Method of integrating an "array" antenna in a medium of a different electromagnetic nature and associated antenna

The field of the invention is that of electromagnetic antennas called “network antennas” used in all types of radiocommunications. These antennas can, in particular, be radars. These antennas can be installed on the ground or on any type of mobile carrier, such as aircraft.

Antennas, in general, are integrated into a medium. This can range from a simple pylon for the cellular telecommunications base station to a mobile carrier, such as an aircraft. The environment surrounding the antenna must be taken into account when designing the latter so as not to disturb the radio performance of the antenna.

The integration of the antenna on a carrier creates a sharp electrical discontinuity which results in an edge diffraction. This diffraction phenomenon disturbs the radiation of the antenna. Edge diffraction also contributes to the electromagnetic signature of the antenna and increases the radar surface area, known by the acronym "SER" of the antenna. Figures 1 to 3 illustrate this problem on a simple example. Figures 1 and 2 show a top view and a side view of a rectangular antenna A of width L x and length L y integrated in an environment M of different electromagnetic nature. Thus, the reflectivity r a of the antenna is different from the reflectivity r mmiddle. The black border B in these two figures represents the discontinuity between the antenna and its middle. FIG. 3 represents in side view the reflection of an incident wave I at the level of this discontinuity B. The incident wave I then generates a specular wave S but also a parasitic retroreflected wave SER linked to the discontinuity B.

Network type electromagnetic antennas, commonly called “array antennas”, consist of a finite set of radiating elements. According to the applications, the constitution of a radiating element varies. In some cases, it can be made entirely of metal. In other cases, it may be made of metal resting on a substrate and surrounded by a superstrate. The term “superstrate” is understood to mean any structure which covers the antenna. A radome is a superstrate.

This structure can be adapted to change the radiation characteristics of the antenna.

Under certain conditions, array antennas can generate surface waves. The surface waves generated by the antenna are diffracted at the edge by the ridges. These waves can reflect off the edges of the antenna cavity and diffract to the other edge of the cavity. A phenomenon of multiple reflection of the surface waves is then observed on the edges of the cavity of the antenna which results in an increase in the SER and a degradation in the performance of the radiation emitted. This phenomenon also contributes to a degradation in the performance of the antenna.

The integration of a network antenna encounters the same type of problem as an antenna. The edges of the edge of the panel create diffraction phenomena which mainly disturb the radiating elements located on the edge of the panel and participate in the SER of the antenna.

Various solutions have been proposed to resolve or to attenuate these problems of integrating the antenna into its environment. A first solution consists in adding, in the environment close to the antenna, materials which absorb electromagnetic waves; this solution is discussed in the publication of EF Knott, JF Schaeffer, and MT Tuley, Radar Cross Section, 2nd edition. Scitech Publishing, 2004. This method makes it possible to reduce cavity reflections and in particular cavity edge reflections due to the presence of surface waves. In addition, these waves create multiple reflections. The presence of absorbents makes it possible to eliminate this phenomenon of reflection of surface waves at the edges of the antenna.

In the case of the integration of finite array antennas, it is possible to add additional dummy radiating elements with dedicated loads similar to the radiating elements at the edge of the panel in order to reduce diffraction linked to surface waves, these elements being called charged radiating elements. This method is described by Ben A. Munk in his book “Finite Antenna Arrays and FSS”, IEEE Press. A Wiley-lnterscience publication. The reduction in surface waves contributes to improving the angular deflection capacity of active antenna arrays and to reducing the RES of the antenna.

A third method is described in application US 20070069940 entitled “Method and Arrangement for Reducing the Radar Cross Section of Integrated Antennas”. It proposes to treat the opening created by the antenna in a medium using resistive materials. This method has the advantage of providing a smooth transition in order to gradually attenuate the surface waves and thus reduce the diffraction due to the edge edges.

These different methods each have their drawbacks.

Solutions based on absorbent materials are generally not sufficient. Absorbents often continue to create a sharp discontinuity between the medium and the antenna. Furthermore, the absorbent materials may be of a different nature than the antenna and do not necessarily operate under the same conditions of temperature, pressure or vibratory environment as those of the antenna.

The solution proposed by Ben A. Munk makes it possible to considerably attenuate the surface waves and consists in adding the charged radiating elements. However, this solution does not solve the problem of structural diffraction generated by the integration of the antenna in its medium. There is always a structural transition between the array antenna and the medium.

The use of progressive resistive layers makes it possible as a first approach to limit the clear discontinuity between the antenna and its medium. However, it is only interested in the variation of a single physical parameter, the resistivity of the material, in order to solve all the diffraction problems. Moreover, this method is not concerned with the performance of the antenna, only with its integration in a metallic environment. In addition, this resistive transition is carried out on a dielectric material, in general the radome of the antenna. It is possible that the antenna does not have dielectric layers with the external environment and therefore makes the use of resistive layers impossible.

These various solutions are therefore not completely satisfactory because they are limited in terms of degree of freedom and only allow processing of a limited number of edge discontinuities.

The method according to the invention does not have the above drawbacks. It optimizes the transition between the antenna and its environment by focusing on the electromagnetic behavior of the discontinuity and aims

thus reducing the effects of diffraction and surface waves resulting from this transition.

More precisely, the subject of the invention is a method of integrating an array antenna in a medium, said antenna comprising a plurality of radiating elements ensuring the transition between the antenna and the medium, the reflectivity of each dependent radiating element. of at least one parameter, the reflectivity being represented by a complex number, the reflectivity of a first element being equal to or close to that of the antenna, the reflectivity of a last radiating element being equal to or close to that of the medium , the reflectivity parameter of the radiating elements included between this first radiating element and this last radiating element varying from one radiating element to the next, characterized in that the method comprises the following steps:

- Step 1: Calculation of a path represented in the complex plane and equal to the sum of the variations of the reflectivity from a radiating element to the following radiating element;

- Step 2: Optimization of the variation of the reflectivity parameter so that the radar equivalent area of ​​the antenna is as small as possible or that at least one of the characteristics of the radiation of the antenna is reached;

- Step 3: Determination of the different radiating elements as a function of said parameter;

- Step 4: Simulation of the overall reflectivity and / or the radiation of the antenna.

Advantageously, the speed of variation of the parameter is minimum between the first element and the next element, minimum between the last element and the previous element and maximum between the two elements furthest from the first element and from the last element.

Advantageously, the reflectivity coefficient is a complex number comprising a real part and an imaginary part and in that the variation of the reflectivity between two radiating elements is equal to the modulus of the variations of the real and imaginary parts of the reflectivity of said radiating elements.

The invention also relates to an array antenna intended to be integrated into a medium and produced according to the preceding method, said antenna comprising a plurality of radiating elements ensuring the transition between the antenna and the medium, the reflectivity of each radiating element depending on 'at least one parameter, the reflectivity being

represented by a complex number, the reflectivity of a first element being equal to or close to that of the antenna, the reflectivity of a last radiating element being equal to or close to that of the medium, characterized in that the reflectivity parameter of radiating elements included between this first radiating element and this last radiating element varies from one radiating element to the next, the speed of variation of the parameter being minimum between the first element and the following element, minimum between the last element and the previous element and maximum between the two elements furthest from the first element and from the last element

Advantageously, the radiating elements being organized in a network, the parameter is the pitch of the network in one direction in space or in two directions in space.

Advantageously, the radiating elements being metallic, the parameter is a geometric parameter of the radiating elements so that the radiating elements have different metallic surfaces.

Advantageously, the parameter is a geometric parameter of the radiating elements so that the radiating elements have different resistive surfaces.

Advantageously, the parameter is a physical characteristic of a substrate constituting the radiating elements.

Advantageously, the parameter is a physical characteristic of a superstrate constituting the radiating elements.

Advantageously, the physical characteristic is the relative permittivity or the permeability of said substrate or of said superstrate.

Advantageously, the radiating elements comprising a plurality of sheets of metallic or resistive patterns, the parameter is the quantity or the arrangement of said sheets present in the radiating elements.

Advantageously, the radiating elements comprising metamaterials, the parameter is the quantity of metamaterials present in the radiating elements.

The invention will be better understood and other advantages will appear on reading the description which follows, given without limitation and thanks to the appended figures, among which:

FIG. 1 represents, in top view, a rectangular antenna according to the prior art integrated in a medium;

FIG. 2 represents, in side view, the previous antenna according to the prior art;

FIG. 3 represents the SER generated at the level of the interface between an antenna according to the prior art and a medium;

FIG. 4 represents, in top view, a rectangular antenna according to the invention integrated in a medium;

FIG. 5 represents, in side view, the preceding antenna according to the invention;

FIG. 6 represents the SER generated at the level of the interface between an antenna according to the invention and a medium;

FIG. 7 represents the variation of the complex reflectivity coefficient between two radiating elements according to the invention;

FIG. 8 represents the variation of the path representative of the variations in reflectivity as a function of successive radiating elements;

FIG. 9 represents the speed of variation of the reflectivity as a function of successive radiating elements;

FIG. 10 represents the variation of the reflectivity coefficient as a function of the variation of the dependence parameter;

FIG. 11 represents the variation of the dependence parameter as a function of the succession of the radiating elements;

FIG. 12 represents a top view of part of an array of radiating elements according to the prior art;

FIG. 13 represents the variation of the complex reflectivity coefficient between two radiating elements in the previous embodiment;

FIG. 14 represents a top view of part of an array of radiating elements in an embodiment according to the invention;

FIG. 15 represents the variation of the path representative of the variations in reflectivity as a function of the successive radiating elements of FIG. 14;

FIG. 16 represents the variation of the path representative of the variations in reflectivity of FIG. 15 as a function of the dependence parameter;

FIG. 17 represents the value of the dependence parameter of FIG. 16 as a function of the radiating element.

By way of example, FIGS. 4 to 6 represent an antenna A according to the invention integrated into its environment M. FIGS. 4 and 5

represent a top view and a side view of a rectangular antenna A of width L x and length L y integrated in an environment M of different electromagnetic nature. As previously, the reflectivity r a of the antenna is different from the reflectivity r m of the medium. This antenna is surrounded by a transition zone T of width L Tx and of length L Ty . This transition zone is made up of radiating elements. The electromagnetic parameters of these elements vary so as to modify their reflectivity coefficient Ty, thus ensuring a smooth transition between the antenna and its medium.

FIG. 6 represents, in side view, the reflection of an incident wave I at the level of the transition zone T. The incident waves then generate specular waves S but also retroreflected waves SER of much smaller amplitudes than in L absence of a transition zone.

In general, the electromagnetic behaviors of the antenna and of the medium are characterized by a surface impedance or reflectivity. There is a passing relationship between these two parameters. It is thus possible to model the antenna and its environment by two plates of different impedances.

Generally, reflectivity is calculated and represented in the complex plane. It depends on the frequency, incidence and polarization of the wave.

As we have seen, the discontinuity caused by the change in impedance modifies the radioelectric behavior of the antenna and induces harmful diffraction phenomena. The integration of a gradual and controlled transition of the reflectivity in one or more directions of space makes it possible to eliminate the effects of this discontinuity. Thus, it is possible to reduce the radar equivalent surface area in significant proportions. It is also possible to optimize one of the characteristics of the radiation of the antenna. Mention will be made, for example, of the overall efficiency of the radiation, but also the shape and distribution of the secondary emission lobes or the gain of the antenna.

The progressive variation of the reflectivity from one radiating element to another can be done on one or more physical parameters of the radiating element which can be:

- The pitch of the network according to one or both directions of the network;

- An intrinsic geometric dimension of the radiating element, such as the opening of a waveguide, a length, a width or a height;

- A physical property of the materials constituting the radiating element such as, for example, the relative permittivity of the substrate which composes it.

To control the gradual variation of the radiating elements at the transition, the reflectivity along the transition can be continuous or discretized. A continuous modification means that the intrinsic property varies within the set of radiating elements of the transition. A discretization of the transition comes down to giving a specific value to each element of the transition. These variations must make it possible to control the surface reflectivity of each radiating element in an appropriate manner.

The method according to the invention makes it possible to reduce the effects of diffraction for a given incidence, polarization and frequency. Although the optimization is carried out for this incidence, this polarization and this determined frequency, it also acts for different incidences, frequencies and polarizations, sometimes according to the same law. Thus, the method is implemented for a typical or average value of the incidence, the polarization and the frequency and applies to a wider range of incidence, polarization and frequency.

It should be noted that the reflectivity does not necessarily vary according to these three parameters. For example, the reflectivity of a metallic plane is equal to -1 whatever the frequency, the polarization and the incidence of the wave.

Either a continuous or discrete set of radiating elements connecting the antenna and its medium, the first element being in contact with the antenna and the last element being in contact with the medium. We denote by n the number of radiating elements and i the serial number of a radiating element, i varying from 0 to n.

The reflectivity of this first element is equal to or close to that of the antenna, the reflectivity of the last radiating element is equal to or close to that of the medium. The reflectivity parameter (s) of the radiating elements included between this first radiating element and this last radiating element vary from one radiating element to the next.

In a first step of the method according to the invention, depending on the choice of the physical parameter (s), an accessible path L in the behavior between the two extreme radiating elements is defined.

If s represents the variation parameter, s varying between two values ​​denoted by a and b, each radiating element has the reflectivity l (s).

This has a real part x and an imaginary part y as shown below.

| x = Æe (r (s))

(y = / m (r (s))

We define the starting point of the path as being the reflectivity of the antenna and the ending point that of the middle. The definition of the reverse also works. The definition of this path gives the variation of the parameterized curve l (s).

The curve in FIG. 7 gives the complex representation of the accessible path as a function of a single physical parameter. The real part x is on the x-axis and the imaginary part y on the y-axis. They are between -1 and +1.

The definition of a standard is necessary if several parameters are chosen. This standard guarantees the progressive variation of the parameters in order to avoid variations of the important parameters without detecting it on the curve l (s).

The parameterized curve T (s) is discretized according to a certain number of elements n of the transition, this discretization can be uniform or non-uniform. A uniform discretization corresponds to the same spacing between each element. In FIG. 7, the point denoted G (0) corresponds to the reflectivity of the antenna and the point denoted G (h) corresponds to the reflectivity of the medium for the nth radiating element. In the case of FIG. 7, this reflectivity is equal to -1.

The length of the parameterized path L rn is worth:

s 0 is the initial value of the physical parameter or of all the parameters when several are taken into account. It corresponds to the value of the parameter of the first radiating element, closest to the antenna.

s n is the final value of the physical parameter or of all the parameters when several are taken into account. It corresponds to the value of the parameter of the last radiating element, closest to the middle.

v (s) is the vector derived from l (s). Its coordinates in the complex plane are

In a second step of the method according to the invention, the masking of the diffraction phenomena is optimized. It is necessary that the norm of the parametric speed noted || p (s) || is weak at the start and end of the transition and large in the center. To do this, it follows mathematical laws that allow this behavior to be obtained. The parametric speed can take different values ​​in the transition.

FIG. 8 presents an example of a mathematical law describing the evolution of the parameterized length L r as a function of the position of the radiating element i. As an indication, the number of radiating elements is 12 in FIGS. 8 and 9. The curve of FIG. 8 shows small variations at the start and at the end so as to obtain low parametric speeds at the ends. The norm of the parametric speed is represented discretely in FIG. 9. It is also expressed as a function of the radiating element i.

Once the law of L r has been defined, the next step of the method consists in going back to the values ​​of the parameter or to all the parameters associated with each value of the length of the parameterized curve.

This determination can be done in different ways: analytically, if there is a passing formula, by means of charts or tabulated values.

FIGS. 10 and 11 represent this step of determining the physical dimensions associated with each element of the transition.

FIG. 10 represents the variation of the length of the path L rn as a function of the maximum value of the parameter s. This figure is represented in a semi-logarithmic frame, the parameter s varying according to a logarithmic law. For a given maximum parameter value, we therefore deduce the value of the corresponding path.

FIG. 11 represents, for a determined maximum parameter value, the value of this parameter for each radiating element. For example, in FIG. 11, the maximum variation of s is 2000 for the first element, 500 for the second, 200 for the third and so on for the following elements.

Once this step is completed, it is possible to represent the reflectivity of all the elements of the transition in the complex plane in order to check the correct distribution of the points on the accessible path determined initially.

By way of non-limiting example, the method is implemented in the case of the integration of an array antenna made up of waveguide openings in a metallic medium. FIG. 12 represents a top view of the antenna A at the level of its separation from the medium M. The openings of the radiating elements ER are all identical, of square shape and of side a. They are regularly arranged.

In the frequency band of interest, the waveguides are said to be “under cut-off”, this results in total reflectivity of the guides, without however having a phase shift of 180 ° like the perfect metal plane. This results in an electrical discontinuity between the array of guides and a metal plate leading to diffraction phenomena. FIG. 13 represents the variation of the reflectivity coefficient between the antenna and its medium in the complex plane. In FIG. 13, the point denoted G (0) corresponds to the reflectivity of the antenna and the point denoted G (h) corresponds to the reflectivity of the medium for the nth radiating element. In the case of FIG. 13, this reflectivity is equal to -1.

The method according to the invention consists in determining a transition zone separating the antenna from its medium so that the problems of parasitic reflectivity are very attenuated.

The radiating elements of this transition zone are of the same nature as those of the antenna but of smaller dimensions. The parameter used to vary the reflectivity of the radiating elements is therefore this dimension. FIG. 14 represents a top view of the antenna at the level of its separation from the medium with the radiating elements ER T of the transition zone. The dimension ai of the first element of the transition zone is therefore less than a 0 , the last element of the antenna, the dimension a 2 of the second element of the transition zone is therefore less than a 0 and so on for the elements following.

FIG. 15 represents the variation of the path representative of the variations in reflectivity as a function of the successive radiating elements of FIG. 14.

FIG. 16 represents the variation of the path representative of the variations in reflectivity as a function of the dependence parameter. In this figure, the parameter a varies between 0 and 7 millimeters.

FIG. 17 represents the value of the dependence parameter as a function of the radiating element.

The simulations of the electromagnetic signature levels with or without said transition zone as defined above show a gain of about 30 dB over several frequency octaves, whatever the polarization of the wave. This gain is all the more important as the incidence approaches the grazing incidence.

The method according to the invention makes it possible to obtain substantial attenuations of the parasitic effects at the cost of reduced additional complexity. In the preceding example of embodiment, the radiating elements of the transition zone are, in fact, of the same nature as those of the antenna and do not pose any implementation problem.

In the previous example, the variable parameter is the size of the radiating elements. There are, however, a number of ways to modify the reflectivity parameter.

Thus, the radiating elements being metallic, the parameter can be a geometric parameter of the radiating elements so that the radiating elements have different metallic surfaces.

The parameter can be a geometric parameter of the radiating elements so that the radiating elements have different resistive surfaces.

The parameter can be a physical characteristic of a substrate or of a superstrate constituting the radiating elements. This physical characteristic may be the relative permittivity or the permeability of said substrate or of said superstrate.

The radiating elements may comprise a plurality of sheets of metallic or resistive patterns, the parameter being the quantity or the arrangement of said sheets present in the radiating elements.

Finally, the radiating elements can include metamaterials, the parameter being the quantity of metamaterials present in the radiating elements. The term metamaterial refers to an artificial composite material which exhibits electromagnetic properties different from those of natural materials. These metamaterials are composed of periodic, dielectric or metallic structures depending on the desired properties.

CLAIMS

1. A method of integrating an array antenna (A) in a medium (M), said antenna comprising a plurality of radiating elements (ER T ) ensuring the transition between the antenna and the medium, the reflectivity of each element radiating dependent on at least one parameter, the reflectivity being represented by a complex number, the reflectivity of a first element being equal to or close to that of the antenna, the reflectivity of a last radiating element being equal to or close to that of the medium, the reflectivity parameter of the radiating elements included between this first radiating element and this last radiating element varying from one radiating element to the next, characterized in that the method comprises the following steps:

- Step 1: Calculation of a path represented in the complex plane and equal to the sum of the variations of the reflectivity from a radiating element to the following radiating element;

- Step 2: Optimization of the variation of the reflectivity parameter so that the radar equivalent area of ​​the antenna is as small as possible or that at least one of the characteristics of the radiation of the antenna is reached;

- Step 3: Determination of the different radiating elements as a function of said parameter;

- Step 4: Simulation of the overall reflectivity and / or the radiation of the antenna.

2. A method of integrating an antenna according to claim 1, characterized in that the speed of variation of the parameter is minimum between the first element and the next element, minimum between the last element and the previous element and maximum between the two elements furthest from the first element and the last element.

3. Method of integrating an antenna according to one of the preceding claims, characterized in that the reflectivity coefficient is a complex number comprising a real part and an imaginary part and in that the variation in reflectivity between two elements radiating is equal to the modulus of the variations of the real and imaginary parts of the reflectivity of said radiating elements.

4. A method of integrating an antenna according to one of the preceding claims, characterized in that, the radiating elements being organized in a network, the parameter is the pitch of the network in one direction of space or two directions of the network. 'space.

5. A method of integrating an antenna according to one of claims 1 to 3, characterized in that the radiating elements being metallic, the parameter is a geometric parameter of the radiating elements so that the radiating elements have different metallic surfaces. .

6. A method of integrating an antenna according to one of claims 1 to 3, characterized in that the parameter is a geometric parameter of the radiating elements so that the radiating elements have different resistive surfaces.

7. A method of integrating an antenna according to one of claims 1 to 3, characterized in that the parameter is a physical characteristic of a substrate constituting the radiating elements.

8. A method of integrating an antenna according to one of claims 1 to 3, characterized in that the parameter is a physical characteristic of a superstrate constituting the radiating elements.

9. A method of integrating an antenna according to one of claims 7 or 8, characterized in that the physical characteristic is the relative permittivity of said substrate or of said superstrate.

10. A method of integrating an antenna according to one of claims 7 or 8, characterized in that the physical characteristic is the permeability of said substrate or of said superstrate.

11. A method of integrating an antenna according to one of claims 1 to 3, characterized in that, the radiating elements comprising a plurality of sheets of metal patterns, the parameter is the quantity or the arrangement of said sheets present in the radiating elements.

12. A method of integrating an antenna according to one of claims 1 to 3, characterized in that, the radiating elements comprising a plurality of sheets of resistive patterns, the parameter is the quantity or the arrangement of said sheets present in. the radiating elements.

13. A method of integrating an antenna according to one of claims 1 to 3, characterized in that, the radiating elements comprising metamaterials, the parameter is the quantity of metamaterials present in the radiating elements.

14. Network antenna intended to be integrated into a medium, said antenna comprising a plurality of radiating elements ensuring the transition between the antenna and the medium, the reflectivity of each radiating element depending on at least one parameter, the reflectivity being represented. by a complex number, the reflectivity of a first element being equal to or close to that of the antenna, the reflectivity of a last radiating element being equal to or close to that of the medium, characterized in that the reflectivity parameter of the elements radiating elements included between this first radiating element and this last radiating element varies from one radiating element to the next, the speed of variation of the parameter being minimum between the first element and the following element,minimum between the last element and the previous element and maximum between the two elements furthest from the first element and the last element.

15. Network antenna according to claim 14, characterized in that the parameter is the pitch of the network in one direction of space or two directions of space.

16. Network antenna according to claim 14, characterized in that the radiating elements being metallic, the parameter is a geometric parameter of the radiating elements so that the radiating elements have different metallic surfaces.

17. Network antenna according to claim 14, characterized in that the parameter is a geometric parameter of the radiating elements so that the radiating elements have different resistive surfaces.

18. Network antenna according to claim 14, characterized in that the parameter is a physical characteristic of a substrate constituting the radiating elements.

19. Network antenna according to claim 14, characterized in that the parameter is a physical characteristic of a superstrate constituting the radiating elements.

20. Network antenna according to one of claims 18 or 19, characterized in that the physical characteristic is the permittivity of said substrate or of said superstrate.

21. Network antenna according to one of claims 18 or 19, characterized in that the physical characteristic is the permeability of said substrate or of said superstrate.

22. Network antenna according to claim 14, characterized in that the radiating elements comprising a plurality of sheets of metal patterns, the parameter is the quantity or the arrangement of said sheets present in the radiating elements.

23. Network antenna according to claim 14, characterized in that the radiating elements comprising a plurality of sheets of resistive patterns, the parameter is the quantity or the arrangement of said sheets present in the radiating elements.

24. Network antenna according to claim 14, characterized in that the radiating elements comprising metamaterials, the parameter is the quantity of metamaterials present in the radiating elements.