Method for integrating a “network” antenna into a different electromagnetic medium, and associated antenna

Abstract

An array antenna (A) in a medium (M) comprises a plurality of radiating elements (ER.sub.T) ensuring the transition between the antenna and the medium, the reflectivity of each element depending on a parameter, the reflectivity of a first element being close to that of the medium, the reflectivity of a last element being close to that of the antenna, the reflectivity parameter of the elements varying from one element to the next. A method comprises calculation of a path equal to the sum of the variations of the reflectivity from one element to the next element, optimization of the variation of the reflectivity parameter so that equivalent radar cross-section of the antenna is the lowest possible or the antenna best observes the radiation objectives, determination of the different elements as a function of said parameter, and simulation of the overall reflectivity and/or of the radiation of the antenna.

Claims

1. A method for incorporating an array antenna (A) in a medium (M), said array antenna comprising a plurality of radiating elements (ER.sub.T) ensuring a progressive transition of reflectivity between the array antenna and the medium, reflectivity of each radiating element depending on at least one parameter, the reflectivity being represented by a complex number, said at least one parameter varying from one radiating element to the next, wherein 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 one radiating element to the next radiating element; Step 2: optimization of the variation of said at least one parameter along said path calculated in the complex plane so that the equivalent radar cross-section of the array antenna is the lowest possible or that at least one characteristic of the radiation of the array antenna is reached; Step 3: determination of the different radiating elements as a function of said at least one parameter; Step 4: simulation of the overall reflectivity and/or of the radiation of the array antenna.

2. The method for incorporating an array antenna as claimed in claim 1, characterized in that the rate of variation of said at least one parameter is minimal between the first radiating element and the next radiating element, minimal between the last radiating element and the preceding radiating element and maximal between the two radiating elements farthest away from the first radiating element and from the last radiating element.

3. The method for incorporating an array antenna as claimed in claim 1, characterized in that said complex number representing the reflectivity comprises 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.

4. The method for incorporating an array antenna as claimed in claim 1, characterized in that, the radiating elements being organized in an array, said at least one parameter is the pitch of the array in one direction of the space or two directions of the space.

5. The method for incorporating an array antenna as claimed in claim 1, characterized in that, the radiating elements being metallic, said at least one parameter is a geometrical parameter of the radiating elements so that the radiating elements have different metallic surfaces.

6. The method for incorporating an array antenna as claimed in claim 1, characterized in that said at least one parameter is a geometrical parameter of the radiating elements so that the radiating elements have different resistive surfaces.

7. The method for incorporating an array antenna as claimed in claim 1, characterized in that said at least one parameter is a physical characteristic of a constituting element constituting the radiating elements, said constituting element being a substrate.

8. The method for incorporating an array antenna as claimed in claim 1, characterized in that said at least one parameter is a physical characteristic of a constituting element constituting the radiating elements, said constituting element being a superstrate.

9. The method for incorporating an array antenna as claimed in claim 7, characterized in that the physical characteristic is the relative permittivity of said constituting element.

10. The method for incorporating an array antenna as claimed in claim 7, characterized in that the physical characteristic is the permeability of said constituting element.

11. The method for incorporating an array antenna as claimed in claim 1, characterized in that, the radiating elements comprising a plurality of sheets of metallic patterns, said at least one parameter is the quantity or the arrangement of said sheets present in the radiating elements.

12. The method for incorporating an array antenna as claimed in claim 1, characterized in that, the radiating elements comprising a plurality of sheets of resistive patterns, said at least one parameter is the quantity or the arrangement of said sheets present in the radiating elements.

13. The method for incorporating an array antenna as claimed in claim 1, characterized in that, the radiating elements comprising metamaterials, said at least one parameter is the quantity of metamaterials present in the radiating elements.

14. An array antenna intended to be incorporated in a medium, said array antenna comprising a plurality of radiating elements ensuring a progressive transition of reflectivity between the array antenna and the medium, reflectivity of each radiating element depending on at least one parameter, the reflectivity being represented by a complex number, that wherein said at least one parameter varies from one radiating element to the next, the rate of variation defined by the derivative of the reflectivity in the complex plane with respect to said at least one parameter being minimal between the first radiating element and the next radiating element, minimal between the last radiating element and the preceding radiating element and maximal between the two radiating elements furthest away from the first radiating element and from the last radiating element.

15. The array antenna as claimed in claim 14, characterized in that said at least one parameter is the pitch of the array in one direction of the space or in two directions of the space.

16. The array antenna as claimed in claim 14, characterized in that, the radiating elements being metallic, said at least one parameter is a geometrical parameter of the radiating elements so that the radiating elements have different metallic surfaces.

17. The array antenna as claimed in claim 14, characterized in that said at least one parameter is a geometrical parameter of the radiating elements so that the radiating elements have different resistive surfaces.

18. The array antenna as claimed in claim 14, characterized in that said at least one parameter is a physical characteristic of a constituting element constituting the radiating elements, said constituting element being a substrate.

19. The array antenna as claimed in claim 14, characterized in that said at least one parameter is a physical characteristic of a constituting element constituting the radiating elements, said constituting element being a superstrate.

20. The array antenna as claimed in claim 18, characterized in that the physical characteristic is the permittivity of said constituting element.

21. The array antenna as claimed in claim 18, characterized in that the physical characteristic is the permeability of said constituting element.

22. The array antenna as claimed in claim 14, characterized in that, the radiating elements comprising a plurality of sheets of metallic patterns, said at least one parameter is the quantity or the arrangement of said sheets present in the radiating elements.

23. The array antenna as claimed in claim 14, characterized in that, the radiating elements comprising a plurality of sheets of resistance patterns, said at least one parameter is the quantity or the arrangement of said sheets present in the radiating elements.

24. The array antenna as claimed in claim 14, characterized in that, the radiating elements comprising metamaterials, said at least one parameter is the quantity of metamaterials present in the radiating elements.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

(1) The invention will be better understood and other advantages will become apparent on reading the following description given in a nonlimiting manner, and from the attached figures in which:

(2) FIG. 1 represents, by a top view, a rectangular antenna according to the prior art incorporated in a medium;

(3) FIG. 2 represents, by a side view, the preceding antenna according to the prior art;

(4) FIG. 3 represents the SER (ERCS) generated at the interface between an antenna according to the prior art and a medium;

(5) FIG. 4 represents, by a top view, a rectangular antenna according to the invention incorporated in a medium;

(6) FIG. 5 represents, by a side view, the preceding antenna according to the invention;

(7) FIG. 6 represents the SER (ERCS) generated at the interface between an antenna according to the invention and a medium;

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

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

(10) FIG. 9 represents the rate of variation of the reflectivity as a function of successive radiating elements;

(11) FIG. 10 represents the variation of the reflectivity coefficient as a function of the variation of the dependency parameter;

(12) FIG. 11 represents the variation of the dependency parameter as a function of the succession of the radiating elements;

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

(14) FIG. 13 represents the variation of the complex reflectivity coefficient between two radiating elements in the preceding embodiment;

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

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

(17) FIG. 16 represents the variation of the path representative of the variations of reflectivity of FIG. 15 as a function of the dependency parameter;

(18) FIG. 17 represents the value of the dependency parameter of FIG. 16 as a function of the radiating element;

(19) FIG. 18 represents a method according to an embodiment.

(20) As an example, FIGS. 4 to 6 represent an antenna A according to the invention incorporated in its environment M. FIGS. 4 and 5 represent a top view and a side view of a rectangular antenna A of width L.sub.x and of length L.sub.y incorporated in an environment M of different electromagnetic nature. As previously, the reflectivity Fa of the antenna is different from the reflectivity Γ.sub.m of the medium. This antenna is surrounded by a transition zone T of width L.sub.Tx and of length L.sub.Ty. This transition zone is composed of radiating elements. The electromagnetic parameters of these elements vary so as to modify their reflectivity coefficient Γ.sub.ij, thus ensuring a soft transition between the antenna and its medium.

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

DETAILED DESCRIPTION OF THE INVENTION

(22) Generally, the electromagnetic behaviors of the antenna and of the medium are characterized by an impedance or a surface reflectivity. There is a transitional relationship between these two parameters. It is thus possible to model the antenna and its medium by two plates of different impedances.

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

(24) As has been seen, the discontinuity brought about by the change of impedance modifies the radio frequency behavior of the antenna and induces detrimental diffraction phenomena. The incorporation of a progressive and controlled transition of the reflectivity in one or more directions of the space makes it possible to make the effects of this discontinuity disappear. Thus, it is possible to reduce the equivalent radar cross-section in significant proportions. It is also possible to optimize one of the characteristics of the radiation of the antenna. Examples that can be cited are the overall efficiency of the radiation, but also, the form and the distribution of the transmission side lobes or the gain of the antenna.

(25) The progressive variation of the reflectivity from one radiating element to another can be made over one or more physical parameters of the radiating element which can be: the pitch of the array in just one or both of the directions of the array; an intrinsic geometrical dimension of the radiating element, such as the aperture of a waveguide, a length, a width or a height; a physical property of the constituent materials of the radiating element such as, for example, the relative permittivity of the substrate of which it is composed.

(26) To control the progressive 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 in all of the radiating elements of the transition. A discretization of the transition amounts to giving a specific value to each element of the transition. These variations need to make it possible to adequately control the surface reflectivity of each radiating element.

(27) The method according to the invention makes it possible to reduce the effects of diffraction for an incidence, a polarization and a determined frequency. Although the optimization is done 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, of the polarization and of the frequency and is applied to a wider incidence, polarization and frequency range.

(28) 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 regardless of the frequency, the polarization and the incidence of the wave.

(29) Take a continuous or discrete assembly of radiating elements linking the antenna and its medium, the first element being in contact with the antenna and the last element being in contact with the medium. The number of radiating elements is denoted n and the order number of a radiating element is denoted i, with i varying from 0 to n.

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

(31) In a first step of the method according to the invention, as a function of the choice of the physical parameter or parameters, an accessible path L in the behavior between the two extreme radiating elements is defined.

(32) If s represents the variation parameter, s varying between two values that are denoted a and b, each radiating element has the reflectivity r(s).

(33) The latter comprises a real part x and an imaginary part y as indicated below.

(34) $\hspace{1em}\{\begin{array}{c}x=\mathrm{Re}\left(\Gamma \left(s\right)\right)\\ y=\mathrm{Im}\left(\Gamma \left(s\right)\right)\end{array}$

(35) The starting point of the path is defined as being the reflectivity of the antenna and the end point is defined as that of the medium. The definition of the reverse also works. The definition of this path gives the variation of the parameterized curve Γ(s).

(36) The curve of 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 is on the y axis. They lie between −1 and +1.

(37) The definition of a norm is necessary if several parameters are chosen. This norm guarantees the progressive variation of the parameters in order to avoid significant variations of the parameters without in any way detecting it on the curve r(s).

(38) The parameterized curve r(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 Γ(0) corresponds to the reflectivity of the antenna and the point denoted Γ(n) corresponds to the reflectivity of the medium for the nth radiating element. In the case of FIG. 7, this reflectivity is equal to −1.

(39) The length of the parameterized path L.sub.Γn has the value:
L.sub.Γn=∫.sub.0.sup.s.sup.n∥v(s)∥ds

(40) s.sub.0 is the initial value of the physical parameter or of all of 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.

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

(42) v(s) is the derivative value of Γ(s). Its coordinates in the complex plane are:

(43) $\hspace{1em}\{\begin{array}{c}{x}^{\prime }\left(s\right)\\ y^{\prime }\left(s\right)\end{array}$

(44) In a second step of the method according to the invention, the masking of the diffraction phenomena is optimized. It is necessary for the norm of the parametric speed denoted ∥v(s)∥ to be low at the start and at the end of the transition and great at the center. For this, it follows mathematical laws which make it possible to obtain this behavior. The parametric speed can take different values in the transition.

(45) FIG. 8 presents an example of the mathematical law describing the changes to the parameterized length L.sub.Γ 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 low 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 expressed also as a function of the radiating element i.

(46) Once the law of L.sub.Γ is defined, the next step of the method consists in working back to the values of the parameter or to all of the parameters associated with each length value of the parameterized curve.

(47) This determination can be made in different ways: analytically, if there is a transition formula, by means of charts or tabulated values.

(48) FIGS. 10 and 11 represent this step of determination of the physical dimensions associated with each element of the transition.

(49) FIG. 10 represents the variation of the length of the path L.sub.Γn as a function of the maximum value of the parameter s. This figure is represented in a semi-logarithmic reference frame, the parameter s varying according to a logarithmic law. For a given maximum parameter value, the value of the corresponding path is therefore deduced therefrom.

(50) 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 subsequent elements.

(51) Once this step is finished, the reflectivity of all of the elements of the transition can be represented in the complex plane to check the correct distribution of the points on the accessible path determined initially.

(52) As a nonlimiting example, the method is implemented in the case of the incorporation of an array antenna composed of waveguide apertures in a metallic medium. FIG. 12 represents, by a top view, the antenna A at its separation with the medium M. The apertures of the radiating elements ER are all identical, of square form and of side a. They are arranged regularly.

(53) In the frequency band of interest, the waveguides are said to be “under cutoff”, which is reflected by a total reflectivity of the guides, without in any way having a phase shift of 180° like the perfect metallic plane. That is reflected by an electrical discontinuity between the array of guides and a plate of metal causing 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 Γ(0) corresponds to the reflectivity of the antenna and the point denoted Γ(n) corresponds to the reflectivity of the medium for the nth radiating element. In the case of FIG. 13, this reflectivity is equal to −1.

(54) The method according to the invention consists in determining a transition zone separating the antenna from its medium so that the issues of spurious reflectivity are highly attenuated.

(55) The radiating elements of this transition zone are of the same nature as those of the antenna but of smaller dimensions. The parameter retained to make the reflectivity of the radiating element vary is therefore this dimension. FIG. 14 represents, by a top view, the antenna at its separation with the medium with the radiating elements ER.sub.T of the transition zone. The dimension a.sub.1 of the first element of the transition zone is therefore less than a.sub.0, the last element of the antenna, the dimension a.sub.2 of the second element of the transition zone is therefore less than a.sub.0 and so on for the subsequent elements.

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

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

(58) FIG. 17 represents the value of the dependency parameter as a function of the radiating element.

(59) FIG. 18 represents a method. At block 1802, there is a calculation of a path represented in the complex plane and equal to the sum of variations of the reflectivity from one radiating element to the next radiating element. At block 1804, there is an optimization of the variation of the reflectivity parameter so that the equivalent radar cross-section of the antenna is the lowest possible or that at least one of the characteristics of the radiation of the antenna is reached. At block 1806, there is a determination of the different radiating elements as a function of said parameter. At block 1808, there is a simulation of the overall reflectivity and/or of the radiation of the antenna.

(60) The simulations of the electromagnetic signature levels with or without said transition zone as defined previously shows a gain of approximately 30 dB over several frequency octaves, regardless of the polarization of the wave. This gain is all the greater when the incidence approaches grazing incidence.

(61) The method according to the invention makes it possible to obtain substantial attenuations of the spurious effects at the cost of reduced additional complexity. In the preceding exemplary embodiment, the radiating elements of the transition zone are, in fact, of the same nature as those of the antenna and pose no production problem.

(62) In the preceding example, the variable parameter is the size of the radiating elements. There are however many ways in which to modify the reflectivity parameter.

(63) Thus, the radiating element being metallic, the parameter can be a geometrical parameter of the radiating element so that the radiating elements have different metallic surfaces.

(64) The parameter can be a geometrical parameter of the radiating elements so that the radiating elements have different resistive surfaces.

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

(66) The radiating elements can 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.

(67) Finally, the radiating elements can comprise metamaterials, the parameter being the quantity of metamaterials present in the radiating elements. The term metamaterial denotes an artificial composite material which has electromagnetic properties different from those of the natural materials. These metamaterials are composed of periodic, dielectric or metallic structures depending on the properties sought.