Anti-reflecting lining structure with a diffraction grating using resonant elements

CA 02856952 2014-05-26 WO 2013/083572 PCT / EP2012 / 074382 1 Anti-reflection coating structure with diffraction grating using resonant elements.

The invention relates to the general field of the reflection of electromagnetic waves, radio waves in particular, and more particularly to that of the prevention of the effects of radio waves reflected by structures such as facades of buildings on the space surrounding such structures.

We are particularly interested in the effects of radio waves reflected by buildings located in airport areas on the proper functioning of radio measurement systems.

When looking at the development of airport areas, an important problem is to determine the best way to locate the buildings necessary for the operation of airport services to minimize the indirect effects of this establishment on sensitive areas. from a radioelectric point of view.

Indeed, such structures generally have large facades, facades which behave like reflectors of radio waves emitted by the various emission sources present in the vicinity or inside the airport area.

However, there are cases where the reflection by the facade of a building of the radioelectric emission produced by a source, distant or not from the building can prove to be extremely annoying insofar as the signal received by the facade is reflected towards a area where it pollutes the radio emissions produced in this area. This is in particular the case if a building located in a zone relatively close to an airstrip reflects towards the runway a radioelectric emission whose frequency band is located in the band occupied by the emissions of the system of ILS landing, in particular in the band occupied by the "Locate" (radio-alignment on the runway center line).

Such parasitic reflection, if it is strong enough, can alter the locator signal and consequently disturb the alignment on the axis of the aircraft runway during the landing phase.

Because of the presence of numerous radio sources, in particular the ILS antennas themselves, the problem of reflections CA 02856952 2014-05-26 WO 2013/083572 PCT / EP2012 / 074382 2 interference from buildings is an important problem whose resolution passes generally by drawing up a site plan comprising areas, in particular the areas relatively close to the tracks where it is prohibited to place any construction of any size.

Given, in particular, urban concentration and the desire to place airport zones at relatively short distances from urban zones, it is becoming increasingly necessary to maximize the occupancy rate of airport zones in terms of surface area.

Consequently finding a solution to the problems of stray reflections of radio signals in sensitive directions appears more than ever current.

The prior art known in this field teaches us that it is theoretically possible to equip the facades of buildings likely to be the fortuitous cause of parasitic reflections in sensitive directions, of reported structures whose object is to create with the wall on which they are installed a diffractor device making it possible to reflect the incident waves, emitted by external electromagnetic sources, in a preferred direction which avoids generating disturbances in a sensitive area.

Such a structure is generally made up of elongate, conductive structural elements, arranged so as to form ribs spaced apart from one another.

The ribs are generally tubular elements having a thickness determined so as to create a phase shift of given value between the wave directly reflected by the wall of the building and that reflected by the ribs.

A Bragg grating type diffraction grating is thus formed which allows, depending on the phase shift induced by the ribs, to diffract the incident wave in a desired direction.

Depending on the wavelength A which it is sought to prevent reflection in a given direction, the ribs which constitute the coating have a given thickness h which may, in the case where the wave in question has a frequency of the order of the hundred megahertz to be relatively large.

CA 02856952 2014-05-26 WO 2013/083572 PCT / EP2012 / 074382 3 such elements therefore prove to be difficult to install on the surface of a facade, especially if the installation of elements is done after the construction of the building.

Furthermore, to ensure correct diffraction of the incident wave, it is generally preferable to equip the facade exposed to the radiation whose effects are to be prevented, with a rib having a length substantially equal to the height or occupying a significant portion of the upper part of the facade.

Consequently, the fixing of its structural elements to the facade considered induces a significant load on the facade, a load which it is sought to minimize by using hollow structural elements having the shape of a tube.

However, such elements remain fragile objects, of which the lack of intrinsic rigidity generally remains the weak point.

Thus, the types of coatings well known in the prior art use structural elements whose mounting on the facade to be fitted is relatively delicate in terms of size, or more exactly of size, weight, weight supported by the facade, and stiffness of the structure formed.

An object of the invention is to propose an alternative structure for forming a Bragg grating type diffraction grating.

This structure is composed of tubular structural elements having both, for the frequency band considered, an intrinsic rigidity greater than that of elements of rectangular section, for a thickness and a width substantially smaller than those of the elements forming the structures. known.

To this end the invention relates to a diffractor device for coating a facade of a building, or any other reflecting wall, exposed to electromagnetic radiation emitted by a distant source, the device comprising a plurality of tubular resonant elements, arranged periodically, substantially parallel, on the facade of said building, so as to form a diffraction grating.

Each resonant element forms a "LC" type resonator configured to re-radiate a wave corresponding to the incident wave affected by a phase shift.

These resonant elements are arranged on the wall such that the incident wave is CA 02856952 2014-05-26 WO 2013/083572 PCT / EP2012 / 074382 4 diffracted in a preferred direction.

Said resonant elements are further oriented in a direction substantially perpendicular to the plane defined by the propagation vectors of the incident and reflected waves.

According to a particular embodiment of the device, the spacing spacing of the various conductive elements is determined as a function of the wavelength A and of the angle of incidence 0 so as to create a diffraction grating, of Bragg grating type, realizing a phase shift of the incident wave such that it is diffracted in a preferred direction.

According to the invention, if the source of the electromagnetic emission is far away the spacing between the various conductive elements is constant along the wall.

On the other hand, if the source of the electromagnetic emission is close, the spacing between the different conductive elements is a function of the local angle of incidence of the electromagnetic wave.

According to the invention, each resonant element consists of a tube of conductive material, the wall of which defines an internal cavity, has a longitudinal opening.

The dimensions and geometry of the cavity defined by the wall, as well as the width of the longitudinal opening formed in this wall, are defined so that the element behaves from an electromagnetic point of view like an LC resonator of resonance frequency Fo and bandwidth AF0.

According to the invention also, each resonant element is preferably configured to have a size which fits into a parallelepipedal volume of given dimensions defined taking into account mechanical constraints.

According to the invention, the resonant elements forming the device can have sections of various polygonal shapes which are all inscribed in a rectangle, the resonant elements having, depending on the section adopted, particular capacitance or inductance values.

CA 02856952 2014-05-26 WO 2013/083572 PCT / EP2012 / 074382 In a particular embodiment, the wall of a resonant element has in cross section the shape of a rectangle whose length has a discontinuity limited by two segments substantially perpendicular to said length, the ends of which are directed towards the inside of the perimeter.

According to the invention also, each resonant element can be configured so as to form a plurality of adjoining tubular cavities.

In a particular embodiment, each resonant element itself consists of a plurality of juxtaposed resonant elements, the side walls of which are in electrical contact.

In a particular embodiment, the internal volume of the resonant elements is empty.

Alternatively, in another embodiment, the cavity defined by the wall of the resonant element is filled with a dielectric material chosen so as to reinforce the rigidity of the element.

According to a variant of this embodiment, the cavity defined by the wall of the resonant element is filled with two superimposed layers of dielectric material, a conductive strip being disposed in the cavity, at the interface between the two layers, opposite of the longitudinal slot formed along the wall.

Alternatively, in another embodiment, the cavity defined by the wall of the resonant element is filled with a non-conductive ferromagnetic material chosen so as to increase the passband AF0 and decrease the frequency Fo of the resonant cavity.

Alternatively still, in another embodiment, the cavity defined by the wall of the resonant element is filled with a dielectric material of high electrical permittivity E, chosen so as to decrease the frequency Fo of the resonant cavity.

CA 02856952 2014-05-26 WO 2013/083572 PCT / EP2012 / 074382 6 The characteristics and advantages of the invention will be better appreciated from the following description, which description is based on the appended figures which show:

- Figure1, an overall schematic representation of the device according to the invention;

- Figure 2, an equivalent diagram of the operation of a structural element constituting the device according to the invention;

- Figures 3 to 8, illustrations of different alternative embodiments of the device according to the invention;

- Figure 9, an illustration highlighting the advantageous character in terms of dimensions of the device according to the invention;

- Figure 10, a curve illustrating the advantages of using the device according to the invention.

Figures 1 and 2 show the general structure of the device according to the invention.

As illustrated in FIG. 1, the covering according to the invention is presented as an arrangement of parallel longilinear structural elements 12 whose length depends on the dimensions of the wall. The structural elements 12 are arranged on the wall 11 so as to form a diffraction grating having a pitch d.

According to the invention, the spacing pitch d of the various conductive elements is conventionally determined as a function of the wavelength A and of the angle of incidence 0 so as to create a diffraction grating realizing a phase shift of the incident wave such that it is diffracted in a preferential direction.

Depending on whether the source of the radio emission received by the wall is considered to be a distant source or not, the step d is a constant step, as in FIG. 1, or a step which varies according to the angle. The structural elements 12 are also arranged on the wall 11 so as to be oriented in a direction substantially perpendicular to the plane defined by the vectors of propagation of the incident and reflected waves.

Thus, in the specific case of a building facade, the structural elements CA 02856952 2014-05-26 WO 2013/083572 PCT / EP2012 / 074382 7 According to the invention, each structural element 12 is a conductive element in the form of a hollow cylinder, a tube, of polygonal shape, having a flat face 15 by which it is fixed to the wall 11, the wall of the tube being provided, on the opposite face 13, with a longitudinal slot 14 of given width e.

The structural elements 12 are made of a conductive material, so that they form LC circuits (ie

inductance - capacitance) with constants L and C distributed. The inductance L is formed by the wall of the tube itself and is a function of the area delimited by the wall of the tube, while the capacitor C is formed by the slot 14 as well as by the cavity 21.

The dimensions of the wall of the tube are also determined so that the values ​​of the inductance L and of the capacitance C make it possible to produce an equivalent resonant circuit having a resonant frequency Fo and a passband AF0 defined in known manner by the following relationships:

F 1 oc __________________________________ [1] and AF .., it L [2] C From the operating point of view, each resonant element, is configured, dimensioned, in such a way that when it is lit by an incident radio wave, it produces a radio wave of the same frequency but affected by a given phase shift, so that the combination of waves reflected by the various structural elements 12 and waves directly reflected by the portions of the wall 11 located between these structural elements forms a diffracted wave in the desired direction.

The device according to the invention therefore behaves like a conventional diffraction grating of the Bragg grating type, comprising simply conductive structural elements.

On the other hand, from a dimensional point of view, the structural elements which constitute it allow CA 02856952 2014-05-26 WO 2013/083572 PCT / EP2012 / 074382 8 to produce a coating advantageously easier to implant on a wall, in particular on a building facade.

Unlike the conductive elements which constitute a conventional diffraction grating, the dimensions of the resonant elements which constitute the device according to the invention are not directly a function of the difference in path to be created between the waves reflected by the wall. 11 itself and those reflected by the structural elements to obtain the desired diffraction, but by the capacitance and inductance values ​​which it is desired to obtain.

Consequently for the same result, that is to say for the same phase shift between the waves reflected by the wall 11 itself and those reflected by the structural elements, the dimensions of the resonant element are substantially smaller than those of a simple conductive element.

FIG. 9 illustrates this structural advantage by presenting the respective dimensions of the structural elements constituting two diffraction gratings intended to control the wave reflected by a wall lit at an incidence of approximately 25 by a radio wave with a frequency of a hundred of megahertz emitted by a distant source, that is to say a source located at a distance from the wall such that the wave received by the facade 10 of the building is a plane wave (Fraunhofer area).

We consider in both cases networks formed of structural elements of rectangular section.

The first network 91 is a conventional network made up of simple conductive elements, while the second network 92 is a device according to the invention, made up of resonant elements 12.

As can be seen in the figure, for the same result, namely producing a phase-shifted wave, the dimensions of a resonant element 12 are very substantially smaller than that of a simple conductive element.

This dimensional characteristic has a double advantage.

The first advantage is of an ergonomic order and consists in that, when mounted on a wall, the diffractor device according to the invention appears to be less prominent and less masking, so that if the wall is a facade provided with windows, outside light enters the building more easily.

The second advantage is mechanical and consists in that, the resonant elements being of smaller dimensions, the problem of their intrinsic rigidity and their possible deformation under CA 02856952 2014-05-26 WO 2013/083572 PCT / EP2012 / 074382 9 the effect of their dead weight arises significantly less acutely when they are mounted on the wall 11 to be coated.

In the following text, various embodiments of the resonant elements constituting the device according to the invention are presented, in which the structural elements have sections of various shapes.

These different embodiments are exposed here in order to demonstrate that, for a given overall size, it is possible to obtain resonant elements having resonant frequencies Fo and different bandwidths AF0.

To clearly highlight the comparative advantages of the different variants, we consider here structural elements whose sections form quadrilaterals which are inscribed in the same rectangle of given length w and given width h.

It should be noted that the variant embodiments described below in no way limit the form, the object or the scope of the invention.

According to a first simple embodiment variant, each tube has, as illustrated in FIG. 2, a rectangular section with two large opposite sides 23 and 25 rectilinear, of length w, corresponding respectively to the internal face 15 and to the external face 13, and two short sides 26, of length h, corresponding to the lateral faces 16.

Furthermore, the side 13 has a discontinuity 24 corresponding to the slot 14.

In this alternative embodiment, the value of the inductance L is in particular determined by the area described by the wall of the tube.

The value of the capacitance C is in turn determined by the width of the slot 14 corresponding on the sectional view to the discontinuity 24 and the dimensions of the internal space delimited by the wall of the element (cf. illustration 2- b).

According to a second alternative embodiment, each resonant element has a section such as that illustrated in FIG. 3, in which the wall of the element 12 has along the slot 14, corresponding to the discontinuity 24 at the level of the section, two edges curved at 900 and facing each other, represented by two segments of identical lengths 31 and 32.

These two edges have lengths defined so as to increase the value CA 02856952 2014-05-26 WO 2013/083572 PCT / EP2012 / 074382 of the capacity C compared to that presented by the previous variant, and to make this capacity C less dependent on the size of the cavity (flat capacitor).

5 According to a third alternative embodiment, illustrated by FIG. 4, the resonant elements 12 used have in section a rectangular section, one of the lengths of which has a discontinuity 24 corresponding to the slot 14.

However, the cavity 17, defined by the wall of the resonant element 11, houses an internal extension 41, integral with the face 10 of the wall 15 opposite the wall 13 comprising the slot 14, and forming a rib 41 projecting from the inside the cavity.

In this alternative embodiment, this rib 41 has, in cross section, a "T" shape, the bar of which is parallel to the lengths 23 and 25 of the rectangle representing the section of the element.

It is configured and arranged inside the cavity so that the "T" bar is placed opposite discontinuity 24, at a distance defined as a function of the resonant frequency Fo and the width of AF0 band expected.

Such a configuration advantageously makes it possible to appreciably increase the value of the capacitance C without appreciably modifying the value of the inductance L.

According to a fourth alternative embodiment, illustrated by FIG. 5, the cavity of the resonant elements 12 used does not simply contain ambient air, but is filled with a dielectric material 51 whose essential role is here to reinforce the mechanical rigidity of the element.

Such a variant is particularly advantageous if the element used is very long, taking into account the dimensions of the wall to be coated.

It should be noted that in this variant embodiment, the material housed in the cavity may also have various electromagnetic properties.

It is thus possible, by using a material having a high permittivity Er, to increase the value of the capacitance C, without changing the dimensions of the cavity or the shape of the wall.

Conversely, by using a material having a high permeability pr, it is possible to increase the value of the inductance L without changing anything else.

CA 02856952 2014-05-26 WO 2013/083572 PCT / EP2012 / 074382 11 It should also be noted that this variant embodiment can be combined with all the preceding variants.

It can in particular include the placement in the material of a longitudinal blade 52 arranged so as to be opposite the slot 14.

This gives a radiant element very similar in its definition of the element of the fourth embodiment described above.

According to a fifth alternative embodiment, illustrated by FIG. 6, a slot 61 is formed on one of the lateral faces 16 of the resonator, the assembly being held by an appropriate dielectric mechanical structure, not shown for reasons of clarity in the figure.

An element 62 having a high inductance is also placed inside the cavity 17, and is electrically connected by each of its ends 63 and 64 to the two edges 65 and 66 of the slot 61.

Thus, the inductance of the LC circuit is increased, which has the effect of lowering its resonance frequency Fo and of increasing its bandwidth AF0.

From the point of view of the embodiment, the element 62 may consist of a single assembly extending over the entire length of the cavity 17 or alternatively, as illustrated in FIG. 6, of several elements arranged one at a time. following the other, each element being connected by its ends to the edges of the slot 61.

According to a sixth alternative embodiment, illustrated in FIG. 7, each resonant element has a slot 61 is provided on one of the lateral faces 16 of the resonator as well as discrete inductances 71, preferably distributed over the entire length of the slot and the terminals each connect the two edges 65 and 66 of the slot 61.

As previously, such an arrangement makes it possible to increase the inductance of the LC circuit, which has the effect of lowering its resonant frequency fo and increasing its bandwidth Afo.

Furthermore, optionally, each resonant element may include discrete capacitive elements 72 implanted, preferably, all along the slot 14 and the terminals of which are connected to the edges of this slot.

According to a seventh variant embodiment, illustrated in FIG. 8, each resonant element constituting the device according to the invention is CA 02856952 2014-05-26 WO 2013/083572 PCT / EP2012 / 074382 12 consisting of two or more elementary resonant elements 11 such as those described above.

Each elementary resonant element is configured so as to have a given resonance frequency Fo and a bandwidth AF0, Fo and AF0 being generally the same for all of the associated resonant elements.

Such an arrangement of contiguous resonant elements tuned to the same frequency Fo has the notable advantage of increasing the power re-radiated by the resonant element, knowing that, in known manner, the value of the re-radiated power mainly depends on the dimensions ( in particular of the width w) of the face of the resonant element which carries the slot 14.

Thus by juxtaposing two or more elementary resonant elements, the power re-emitted by the device is significantly increased, without significantly changing the operating parameters of the resonant circuit itself.

In a simple embodiment, illustrated in FIG. 8, the composite elements thus formed can be produced from a single tubular structure 81 of rectangular section whose internal cavity is separated into elementary cavities 82 by intermediate partitions 83, each elementary cavity 82 being provided with a longitudinal slot materialized in the sectional view of FIG. 7 by the discontinuities 84.

Thus, as can be seen through the embodiments described in the foregoing text, the device according to the invention proposes, for coating a wall subjected to a radioelectric emission, the reflection of which in a given direction to be avoided, an advantageous alternative solution to the use of diffraction gratings with simple conductive structural elements of the prior art, as regards both the installation and the operation.

FIG. 10 presents on the same reference frame (angle of observation, equivalent surface of the wall) the curves of the radar equivalent surface of a wall constituted by the facade of a building considering three states; a first state (curve 101) corresponding to a bare wall, a second state (curve 102) corresponding to a wall coated with a diffraction grating according to the prior art and a third state (curve 103) corresponding to a wall coated with device according to the invention.

As can be seen in CA 02856952 2014-05-26 WO 2013/083572 PCT / EP2012 / 074382 13 the figure, although being less effective than the devices of the prior art (curve 102), the device according to the he invention makes it possible to significantly reduce the equivalent surface of the wall.

As a result it makes it possible to sufficiently reduce the level of nuisance caused by the reflection of unwanted radio waves, while advantageously having a footprint much smaller than that presented by the devices according to the prior art, which makes it in particular less massive and easier to install.