EP1972882A1 - Electromagnetic window. - Google Patents

Abstract

The window has rigid/flexible flat supports (1, 3) made of dielectric material, and conductive pattern networks (2, 4) for conducting electricity carried by the supports, respectively, where patterns of each network are insulated. Jacks (10, 10') act on the supports for selectively arranging the supports in one of two positions corresponding to passing and blocked states, respectively. The networks are electrically decoupled and coupled in passing and blocked states, respectively. The networks authorize the circulation of electric currents on their surfaces when the networks are coupled.

Description

The present invention relates to an electromagnetic window with variable and controlled electromagnetic transparency and, more particularly, to such a switchable window between first and second so-called passing states and blocked respectively.

Such a window can be used, for example, to temporarily protect a device sensitive to a disturbing external electromagnetic field. In the military field, the blocking of the window makes it possible to erase the radar signature of an antenna, for example, placed behind the window.

Such windows are known comprising walls of metallic material, these walls being movable between a closed position in which they stop the electromagnetic radiation and an open position in which they pass this radiation. These windows have several disadvantages. In the first place, the passage from one position to another using mechanical means for moving the metal walls, this results in a relatively long switching time between the two states of the window. In addition, the electrical contact between the different metal walls of the window may be defective and generates leakage of the electromagnetic field through the window.

Document is known WO 94/00892 A a frequency guide type device which has at least one frequency selection surface and which can be used as an antenna or radome in the communications systems. The frequency selection surface may include first and second rows of electrically conductive elements on a dielectric substrate or dielectric elements on an electrically conductive substrate. By moving the first row of elements relative to the second row of elements, it is possible to adjust the frequency response.

Document is known FR 2,711,846 a concealment device for a radar antenna. The device has several hollow elements in the form of concentric mounted cones, which can be moved relative to each other by means of adjusting jacks. The cavities between the concentric cones each communicate with a dielectric fluid reservoir. The adjustment of the cone position is achieved by mechanical means. Depending on the position and thickness of each cone, the device filters certain frequencies. Such a device is specifically adapted to be made in the outer carcass of a flying machine and is expensive to produce.

Other types of controlled electromagnetic transparency screens have been designed, particularly in the field of radar antennas. These screens make use of networks of electronic switches such as transistors or PIN diodes, for example. In this respect it is possible to consult the patent EP 595 726 or the article entitled " The phase-switched screen "authors B. Chambers and A. Tennant, published in the journal IEEE Antennas and Propagation Magazine, Volume 46, No. 6 of December 2004 . Other screens make use of micro-electromechanical components (or MEMS). Such screens are, by their complex structure, sometimes fragile and costly realization. In addition, their bandwidth is often limited.

It has also been proposed, in an application to radar antennas in the X-band, the use of a plasma to reflect microwaves (see the doctoral thesis entitled " Experimental study and modeling of a plasma reflector in helium for radar applications "author: Lise Caillault, born in Versailles on January 25th, 2002, n ° report ONERA: TH 2002-002 ). Again it is a costly device and limited bandwidth (8-12 GHz).

The present invention is specifically intended to provide a switchable electromagnetic window between on and off states that does not have these disadvantages. More specifically, the object of the invention is to provide such a window which is inexpensive and has a very wide bandwidth, for example from 0 to 40 GHz.

This object of the invention is achieved, as well as others which will appear on reading the description which follows, with a window of the type described in the preamble of the present description, remarkable in that it comprises at least first and second supports of dielectric material, first and second networks of electrically conductive patterns carried by said first and second supports respectively, the patterns of each array being electrically isolated from each other, and means acting on said carriers for selectively disposing in one or the other of first and second relative positions, corresponding to said passing states and respectively blocked, positions in which said networks of conductive patterns are electrically decoupled or coupled respectively, said networks allowing the circulation of electric currents on any their surface when electrically coupled in their second relative position.

Thus, the window is opaque or transparent to electromagnetic radiation depending on whether these networks are coupled or decoupled.

Thanks to its simple structure, this window is inexpensive.

Furthermore, the supports made of dielectric material, on which act the means for arranging them in one position or another, make it possible to overcome the problems of electromagnetic leakage of the known devices mentioned above. In addition, depending on the means chosen to arrange the supports in one position or another, it is possible to achieve switching times between the two very short window states.

In addition, as will be seen below, its bandwidth is very wide.

• dans la deuxième position relative, les réseaux sont couplés par contact électrique,
• en variante, dans la deuxième position relative, les réseaux sont couplés par effet capacitif,
• dans la première position relative, les réseaux sont séparés par une distance suffisante pour empêcher tout couplage capacitif sensible,
• les dimensions des motifs conducteurs sont inférieures à la moitié de la longueur d'onde de la fréquence maximum de fonctionnement assignée à la fenêtre,
• dans leur deuxième position relative, les motifs conducteurs des réseaux délimitent ensemble des motifs intercalaires où ils ne sont pas superposés, que le périmètre de ces motifs intercalaires étant inférieur à la plus petite longueur d'onde de fonctionnement assignée à la fenêtre,
• les supports de motifs conducteurs sont plans et disposés parallèlement l'un à l'autre,
• en variante, les supports de motifs conducteurs sont coniques et disposés coaxialement l'un à l'autre,
• les moyens agissant sur la position relative des supports de motifs conducteurs sont des moyens mécaniques,
• en variante ces moyens sont des moyens pneumatiques,
• les supports sont rigides,
• en variante, ces supports sont souples.

According to other features of the invention:

• in the second relative position, the networks are coupled by electrical contact,
• alternatively, in the second relative position, the networks are capacitively coupled,
• in the first relative position, the networks are separated by a distance sufficient to prevent any sensible capacitive coupling,
• the dimensions of the conductive patterns are less than half the wavelength of the maximum operating frequency assigned to the window,
• in their second relative position, the conductive patterns of the networks together delimit intercalary patterns where they are not superimposed, the perimeter of these spacer units being less than the smallest operating wavelength assigned to the window,
• the conductive pattern supports are planar and arranged parallel to each other,
• alternatively, the conductive pattern supports are tapered and arranged coaxially with each other,
• the means acting on the relative position of the conductive pattern supports are mechanical means,
• alternatively these means are pneumatic means,
• the supports are rigid,
• alternatively, these supports are flexible.

The invention also relates to the use of such a window for the temporary deletion of the radar signature of a device placed behind this window.

• la figure 1 illustre schématiquement les fonctions de la fenêtre électromagnétique suivant l'invention,
• les figures 2A et 2B d'une part, 2C et 2D d'autre part, représentent schématiquement les premier et deuxième réseaux de motifs conducteurs et les premier et deuxième supports qui les portent respectivement, en plan et en coupe respectivement,
• la figure 3A illustre le dimensionnement des motifs conducteurs,
• la figure 3B est une vue schématique, en plan, des deux réseaux superposés, faisant apparaître la géométrie des motifs intercalaires dans lesquels les deux réseaux ne se superposent pas alors qu'ils sont dans leur deuxième position relative,
• les figures 3C et 3D représentent schématiquement, en coupe, les deux réseaux dans leurs première et deuxième positions relatives respectivement,
• la figure 4 représente schématiquement un mode de réalisation de la fenêtre suivant l'invention, dans lequel les moyens agissant sur les supports des réseaux pour les disposer sélectivement dans l'une ou l'autre de leur deux positions relatives sont des moyens mécaniques,
• la figure 5 représente un autre mode de réalisation de l'invention dans lequel les moyens d'actionnement sont des moyens pneumatiques,
• les figures 6 et 7 représentent schématiquement encore un autre mode de réalisation de la fenêtre suivant l'invention, à moyens d'actionnement pneumatiques et supports souples, dans leurs première et deuxième positions relatives respectivement,
• les figures 8 et 9 représentent schématiquement encore un autre mode de réalisation de l'invention, à supports coniques coaxiaux, dans leurs première et deuxième positions relatives respectivement, et
• les figures 10 et 11 sont des graphes illustrant les performances de la fenêtre suivant l'invention, dans ses états passant et bloqué respectivement.

Other features and advantages of the present invention will appear on reading the description which follows and on examining the appended drawing in which:

• the figure 1 schematically illustrates the functions of the electromagnetic window according to the invention,
• the Figures 2A and 2B on the one hand, 2C and 2D, on the other hand, represent schematically the first and second networks of conductive patterns and the first and second supports respectively carrying them, in plan and in section respectively,
• the figure 3A illustrates the sizing of the conductive patterns,
• the figure 3B is a schematic view, in plan, of the two superimposed networks, showing the geometry of the intermediate patterns in which the two networks do not overlap while they are in their second relative position,
• the Figures 3C and 3D schematically represent, in section, the two networks in their first and second relative positions respectively,
• the figure 4 schematically represents an embodiment of the window according to the invention, in which the means acting on the supports of the networks to arrange them selectively in one or the other of their two relative positions are mechanical means,
• the figure 5 represents another embodiment of the invention in which the actuating means are pneumatic means,
• the Figures 6 and 7 schematically represent yet another embodiment of the window according to the invention, with pneumatic actuating means and flexible supports, in their first and second relative positions respectively,
• the Figures 8 and 9 schematically represent yet another embodiment of the invention, with coaxial conical supports, in their first and second relative positions respectively, and
• the Figures 10 and 11 are graphs illustrating the performance of the window according to the invention, in its passing states and blocked respectively.

In all these figures, reference numerals identical to references used in the preceding figures identify identical or similar elements or members.

We refer to the figure 1 of the accompanying drawing which illustrates the operation of the electromagnetic window according to the present invention. This is switchable between two so-called "passing" and "blocked" states respectively, by means which will be described later. In the on state, the wave And transmitted by the window F is of amplitude substantially equal to that of the incident wave Ei. In the off state the amplitude of Et is almost zero while the amplitude of the reflected wave Er is substantially equal to that of Ei.

The Figures 2A to 2D schematically represent the structure of the essential elements of the window according to the invention. The latter thus comprises first and second planar supports 1 and 3 respectively, made of dielectric material, carrying pattern arrays such as 2 and 4 respectively, of electrically conductive material. In the embodiment shown, these patterns 2 and 4 are in the form of conformal elongated rectangles, arranged in parallel and equidistant lines, each pattern being physically separated, and therefore electrically isolated, from the adjacent patterns so as to constitute an electric conduction island. separate.

Such patterns of conductive patterns can be achieved cheaply by conventional photoengraving techniques, well known in the manufacture of printed circuits.

On the Figures 2A and 2C the supports 1 and 3 are arranged in such a way that the patterns of conductive patterns they carry are oriented orthogonally to one another. It is conceivable that by superimposing on one another the faces of these supports which carry networks thus oriented (see Figs. 3B and 3D ), an electrical coupling is established between the patterns of the two networks. If the patterns of the two networks are in physical contact with each other, this coupling establishes an electrical conduction between these patterns. If the patterns of the two networks remain opposite, a small distance in a dielectric medium 5, there is established a capacitive effect by electrostatic influence between the two networks. In both cases, currents can circulate in all directions and over the entire surface of these networks, thanks to the close and continuous interweaving of the reasons which constitute them, as it is clear from the Figures 3B and 3D . It is this conduction that prevents the transmission of electromagnetic waves through the window according to the invention.

To restore this transmission, it is sufficient to separate the two supports by a small distance, as shown in FIG. figure 3C , so as to eliminate any electrical coupling, by conduction or capacitive effect, between the two networks. The electric currents carried by each network thus become almost indifferent to the presence of the other network. The dielectric medium which separates them then can be the ambient medium or a specific gas. In this case the transmission attenuation in the crossing of the window is only the product of attenuations due to the two supports and networks they carry. She can be weak.

It is clear that the blocking of the transmission of electromagnetic waves is all the more complete as the two networks ensure, in combination, the most complete coverage of the surface of the supports opposite. On the figure 3B the shape of a spacer pattern 6 has been shown which is repeated over this entire surface and which corresponds to a defect of coverage of this surface by the superimposed networks. It is obviously necessary that this defect is of surface as small as possible and compatible with the expected performances of the window as regards bandwidth. For this purpose, a geometry of the networks giving the perimeter of the intermediate pattern 6 a value smaller than the smallest wavelength included in this bandwidth will advantageously be chosen.

This bandwidth also determines the sizing of the conductive patterns. Thus, the dimensions of these must be less than half the wavelength corresponding to the maximum frequency limiting this bandwidth.

• longueur L1 des motifs conducteurs : 1,75 mm,
• largeur L2 de ces motifs : 0,25 mm,
• distance L3 entre motifs : 0,25 mm.

By way of example, to produce a window with controlled transparency with respect to electromagnetic waves with a frequency of less than 20 GHz, it will be possible to retain the following dimensioning of the patterns (see FIG. figure 3A ):

• length L1 of the conductive patterns: 1.75 mm,
• width L2 of these patterns: 0.25 mm,
• distance L3 between patterns: 0.25 mm.

The supports 1 and 3 may consist, for example, of a polyimide material of 50 .mu.m thick. The distance or distance 8 between the supports varies from the contact for the blocked state to a distance greater than 1 mm for the on state.

We have shown figure 4 a first embodiment of the window according to the invention, showing the means which act on the window to change it from its state to its off state or vice versa. The supports 1 and 3 and the patterns networks 2 and 4 are shaped as described above and are fixed on jaws 9 and 9 'respectively, of dielectric material, arranged parallel to each other. Cylinders 10, 10 'make it possible to pass their spacing 8, in the medium 5, from a substantially zero value corresponding to the locked state to a predetermined value, advantageously greater than 1 mm, which eliminates any coupling between these networks for establish the passing state.

We have shown figure 5 another embodiment of the invention in which the network supports 1 and 3 define, with a flexible membrane 11 device, an enclosure isolated from the surrounding medium. Pneumatic means such as a pump 12 are connected to this chamber to selectively vary the pressure Pi prevailing therein with respect to the external pressure Pe. By establishing in the enclosure a pressure Pi much lower than Pe, the enclosure contracts and the two networks of conductive patterns are pressed against each other. The window is then in its locked state. By raising the pressure Pi above Pe, it causes the swelling of the enclosure and the separation of the two networks, which restores the passing state of the window.

In the embodiment of the figure 5 , the network supports are rigid. Representatives Figures 6 and 7 another embodiment of the invention wherein these supports must, on the contrary, be flexible. By way of nonlimiting example, it can then be used to form these supports 1 and 3 a polyimide film of a few tens of microns thick. Here again, an enclosure is formed by surrounding the supports 1 and 3 with a flange 13 cooperating with a shim 14 open at 14a resting on the peripheral metallizations 15, 15 'facing the supports on which the pattern gratings are formed. conductors. A pump 12 makes it possible to act on the pressure of the dielectric medium 5 contained in the enclosure with respect to the pressure prevailing outside this enclosure. This is how the supports can move from the schematic position to the figure 6 where they are separated (passing state) at the position of the figure 7 where, thanks to their flexibility, they are joined by a suitable suction developed by the pump 12 (blocked state).

The embodiment of the invention whose structure and operation are illustrated by the Figures 8 and 9 is adapted to the protection of an electromagnetic sensor 16 protected by a conically shaped radome 17. This is typically the case of an aircraft radar or guided missile system. When such an installation is inactive, it is often desirable to remove the radar signature. This is precisely the role of the embodiment of the window according to the invention represented by the Figures 8 and 9 .

The supports 1 and 3, both conically shaped, are thus fixed on the inner face of the radome 17 and on the outer face of a central rigid conical part 18 respectively. The conical part 18 is movable coaxially with the radome 17, by sliding on an axis 19. Here again a sealed enclosure is defined by the radome 17, the part 18 and a flexible peripheral membrane 11.

A pump 12 connected to this enclosure by a conduit 20 of dielectric material controls the pressure in the enclosure. It will be noted that this pump 12 is placed behind the sensor 16 so as not to disturb the propagation of electromagnetic waves in the cavity located in front of this sensor.

As previously described in connection with Figures 5 to 7 by suitably adjusting the pressure in the chamber with respect to the external pressure, the position shown in FIG. figure 8 , in which the patterns of conductive patterns carried by the supports 1 and 3 are spaced from each other (on state) at the position of the figure 9 in which these networks are in contact with each other (blocked state) or vice versa. It is understood that a passage of the pressure in the chamber below the external pressure causes a sliding to the right (from the point of view of the figure 9 ) of the conical part 18 and the network that it carries, which then comes close to the facing network carried by the inner surface of the radome 17. A rise in the pressure in the chamber above the external pressure recalls the conical part in the position shown in figure 8 .

The performances of the electromagnetic window according to the invention, in its passing and blocked states, are illustrated by the graphs of the Figures 10 and 11 respectively.

To the figure 10 the variations, as a function of the frequency of the incident wave, of the reflection coefficients R and of the transmission T of this wave on a window in the on state such as that represented in FIGS. Figures 3A to 3D .

$$\mathrm{T}\left(\mathrm{dB}\right)\mathrm{=}\mathrm{20}\log \left(\mathrm{IEtl}\mathrm{/}\mathrm{IEil}\right)$$

Er, Et et Ei étant définis comme décrit ci-dessus en liaison avec la figure 1.The graphs give the ratio between the intensity of the reflected or transmitted fields and the incident field, in dB: $$\mathrm{R}\left(\mathrm{dB}\right)\mathrm{=}\mathrm{20}\log \left(\mathrm{IErl}\mathrm{/}\mathrm{IEIL}\right)$$

$$\mathrm{T}\left(\mathrm{dB}\right)\mathrm{=}\mathrm{20}\log \left(\mathrm{IETL}\mathrm{/}\mathrm{IEIL}\right)$$

Er, Et and Ei being defined as described above in connection with the figure 1 .

On the graph of the figure 10 , it appears that the attenuation in transmission is of the order of -0.5 dB at 10 GHz and of the order of -2.5 dB at 30 GHz. The window structure according to the invention thus very effectively minimizes the lack of transparency in the on state.

To the figure 11 the evolution of the same coefficients for the window in the off state is represented. It appears that the window according to the invention remains substantially opaque up to 40 GHz, with a transmission lower than -14 dB up to this frequency. This is a much better result than those that can be expected electromagnetic screens of the prior art, discussed in the preamble of this description.

It now appears that the present invention makes it possible to achieve the goals announced, namely to provide an electromagnetic window that can be performed cost-cost, while having high performance, especially in terms of bandwidth.

Naturally, the invention is not limited to the embodiments described and shown which have been given by way of example only. Thus, patterns of patterns of different shape from that (rectangular) represented in the figure 3A could be substituted for those described, provided that the constraints are met, especially those concerning the necessary electrical continuity to establish to block the window and sizing to obtain the bandwidth sought.

Claims (14)

Electromagnetic window (F) switchable between first and second states passing and blocked respectively, with respect to the transmission of electromagnetic waves through said window, characterized in that it comprises at least first (1) and second (3) supports of dielectric material, first and second networks of conductive patterns (2,4) of electricity carried by said first (1) and second (3) supports respectively, the patterns (2,4) of each network being electrically isolated from one another, and means acting on said supports (1,3) for selectively disposing them in one or the other of first and second relative positions, corresponding to said on and off states respectively, positions in wherein said conductive pattern arrays (2, 4) are electrically decoupled and coupled respectively, said arrays permitting the circulation of electric currents over their entire area. urface when electrically coupled in their second relative position. Electromagnetic window according to claim 1, characterized in that , in said second relative position, said networks are coupled by electrical contact. Electromagnetic window according to claim 1, characterized in that , in said second relative position, said gratings are capacitively coupled. Electromagnetic window according to any one of claims 1 to 3, characterized in that , in said first relative position, said gratings are separated by a distance sufficient to prevent any sensitive capacitive coupling. Electromagnetic window according to any one of the preceding claims, characterized in that the dimensions of said conductive patterns (2,4) are less than half the wavelength of the maximum operating frequency assigned to said window. Electromagnetic window according to any one of the preceding claims, characterized in that, in their second relative position, said conductive patterns (2,4) of said arrays together define spacers units (6) where they are not superimposed, and that the perimeter of said intermediate units is less than the smallest operating wavelength assigned to said window. Electromagnetic window according to any one of the preceding claims, characterized in that said carriers (1,3) of conductive patterns are planar and arranged parallel to each other. Electromagnetic window according to any one of claims 1 to 6, characterized in that said carriers (1,3) of conductive patterns are conical and arranged coaxially with each other. Electromagnetic window according to any one of the preceding claims, characterized in that said means acting on the relative position of said conductive pattern supports are mechanical means (10). Electromagnetic window according to any one of claims 1 to 8, characterized in that said means acting on the relative position of said conductive pattern supports are pneumatic means (12). Electromagnetic window according to claim 10, characterized in that said supports (1,3) are rigid. Electromagnetic window according to claim 10, characterized in that said supports (1,3) are flexible. Electromagnetic window according to any one of the preceding claims, designed to operate in the frequency band 0 to 20 GHz, characterized in that said conductive patterns are of rectangular shape, length (L1) and width (L2) equal. at 1.75 mm and 0.25 mm respectively, the distance (L3) between adjacent conductive patterns being 0.25 mm. Use of the window according to any one of the preceding claims, for the temporary deletion of the radar signature of an apparatus placed behind said window.

Images