS SYSTEM

Abstract

System for reducing the reflectivity of an electromagnetic wave incident on a surface and device implementing this system A system for controlling the reflection of an electromagnetic wave incident on a surface characterized in that the system is integrated into the surface, the system is equipped with a checkerboard comprising first facets and second facets, each first facet comprises a first non-conductive zone and a first electrically conductive zone, said first zones being configured so that the first facet has a series equivalent resonant circuit Zs, each second facet comprises a second non-conductive zone and a second electrically conductive zone, said second zones being configured so that the second facet has a parallel equivalent resonant circuit Zp, the first impedance Zs and the second impedance Zp being such that Zp*Zs=Z0², with Z0 the impedance of the vacuum.Figure for abstract: Figure 1.

Description

Title of the invention: System for reducing the reflectivity of an electromagnetic wave incident on a surface and device implementing this system FIELD OF THE INVENTION

[0001] The present invention relates to a system for reducing the reflectivity of an electromagnetic wave incident on a surface. BACKGROUND OF THE INVENTION

[0002] The increase in communication and connectivity needs leads to the multiplication of systems interacting with the electromagnetic environment such as antennas, absorbing devices, electromagnetic shielding devices, reflectors. This leads to an increase in the complexity of integrating these systems into carriers such as vehicles or buildings. The interactions between these systems themselves and between these systems and the surrounding infrastructures lead to disruptions in their operations. Furthermore, the need to increase the stealth of targets generates the need to reduce their reflectivity.

[0003] Satisfaction of these needs can be achieved by the use of materials having controlled and the lowest possible electromagnetic reflectivity.

[0004] In particular, panels that absorb electromagnetic radiation to attenuate interference and absorbent paints are known.

[0005] However, these systems have limited performance.

[0006] Indeed, they only operate over a limited frequency range and for incidences close to normal to their external surface. In addition, these systems can degrade when exposed to the environment in which they are placed, for example by corrosion. Summary of the invention

[0007] There is a need for devices capable of interacting with electromagnetic waves allowing better integration of radiating means on a carrier such as a vehicle or a surface vessel or improving the stealth of targets.

[0008] For this purpose, a system for controlling the reflection of an incident electromagnetic wave on a surface is described below. The system is integrated into the surface and is equipped with a checkerboard composed of first facets and second facets, each first facet is composed by the repetition of a first pattern comprising a first dielectric zone and a first electrically conductive zone, said first zones being configured so that the first facet comprises a circuit series equivalent resonant having an impedance Zs. Each second facet is composed by the repetition of a second pattern comprising a second dielectric zone and a second electrically conductive zone, said second zones being configured so that the second facet comprises a parallel equivalent resonant circuit having an impedance Zp, the first impedance Zs and the second impedance Zp being such that Zp*Zs=Z02, with Zo the impedance of the vacuum.

[0009] This makes it possible to obtain cancellation of the backscattered field in the reflection axis over a wide frequency band.

[0010] According to particular embodiments, the reflection system has one or more of the following characteristics, taken in isolation or in all technically possible combinations:

[0011] - The first dielectric zone has a first shape and the first electric zone electrically conductive zone has a second shape, furthermore the second dielectric zone has the second shape and the second electrically conductive zone has the first shape. This has the advantage of having complementary first and second facets to achieve cancellation of the backscattered field in the reflection axis over a wide frequency band.

[0012] - the first patterns and the second patterns have the same geometry and the first electrically conductive areas are connected by electrical connection means so as to be short-circuited. This has the advantage of having self-complementary first and second facets to achieve cancellation of the backscattered field in the reflection axis over a wide frequency band.

[0013] - a pair rate Tp of the checkerboard is defined by:

[0014] Tp = min(number of first facets, number of second facets), max(number of first facets, number of second facets) pair ratio being greater than or equal to 0.95, preferably greater than or equal to 0.98. In order to adapt to the geometry of the surface to be treated, each first facet is not necessarily associated with a second complementary facet. To obtain a satisfactory result, the pair ratio must be sufficiently high to ensure a negligible residue of the backscattered field.

[0015] - the areas of the first facets and the second facets are equal. This allows to use particularly economical manufacturing processes.

[0016] The description also relates to a device implementing the system for controlling the reflection of an incident electromagnetic wave on a surface comprising a first dielectric layer, a checkerboard, a second dielectric layer and a conductive plane arranged so as to act as a ground plane and stacked in the following order: the first layer then the checkerboard then the second layer then the conductive plane. This arrangement makes it possible in particular to protect the checkerboard from attacks from the external environment, improving for example resistance to shocks and corrosion.

[0017] According to particular embodiments, the device has one or more of the following characteristics, taken in isolation or in all technically possible combinations:

[0018] - a third dielectric layer arranged so that the stack begins with a third layer, then the first layer, then the checkerboard, then the second layer, then the conductive plane. This additional layer helps reduce backscattering by improving the device's adaptation to the vacuum impedance over a wider band.

[0019] - an additional grid can also advantageously be placed between the third layer and the first layer to improve the performance of the device at large angles of incidence. BRIEF DESCRIPTION OF THE FIGURES

[0020] Characteristics and advantages of the invention will appear on reading the description which follows, given solely by way of non-limiting example, and made with reference to the appended drawings, in which:

[0021] [Fig.l] [Fig.l] shows a detail of the reflection system

[0022] [Fig. 2] [Fig.2a] and 2b show elementary and complementary patterns in the form of slits and rings.

[0023] [Fig.3] [Fig.3] shows a checkerboard composed of first and second facets constituting the invention

[0024] [Fig.4] [Fig.5] Figures 4 and 5 show embodiments of checkerboards res respectively multi-periodic and symmetrical and non-periodic and non-symmetrical

[0025] [Fig.6] [Fig.6] shows a graph illustrating the attenuation performance measured as a function of frequency.

[0026] [Fig.7a][Fig.7b] Figures 7a and 7b show a particular embodiment of the so-called self-complementary system.

[0027] [Fig.8] [Fig.8] is a graph illustrating the phase variation obtained for each of the facets as well as their difference calculated on the embodiment of Figure 7 as a function of the frequency.

[0028] [Fig.9] [Fig.9] shows a longitudinal sectional view of an example of the device implementing the system of the invention.

[0029] [Fig.l] shows a checkerboard 1 according to the invention. The checkerboard 1 comprises first facets 2 and second facets 3.

[0030] [Fig.2a] shows a basic pattern of the first facet 2. [Fig.2b] shows a basic pattern of the second facet 3.

[0031] According to [Fig.2a], the basic pattern of the first facet 2 consists of a first non-conductive zone 4 and a first conductive zone 5 while according to [Fig.2b], the basic pattern of the second facet 3 consists of a second conductive zone having the shape of the non-conductive zone 4 of the first pattern and a second non-conductive zone having the shape of the conductive zone 5 of the first pattern.

[0032] The first and second facets 2, 3 are said to be complementary: the superposition of the conductive zones of the elementary patterns of the first and second facets gives a fully conductive surface.

[0033] Any other set of complementary shapes can be considered.

[0034] Each facet is formed by repeating an elementary pattern so as to form a high impedance surface (HIS). The patterns are repeated, for example, periodically in a facet. In the embodiment of Figures 2a and 2b, the elementary patterns are square in shape.

[0035] In one embodiment, the conductive zones are made with, for example, a metal such as copper.

[0036] Any other electrically conductive material can be considered for producing these conductive zones and in particular conductive inks.

[0037] In another embodiment, the non-conductive areas are formed by spaces occupied by air, vacuum or a dielectric material, for example a polyurethane resin, filling this space during integration into a complete device. Thus in the embodiment of [Fig.2a], the first pattern forms a slot while in the embodiment of [Fig.2b], the second pattern forms a ring.

[0038] We call Zs the impedance of a series resonant circuit equivalent to the first facet 2 and Zp the impedance of a parallel resonant circuit equivalent to the second facet 3 complementary to the first facet.

[0039] The localized elements of these two circuits are connected by the following relation 1:

[0040] ZpZç = Z2

[0041] where Zo is the vacuum impedance.

[0042] The phase difference between the reflection coefficients of the two circuits is then equal to 180° regardless of any losses and the frequency (including at resonance).

[0043] In the lossless case the amplitudes of the backscattered fields are equal to 1 while in the lossy case they are less than 1 but remain equal to each other. This results in a cancellation of the backscattered field in the axis, i.e. the wave electromagnetic radiation incident on the surface of the system is cancelled during its reflection, thus improving the integration of antennas or other radiating devices by reducing interference between them by means of separators implementing the system of the invention or reducing the backscattering of the wave emitted by a radar and therefore improving the stealth of the carrier.

[0044] Relation 1 above implies the following relation 2 between the localized elements (Rs, Ls, Cs) and (Rp, Lp, Cp) of the two circuits:

[0045] y 2 _ Rs p -p _ Lp _ Ls ~ R c~CP * P

[0046] These relationships remain true regardless of the normalization impedance Z„.

[0047] The dimensioning therefore consists of using a normalization impedance which is compatible with the known inductances and capacities of achievable grids. The cancellation of the reflection coefficient of the surface oT representable by Zs the impedance of a series circuit equivalent to facet 2 and by Yp, the admittance of a parallel circuit equivalent to a second facet 3 is written below.

[0048] If ZT is the equivalent impedance of the total surface of the system, i.e. the combined surface of the first and second facets 2 and 3, then the cancellation of the total reflected field amounts to ZT = Zo, i.e. the surface is adapted to the impedance of the vacuum. This results in the following relation 3: [0049 1 „_1[Z,-Zo Z r -Z () ^-2[z,+z0-y p +Y0l -Z^+Zo-'- 1

[0050] Note that relation 1 is written in normal incidence and that it becomes in oblique incidence the following relation 4: [° 051 ] z^^z^cos^e

[0052] with p = + 1 in Transverse Electric mode and p = - 1 in Transverse Magnetic mode, as well as 0 the angle of incidence.

[0053] The first and second facets 2 and 3 are associated in pairs to obtain the effect of canceling the backscattered field in the axis.

[0054] The result of the following relation 5 is called the pair rate:

[0055] Tp = min(number of first facets, number of second facets) max(number of first facets, number of second facets)

[0056] A TP pair ratio of 0.95, preferably greater than or equal to 0.98, makes it possible to obtain good absorption results with the reflection system of the invention. Preferably, the reflection system has a pair ratio equal to 1 or as close as possible to 1.

[0057] [Fig.3] shows an embodiment where the checkerboard 1 has a set of facets 2 and 3 arranged periodically. The x and y directions are the transverse directions in the plane of the checkerboard 1. The first and second facets 2 and 3 alternate regularly without the facet surfaces varying in any direction. Each first facet and each second facet have the same geometry. Their areas are equal.

[0058] Figures 4 and 5 show two embodiments of the invention for which the checkerboard 1 is non-periodic. The x and y directions are the transverse directions in the plane of the checkerboard 1. In the embodiment of [Fig.4], the checkerboard 1 formed is multi-periodic and symmetrical, the first and second facets 2, 3 are of different dimensions depending on their location in the network.

[0059] Thus on the first line of checkerboard 1, after a first and a second square-shaped facet, the first and the second facet have a larger dimension in the x direction than in the y dimension. Similarly on the first column of checkerboard 1, after a first and a second square-shaped facet, the first and the second facet have a larger dimension in the y direction than in the x dimension.

[0060] In the embodiment of [Fig.5], the checkerboard 1 is non-symmetrical, the alternation of the first and second facets is not regular. On the first line, the arrangement begins with three second facets 3 then two first facets 2, followed by a second facet 3, three first facets 2 and finally a second facet 3.

[0061] These multi-periodic or non-periodic arrangements make it possible to treat the network lobes, so as to minimize them, or even eliminate them. The type of non-periodic arrangement can be optimized according to the frequencies, incidences and observation zones of the electromagnetic waves considered.

[0062] [Fig.6] shows a graph illustrating the reduction of the radar cross section (RCS) as a function of frequency.

[0063] The SER is the capacity of the surface to backscatter the incident light towards a given direction (bistatic) or in particular towards the emission point (monostatic). The measurement is carried out at normal incidence. The curve is the result obtained for a device implementing the system of the invention having the following characteristics: non-periodic checkerboard, with dimensions 244.8 mm x 244.8 mm.

[0064] A first reduction band at -17 dB is observed from 6.8 GHz up to 9.8 GHz and greater than -23 dB over a wide frequency band between 9.8 GHz and 16.5.

[0065] It is also noted that the device implementing the system of the invention achieves attenuations greater than -25 dB for several frequencies.

[0066] The invention therefore allows excellent attenuation as well as attenuation beyond -20 dB over a very wide frequency band.

[0067] Figures 7a and 7b show a particular embodiment of the system of the invention. [Fig.7a] illustrates a square elementary pattern composed of a conductive zone 5 and a non-conductive zone 4.

[0068] In the embodiment, the conductive zone 5 is a square and the non-conductive zone 4 is made up of triangles attached to each side of the square of the conductive zone 5. [Fig.7b] shows the checkerboard 1 which comprises a set of first facets 2 and second facets 3 arranged periodically. The first facet 2 comprises meshes made up of patterns composed of a conductive zone 5 and a non-conductive zone 4. It is understood that four elementary patterns are gathered for each non-conductive zone 4. The conductive zones 5 are electrically connected to each other by electrical connection means 6.

[0069] The conductive zones 5 are therefore short-circuited. The second facet 3 has the same arrangement of patterns as the first facet 2. The first facet 2 and the second facet 3 therefore have the same geometry. On the other hand, the conductive zones 5 of the second facet 3 are not connected to each other. They are in open circuit. The first facet 2 is equivalent to a series resonant circuit and the second facet 3 is equivalent to a parallel resonant circuit.

[0070] It is then said that the first and second facets 2, 3 are self-complementary. This embodiment also makes it possible to obtain the cancellation of the backscattered field in the axis regardless of the frequency and possible losses.

[0071] For self-complementary facets, the pattern of the second facet 3 is generally obtained by a rotation, a symmetry or a translation of the pattern of the first facet 2. The conductive zones 5 are for example metallic, such as copper or conductive ink. Any other electrically conductive material can be considered to produce these conductive zones.

[0072] In another embodiment, the non-conductive zones 4 are for example made by spaces occupied by air, vacuum or by a dielectric material, for example a polyurethane resin, filling this space during integration into a complete device.

[0073] [Fig. 8] shows a graph illustrating the Fonde phase reflected by the surface of the system of the embodiment of Figure 7 versus the incident Fonde phase as a function of frequency. The dashed line represents the phase shift of Fonde reflected by the first facets 2, i.e. the short-circuited facets.

[0074] The dotted line represents the phase shift of the reflected Fonde by the second facets 3, i.e. the open-circuit facets. In both cases, it can be seen that the phase shift applied to the reflected Fonde evolves continuously as a function of the frequency.

[0075] These two types of reflected waves have a substantially constant phase shift equal to 180° over a frequency band ranging from 4 GHz to 18 GHz.

[0076] This results in cancellation of the waves reflected by the different facets of the surface of the system of the invention. That is to say that the system of the invention does not backscatter the electromagnetic waves directed towards it in the frequency band of its operation.

[0077] [Fig.9] illustrates a device implementing the system of the invention. The device is a stack consisting of the conductive plane 9, the dielectric layer 8, the checkerboard 1, the second layer 7. The first and second facets 2, 3 of the checkerboard 1 are visible in the form of long dashes for the first facet 2 and short dashes for the second facet 3. The conductive plane 9 is also called the reflector plane or ground plane and allows incident waves to be reflected on the surface of the device. The conductive plane 9 is a metallic surface for example which comprises copper or a composite such as a ply of carbon fibers.

[0078] The first and second layers 7, 8 comprise dielectric materials, for example resins or composite materials. These resins may or may not comprise reinforcing fillers to improve the mechanical strength of the device. The resins may, for example, be chosen from the family of polyesters or vinyl esters.

[0079] The first and second layers 7, 8 also make it possible to protect the checkerboard 1 from attacks from the environment in which the device implementing the system of the invention is used.

[0080] In a specific embodiment, the first layer 7 comprises a vinylester resin and woven polyethylene fibers and the second layer 8 comprises a polyester resin and high modulus S2 glass fibers.

[0081] This makes it possible, for example, to have a first layer 7 having a low dielectric permittivity as well as low losses and a second layer 8 having a stiffness allowing the use of the stack of the device as a structural panel.

[0082] In the embodiment of [Fig.9], the first layer 7 has a thickness of 4.1 mm and a dielectric permittivity of 2.6. The second layer 8 has a thickness of 3.5 mm and a dielectric permittivity of 4.

[0083] The first layer 7 is configured so as to function as an impedance transformer and transform the impedance of the surface of the checkerboard ZT in order to bring its value closer to that of the impedance of the vacuum Zoen by varying in particular the thickness of this layer as a function of the permittivity of said layer.

[0084] The thickness of the second layer 8 is configured so as to optimize the performance of the device over a wide operating band. This thickness is chosen as a function of the permittivity of the material. The thickness is generally substantially close to a quarter of the wavelength of the central frequency of the band of frequency considered.

[0085] In a particular embodiment, the impedance transformer can be completed by a third layer, not shown, we then have a stack comprising the third layer then the first layer 7, the checkerboard 1, the second layer 8 and the conductive plane 9. The third layer completes the role of impedance transformer and its thickness and dielectric permittivity characteristics are chosen so as to raise the impedance of the surface of the checkerboard ZT towards the impedance of the vacuum Zo.

[0086] In another particular embodiment, the device may be supplemented by a grid in addition to the third layer. The grid, not shown, may have a regular geometry in the form of a plate pierced with recesses at regular intervals. This grid makes it possible to extend the operation of the device to large angles of incidence by straightening the incident wave.

[0087] According to an embodiment not shown, the different materials used can be lossless. This means that when the electromagnetic wave passes through them, its amplitude does not change.

[0088] This can have the advantage of simplifying the implementation of materials and reducing their cost.

[0089] According to another embodiment, the materials used for the non-conductive and / or conductive patterns are lossy. That is to say, when the electromagnetic wave passes through them, its amplitude is attenuated.

[0090] This has, for example, the advantage of adding a phenomenon of absorption of electromagnetic waves by the material itself, leading to the improvement of certain performances of the material and obtaining a more homogeneous absorption on the surface, making it possible, for example, to treat the network lobes more effectively.

[0091] In another embodiment, some areas of the device implementing the reflection control system of the invention comprise lossless materials while other areas comprise lossy materials. For example, the edges of the device comprise lossy materials to address edge effects and grating lobes.

Claims

Claims

1. System for controlling the reflection of an electromagnetic wave incident on a surface, characterized in that the system is integrated into the surface, and the system is equipped with a checkerboard (1) comprising first facets (2) and second facets (3), each first facet (2) comprises a first non-conductive zone and a first electrically conductive zone, said first zones being configured so that the first facet (2) has a series equivalent resonant circuit Zs, each second facet (3) comprises a second non-conductive zone and a second electrically conductive zone, said second zones being configured so that the second facet (3) has a parallel equivalent resonant circuit Zp, the first impedance Zs and the second impedance Zp being such that Zp * Zs = Z02, with Zo the vacuum impedance and in that a pair ratio TP of the checkerboard (1) is defined by TP = min (number of first facets,number of second facets') max (number of first facets, number of second facets), the pair rate being greater than or equal to 0.95, preferably greater than or equal to 0.98.,

2. System for controlling the reflection of an electromagnetic wave incident on a surface according to claim 1, characterized in that for each first facet (2) the first non-conductive zone forms a first pattern (4) and the first electrically conductive zone forms a second pattern (5) and that for each second facet (3), the second non-conductive zone has a shape corresponding to the second pattern (5) and the second electrically conductive zone has a shape corresponding to the first pattern (4).

3. System for controlling the reflection of an electromagnetic wave incident on a surface according to claim 1, characterized in that each first facet (2) and each second facet (3) comprise a non-conductive zone forming a first pattern (4) and an electrically conductive zone forming a second pattern (5), and in that the electrically conductive zones of said first facets (2) are connected by electrical connection means (6) so as to be short-circuited.

4. System for controlling the reflection of an electromagnetic wave incident on a surface according to one of the preceding claims, characterized in that the areas of the first facets and the second facets are equal.

5. Device implementing the system for controlling the reflection of an incident electromagnetic wave on a surface according to claim 1, characterized in that it comprises a first dielectric layer (7), a second dielectric layer (8) and a conductive plane (9) arranged so as to act as a ground plane, stacked in the following order: the first layer (7) then the checkerboard (1) then the second layer (8) then the conductive plane (9).

6. Device implementing the system for controlling the reflection of an incident electromagnetic wave on a surface according to the preceding claim, characterized in that it further comprises a third dielectric layer arranged so that the stacking begins with the third layer, then the first layer (7) then the checkerboard (1) then the second layer (8) then the conductive plane (9).

7. Device implementing the system for controlling the reflection of an incident electromagnetic wave on a surface according to the preceding claim, characterized in that it further comprises a capacitive grid placed between the third layer and the first layer (7) so as to improve the adaptation of the device to large angles of incidence.

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