WO2013083571A1

WO2013083571A1 - Structure for lining a wall for redirecting waves received by said wall

Info

Publication number: WO2013083571A1
Application number: PCT/EP2012/074380
Authority: WO
Inventors: Andrew THAIN; Laurent EVAIN; Guillaume CAMBON; Gilles PERES
Original assignee: European Aeronautic Defence And Space Company Eads France; Airbus
Priority date: 2011-12-06
Filing date: 2012-12-04
Publication date: 2013-06-13
Also published as: FR2983578B1; FR2983578A1

Abstract

The invention consists of a diffractive device for fitting to a façade (31) of a building (30), or to any other reflective wall. The device comprises a plurality of conductive structural elements (11) arranged periodically and in a substantially parallel manner on the façade of said building, in such a way as to form a diffraction grating. Said conductive elements (11) are cylindrical elements of trapezoidal cross-section (12) whereof the height (h) and the width at the top (w) are determined on the basis of the frequency of the incident wave. In order to achieve this, various values of the width of the base (b) are considered, and the element (23), whereof the cross-section minimises the reflection of the incident wave in the specular direction, is determined for each of said values while considering various height (h) values. One of said elements (23) determined in this way is then selected on the basis of predefined mechanical criteria.

Description

Wall covering structure for redirecting waves received by said wall. 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 building facades on the surrounding space of such structures. In particular, the effects of radio waves reflected by buildings located in airport areas on the proper functioning of radio measurement systems are of particular interest.

When considering the development of airport areas, an important problem is to determine the best way to locate the buildings necessary for the operation of the airport services, to minimize the indirect effects of this implementation on the zones. responsive from a radio point of view. Indeed, such structures generally have facades of large dimensions, facades that behave as reflectors of radio waves emitted by the different sources of emissions present in the vicinity or inside the area of the airport. However, there are cases where the reflection by the facade of a building of the radio emission produced by a source, distant or not of the building, can be extremely inconvenient insofar as the signal received by the facade is reflected towards an area where it comes to pollute the radio emissions realized in this zone.

This is particularly the case if the facade of a building located in an area relatively close to an airstrip reflects towards the track a radio transmission whose frequency band is in the band occupied by the broadcasts. of the ILS landing system, in the band occupied by the "Localizer" (tracking system on the runway axis) in particular. Such a parasitic reflection, if it is strong enough, can alter the signal of the localizer and consequently disrupt the alignment on the axis of the runway of the aircraft in the landing phase. As an airport hosts many radio sources (including the ILS antennas themselves), the problem parasitic reflections by buildings is a major problem whose resolution usually involves the development of a site plan including areas, including areas relatively close to the tracks where it is prohibited to place any construction of size a little bit important.

Given, in particular, urban concentration and the desire to place airport areas at relatively short distances from urban areas, it is becoming increasingly necessary to maximize the occupancy rate of airport areas in terms of area. As a result, finding a solution to the problems of spurious reflections of radio signals in sensitive directions appears more than ever current.

The known prior art in this field is to equip the facades of buildings that may be the accidental cause of parasitic reflections in sensitive directions, reported structures whose object is to create with the wall on which they are installed a device diffraction device for reflecting the incident waves, emitted by external electromagnetic sources, in a preferred direction, towards the source for example, which avoids generating disturbances in a sensitive area.

Such a structure is generally constituted by elongate structural elements arranged so as to form ribs spaced apart from each other. These ribs are generally tubular elements having a determined height so as to create a phase shift of a given value between the wave directly reflected by the wall of the building and that reflected by the ribs. A diffraction grating of Bragg grating type is thus formed, which makes it possible, according to the phase difference induced by the ribs, to diffract the incident wave in the desired direction.

In order 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 height substantially equal to that of the facade or occupying, at least, a proportion significant of the upper part of the building. As a result fixation its structural elements on the facade considered induces a significant load on the facade load that is sought to minimize by using hollow structural elements having a tube shape. The section of these tubes is commonly triangular or rectangular and the thickness of their wall is thin with respect to their length. However, a triangular section gives the structural element a good rigidity, but instead requires that the element has a high height, height which corresponds to the height of the triangle. This height is a direct function of the frequency of the signal to be diffracted. A rectangular section allows for it, for similar constraints of frequency, direction and reflected power, to use structural elements of smaller thickness, thickness which corresponds to the width of the rectangle, but has a lower rigidity. Thus in both cases we use structural elements whose mounting on the facade to be equipped is relatively delicate in terms of weight, weight supported by the facade, and rigidity of the structure formed.

An object of the invention is to provide an alternative structure for forming a coating behaving like a diffraction grating, composed of tubular structural elements having both, for the frequency band considered, intrinsic rigidity greater than that of elements. of rectangular section and a smaller thickness than those of triangular section elements, while ensuring a reflected power level sufficient to achieve the desired diffraction effect.

Another object of the invention is to provide a method for determining in a simple manner the dimensional parameters of the structural elements constituting the coating according to the invention.

For this purpose the invention relates to a diffraction device forming a coating for a facade of a building, or any other reflective wall, exposed to electromagnetic radiation emitted by a remote source. This coating comprises a plurality of elongate conductive structural elements arranged periodically on the facade of said building, substantially parallel, so as to form a diffraction grating. Said conductive elements are cylindrical elements of trapezoidal section, oriented in a direction substantially perpendicular to the plane defined by the incident and reflected wave propagation vectors.

According to the invention, the spacing pitch of the various conductive elements is determined as a function of the wavelength λ and of the angle of incidence Θ so as to create a diffraction grating producing a phase shift of the incident wave such that it is diffracted in a preferred direction. In a particular embodiment, adapted to the case where the source of the electromagnetic emission is distant, the spacing between the different conductive elements is constant.

In a particular embodiment, adapted to the case where the source of electromagnetic remission is close, the spacing between the different conductive elements is a function of the angle of incidence of the electromagnetic wave at the location of the wall considered.

In a preferred embodiment of the invention, the height and the width at the top of the conductive elements are defined, as a function of the frequency of the incident wave, by determining, for different base widths b, chosen a priori and for a plurality of height values h of the trapezoidal section, the value of the vertex width w which minimizes the reflection of the incident wave in the specular direction, this determination being made by electromagnetic simulation, then selecting one of the trapezoidal sections retained.

According to the invention, the selection of one of the retained trapezoidal sections is carried out mainly on the basis of predefined mechanical criteria.

In a particular form of implementation; the mechanical criterion used is the mass to stiffness ratio. According to a particular embodiment of the device according to the invention, the conductive structural elements are made in the material constituting the facade or more generally the wall. The invention advantageously makes it possible to produce a diffractive device capable of coating a wall from structural elements whose height is lower than that of triangular section structural elements and whose rigidity is greater, for radioelectric properties equivalent to that of structural elements of rectangular section.

The proposed determination method, well adapted to the determination of the dimensions of the trapezoidal section structural elements constituting the wall covering according to the invention, also advantageously makes it possible to determine these dimensions in a simple manner, without solving complex systems.

The characteristics and advantages of the invention will be better appreciated thanks to the following description, which is based on the appended figures which show:

the figurel, a schematic illustration of the structure of the coating according to the invention;

- Figure 2, a curve illustrating the principle of determining the dimensions of the structural elements forming the coating according to the invention.

- Figure 3, a schematic representation of an example of arrangement of the coating according to the invention on the facade of a building.

The following description presents the system according to the invention in a particular application, that of the lining of the walls of an airport building, a building located near an airstrip equipped with a system allowing the landing of an instrument aircraft (ILS) for example.

This example of application, which makes it possible to highlight the advantages of the invention, however, should not be considered as limiting the scope or extent thereof. The coating according to the invention comprises a set of conductive structural elements, or ribs, arranged on all or part of the facades of the building in question. The facades to be protected are here those likely to be subjected to radio emissions and to reflect these emissions in a direction for which they constitute a source of alteration of a useful signal, the zone of exploitation of the signal emitted by the " Localizer "of an ILS system in the example taken here.

According to the invention, as shown in Figure 1, the conductive elements are hollow elongate elements which are arranged relative to each other so as to form, in known manner, a diffraction grating, Bragg grating type.

Each conductive element has a tubular structure whose section has dimensions which are mainly a function of the frequency of the waves that it is desired to diffract.

In known manner, the shape of the selected section is generally a simple polygon of rectangle or triangle type, chosen form in particular because the determination of the dimensions of such a section is simple. It should be noted that the triangular shape is generally preferred in that, for a length and a wall thickness equal, a triangular section structural element has a much greater intrinsic rigidity than a structural element of rectangular section. However, for a relatively low radio frequency, of the order of one hundred megahertz for example, the triangular ribs have the disadvantage of having a large height, typically several tens of centimeters, so that the coating appears to be very salient on the facade and the structural elements are difficult to rigidly fix on it.

On the other hand, the coating according to the invention is constituted by conductive structural elements 1 1 of trapezoidal section, the dimensions of which, height h and width at the vertex w in particular, take into account both the radio requirements related to the need to diffract the wave received by the facade 10, and architecture and civil engineering requirements, particularly related to the means that can be implemented, given the structure of the facade, to ensure the mounting of the structural elements 1 1. According to the invention, the spacing d between the ribs 1 1, (not the diffraction grating) is conventionally determined, for a remote transmission source, by the following relation: ά = λΙ (2ύη (θ) [1]

Where Θ is the direction of the incident wave from the normal to the facade.

Here is meant by remote source a source located at a distance from the building such that the wave received by the facade 10 of the building is a plane wave (Fraunhofer Zone). Usually the distance D beyond which one considers that the source is really far away is given by the following formula:

2 · J ²

° -ΊΓ ¹²¹

Where L represents, in the application considered, the length of the facade and λ the emitted wavelength.

In the case where the source which one tries to protect itself can not be considered as distant, the step d can be a non-constant step, the value of d separating two structural elements 1 1 consecutive, two ribs, being then function of their positions on the facade for an angle of arrival of the given incident wave. In other words, the angle of arrival of the incident wave on the facade can not be considered as constant in first approximation so that step d is then a function of the local angle of incidence of the wave received by the facade 10. According to the invention also, the dimensions of the structural elements, the dimensions of the section 12 mainly, are determined so as to produce between the waves reflected by the facade itself (between two structural elements 1 1) and those reflected by the structural elements a phase shift such that we produce a significant attenuation of the wave refracted globally in the specular direction. With regard here to trapezoidal section elements, fixed to the facade in such a way that they rest on the face of the wall corresponding to the large base of the trapezium, we are particularly interested in the height h of the trapezium and the width w of the small base.

In general, the determination of the optimal measurements of the section 12 of the ribs for a polygonal section, is not easily determinable by analytical calculation. This is why one generally limits oneself to simple forms such as the rectangle and the triangle even if these two forms induce morphological and functional limitations on the coating. Indeed these two simple shapes can be described by only two sizes, base width and height for the triangle or length and width for the rectangle, the two quantities bringing to the overall operation separate contributions, independent.

Therefore, insofar as the coating according to the invention comprises ribs January 1 of trapezoidal section 12, it is necessary to have a suitable means for determining in a simple and fast way the measurements of the trapezoidal section most appropriate to the case. considered. In the following text describes a method for optimizing the determination, for a given application, optimal measurements of the trapezoidal section of the tubes constituting the ribs January 1. This method is illustrated in FIG. 2 which takes as an example the determination of the optimal measurements of the trapezoidal section 12 of a structural element 1 1 constituting the covering of a facade subjected to a radio transmission with a frequency of 1 10 MHz , whose direction of arrival is substantially oriented at 45 ° by rappat normal to the facade 10.

The method according to the invention consists in determining the measurements of the sections of the structural elements 1 1 by proceeding in successive steps:

in a first step, a value of width of the large base is determined for a given incident frequency. This value is chosen a priori, depending in particular on the desired rigidity and the mounting stresses of the ribs January 1 on the facade. in a second step, for a plurality of height values h of the trapezoidal section 12, determined a priori also, the value of the width at the vertex w, the width of the small base of the trapezium, which, taking into account the pitch d of the diffraction grating that constitutes the coating, minimizes the reflection of the incident wave in the desired direction. This direction may be, for example, the specular direction, that is to say the direction symmetrical of the direction of arrival of the wave relative to the plane of the facade.

Thus, for each of the heights h chosen, heights between a maximum value and a minimum value, a width at the top w, the trapezoids corresponding to the pairs of values (w, h) can thus be represented in plane (height, width at vertex) by points 21 defining a curve 22.

As can be seen in FIG. 2, each point 21 thus corresponds to a particular trapezoidal shape 23 with a height and a width at the apex specific to this shape. The curve 22 is intended to define the location of the trapezoidal sections defining ribs having a minimum reflection in the desired direction.

in a third step, one chooses that of points 21 of the curve that best meets all the constraints imposed on the structural elements constituting the coating according to the invention. At this stage, the electromagnetic constraints having already been taken into account in the implementation of the first steps, the selection criteria are mainly related to mounting constraints in relation to both the length and rigidity of the structural elements themselves. and with considerations relating to the structure of the facade itself.

This original determination method can be implemented in different ways.

For example, different models of the final coating can be made, each model having structural elements whose section has specific measurements and using the measurements of the models having a minimum reflection to establish the curve 22.

Alternatively, if the appropriate computer resources are available, one can proceed by simulation of the electromagnetic behavior, at the frequency of computer models of structural elements, and select those models with minimal reflection at this frequency. It should be noted that in the case where the determination method is implemented on simulation models, it is advantageous to consider in the first step several possible values for the width of the large base of the trapezium. In this way, the candidates selected during the second step may then also have different basic widths, provided that these widths remain compatible with the requirement of the desired maximum refractive direction. In this case, the plane curve 22 of FIG. 2 becomes a three-dimensional curve and leaves a wider choice to take into account the constraints that are to be considered during the third step of the method (selection of the optimal section).

It should also be noted that, in the case of simulation, it is also possible to use an optimizer to find optimal dimensions of the ribs. With regard to the implementation of the coating according to the invention on the wall considered, the facade of a building in the example detailed here, It should first be considered how are arranged according to the invention the elements structural elements that constitute it.

The coating according to the invention is an arrangement of parallel elongate elements whose length depends on the height of the wall to be coated. The structural elements are arranged to be oriented in a direction substantially perpendicular to the plane defined by the incident and reflected wave propagation vectors. Thus, in the specific case of a building facade the structural elements are arranged vertically.

As regards the length of the structural elements, this results above all from a compromise between the cost of implantation and the effectiveness of the suppression of disturbances. The best compromise can be evaluated by simulation, using specific software or from measurements made on scale models. Figure 3 illustrates a example of placing a coating according to the invention on a facade 31 of a building 30, provided with windows 32 and access 33 and 34. In this example the coating according to the invention is set up at level of the upper part of the facade.

Claims

1. Diffraction device for equipping a facade (31) of a building (30), or any other reflecting wall, exposed to electromagnetic radiation emitted by a source located at a distance from the building, the device comprising a plurality of structural elements arranged on the facade of said building, characterized in that these structural elements are conductive elements (1 1) arranged substantially parallel to the facade of said building and arranged to form a diffraction grating, said conductive elements (1 1) being elements cylindrical trapezoidal section (12), oriented in a direction substantially perpendicular to the plane defined by the incident and reflected electromagnetic wave propagation vectors.

2. Device according to claim 1, characterized in that the spacing pitch, d, of the various conductive elements (1 1) is determined as a function of the wavelength λ and the angle of incidence Θ so to create a diffraction grating providing a phase shift of the incident wave such that it is diffracted in a preferred direction.

3. Device according to one of claims 1 or 2, characterized in that, if the source of the electromagnetic emission is distant, the spacing d between the various conductive elements (1 1) is constant.

4. Device according to any one of claims 1 or 2, characterized in that, if the source of the electromagnetic emission is close, the spacing d between the various conductive elements (1 1) is a function of the angle d local incidence of the electromagnetic wave.

5. Device according to any one of the preceding claims, characterized in that the height h and the width at the summit w of conductive elements (1 1) are defined, according to the frequency of the incident wave, by determining, for different base widths b, chosen a priori, and for a plurality of height values h of the trapezoidal section (12) , the value of the width at the aperture w which minimizes the reflection of the incident wave in the specular direction, this determination being carried out by electromagnetic simulation, and by selecting one of the retained trapezoidal sections (23).

6. Device according to claim 5, characterized in that the selection of one of the trapezoidal sections (23) is performed on the basis of predefined mechanical criteria.

7. Device according to claim 6, characterized in that the criterion used is the mass to stiffness ratio.

8. Device according to one of the preceding claims, characterized in that the conductive elements (1 1) are made in the material of the facade.