EP2564466B1 - Compact radiating element having resonant cavities - Google Patents

Description

The present invention relates to the field of radiating elements, in particular for the low frequency bands, more particularly the frequency bands lying below the S band, and used in applications requiring the radiating of power, which can also be used in network antennas. It applies in particular to antennas used in telecommunication satellites.

The term "radiating element" denotes a combination of at least one radiating ground plane, excitation means intended to be supplied with signals, and a charged resonant cavity, of radiating energy representative of these signals according to a wavelength λ ₀ chosen.

The radiating elements used in network antennas must typically have at least one of the following characteristics: a high surface efficiency and / or a small footprint and a low mass and / or the ability to be excited compactly in single or dual -polarization and / or a bandwidth compatible with the intended application.

The feature of high surface efficiency is particularly important in uses of radiating elements in network antennas, because it optimizes the gain and reduces the levels of side lobes and lattice lobes. However, as explained below, this characteristic is hardly compatible with some of the other characteristics, and in particular those of compactness and integration, regardless of the frequency band concerned.

The term "network antenna" designates both active direct radiation network antennas and focal network antennas, the latter having one or more focusing reflectors, with an array of elementary sources placed in the focal zone. Such an antenna geometry is commonly referred to by the acronym FAFR corresponding to the English terminology "Focal Array Fed Reflector". Within such an antenna, each beam or "spot" is achieved by coherently grouping the signals of a subset of the elementary sources, with appropriate amplitudes and phases to obtain the desired antenna pattern, including the size and direction of aiming of the main lobe of radiation.

In the low frequency bands, such as the L or S band, the radiating elements, whatever the applications for which they are intended, are intended to supply the cones, too bulky. The most compact horns are of the Potter horn type; they have a longitudinal dimension typically greater than 3λ ₀ , where λ ₀ is the wavelength in vacuum; for example, λ ₀ is of the order of 150 mm in S-band. These Potter's horns are limited in radiating aperture, and thus in gain. Larger dimensions require longer lengths. As a result, Potter's turbinates have significant longitudinal bulk, as well as a large mass.

Subnets, for example planar in the case of space applications, are also not satisfactory, in terms of losses and compatibility with high power operations.

A first type of planar subnetwork consists of radiating elements of cobblestone type, also called "patches" according to English terminology, connected by a triplate splitter. This splitter is relatively complex and makes it difficult to achieve a sub-network allowing bipolarization or even a dual-band operation. The losses generated in this network can also be significant.

A second type of sub-network, in particular described in the French patent application published under the reference FR2767970 , consists of the combination of an exciter type resonator and cobblestones which constitute radiating elements known under the acronym ERDV, for "Radiant Element with Variable Directivity". This second type makes it possible to dispense with the splitter, and thus to significantly simplify its definition, as well as to repolarize in circular fields when the blocks, or "patches", are chamfered and the polarization is circular. But, its implementation for openings greater than 1.5 times the nominal operating wavelength is complex. This concept is also based on a micro-ribbon technology that can be incompatible with high power.

A simplification to the subnetworks of the second type has been proposed. It consists in replacing, on the one hand, the parasitic blocks by a metal grid realizing a semi-reflective interface facilitating the establishment of the electromagnetic field in the cavity, and on the other hand, the exciter pad by a guided exciter, so to define a Pérot-Fabry cavity, as in the case of an ERDV. The radiating element is then entirely metallic, compatible with applications requiring a high power, much simpler to define than a conventional ERDV element, and makes it possible to reach larger radiating openings than a conventional ERDV element. However, such a radiating element has two disadvantages: obtaining large radiating openings requires grids of high reflectivities, so that the electromagnetic field is established in the Perot-Fabry type cavity. The use of these strong reflectivities generates a significant return of the signal towards the access guide, and the adaptation of the radiating element is very delicate and valid only on a very narrow frequency band. On the other hand, when a high surface efficiency is required, it is then necessary, to insert the radiating element in a network antenna, to constrain the expansion of the electromagnetic field in the cavity, via metal walls. . These induce a non-uniform distribution of the field in the metal cavity. Admittedly, the use of variable pitch grids makes it possible to improve the distribution of the field by causing greater reflection at the center than at the periphery, but then the complete structure becomes very difficult to adapt.

A solution is proposed in the French patent application published under the reference FR2901062 . One of the embodiments presented therein, described below in detail with reference to the figure 2 , includes a stack two air cavities type Perot-Fabry, allowing great compactness, while giving a high surface efficiency and compatibility with high power signals. The stack of two cavities makes it possible to release the coefficient of overvoltage of the exciter cavity, and thus to reduce the returns in the access, to allow a better adaptation. However such a structure is conducive to the excitation of higher modes, in particular generated by the discontinuity present at the interface of the two stacked cavities. These higher modes interfere with the antenna's radiation pattern. The patent application FR2901062 above proposes to overcome this problem by the use of side walls for the cavities, in which are made adequate reliefs. The reliefs may for example be made in the form of longitudinal corrugations. Nevertheless, such corrugations are difficult to achieve, and are relatively bulky. In addition, it may be necessary in practice to load these corrugations of a dielectric, which makes their realization more complex, and can generate problems in a space environment, or in which it is necessary to process strong signals. power.

Finally, it is necessary to associate polarization devices with antennas. For example, the radiating elements must be able to be excited in single polarization and / or in bipolarization and / or in circular polarization. Typically, in antennas comprising horn type radiators, the size of the polarizer is of the same order of magnitude as the size of the horn. Thus, the size of the antennas is strongly impacted by the addition of polarizers.

An object of the present invention is to overcome at least the aforementioned drawbacks, by proposing a radiating element with resonant cavities with high surface efficiency, whose structure is particularly compact, and confers an optimal compromise between a high surface efficiency, a low congestion and low mass, as well as the ability to be excited in single polarization or bipolarization.

For this purpose, the subject of the present invention is a radiating element comprising at least two concentric resonant cavities, formed by a lower cavity fed by excitation means, and an upper cavity stacked on the lower cavity, each of said resonant cavities being delimited. in its lower part by a ground plane, in its lateral part by a substantially cylindrical or conical side wall, at least the upper cavity being delimited at its upper part by a first substantially plane cover, the radiating element being characterized in that essentially cylindrical and concentric corrugations of the resonant cavities are formed substantially below the first ground plane of the upper resonant cavity.

In one embodiment of the invention, the sidewalls may be substantially cylindrical in shape.

In one embodiment of the invention, the sidewalls may be substantially conical in shape.

In one embodiment of the invention, the lower cavity may also be delimited at its upper part, substantially at the level of the lower part of the upper cavity, by a second cap.

In one embodiment of the invention, the ground planes, the covers, the side walls and the corrugations may be essentially made of a metallic material.

In one embodiment of the invention, the covers may be formed by a partially reflecting surface.

In one embodiment of the invention, the covers may be formed by a metal grid.

In one embodiment of the invention, the covers may be formed by a dielectric material.

In one embodiment of the invention, the radiating element may be characterized in that a polarizing radome is formed in the upper part of the upper cavity.

In one embodiment of the invention, the polarizing radome may be formed by two polarizing frequency selective surfaces called FSS. polarizers substantially planar, arranged parallel to each other, and parallel and substantially above said first hood.

In one embodiment of the invention, each polarizing FSS may be formed by a metal plate having a plurality of slots.

In one embodiment of the invention, each polarizing FSS may be formed by a metal plate comprising a plurality of cross-slot cells.

In one embodiment of the invention, each polarizing FSS may be formed by a metal plate comprising a plurality of cross-slotted cells arranged in a periodic pattern on the surface of the metal plate.

In one embodiment of the invention, the side walls and the corrugations may be cylindrical with circular section.

In one embodiment of the invention, said excitation means may comprise at least one concentric supply guide of the resonant cavities and opening directly, or via adaptation means, into the lower cavity.

In one embodiment of the invention, said excitation means may comprise at least one double feed formed by two lateral waveguides opening symmetrically with respect to the main axis of the lower cavity, substantially at the level of the side wall of the lower cavity, the signals conveyed by the excitation means being tuned in phase so that the unwanted upper modes are filtered.

In one embodiment of the invention, said excitation means may comprise at least one concentric supply guide of the resonant cavities and opening directly, or via adaptation means, into the lower cavity, and at least one power supply. double formed by two lateral waveguides opening symmetrically with respect to the main axis of the lower cavity, substantially at the sidewall of the lower cavity, the signals conveyed by the excitation means being tuned in phase so that the unwanted top modes are filtered.

In one embodiment of the invention, a polarizing radome can be made above the upper cavity, the polarizing radome being substantially cylindrical and concentric resonant cavities.

In one embodiment of the invention, the polarizing radome may be substantially cylindrical in shape with a square section.

The present invention also relates to a network antenna characterized in that it comprises one or a plurality of radiating elements as described above.

• la figure 1, un élément rayonnant à cavité à air unique, de structure en elle-même connue de l'état de la technique ;
• la figure 2, un élément rayonnant à empilement de deux cavités à air, de structure en elle-même connue de l'état de la technique ;
• les figures 3a et 3b, un élément rayonnant selon un exemple de réalisation de l'invention, respectivement en vue en coupe latérale et en vue de dessus ;
• la figure 4, un élément rayonnant selon un autre exemple de réalisation de l'invention, dans une vue en coupe latérale ;
• la figure 5, un élément rayonnant selon un autre exemple de réalisation de l'invention, dans une vue en coupe latérale ;
• les figures 6a et 6b, un élément rayonnant selon un autre exemple de réalisation de l'invention, respectivement dans une vue en coupe latérale, et dans une vue en perspective.

Other features and advantages of the invention will appear on reading the description, given by way of example, with reference to the appended drawings which represent:

• the figure 1 a radiating element with a single air cavity, of a structure that is itself known from the state of the art;
• the figure 2 a radiating element with a stack of two air cavities, with a structure of itself known from the state of the art;
• the Figures 3a and 3b , a radiating element according to an exemplary embodiment of the invention, respectively in lateral sectional view and in top view;
• the figure 4 , a radiating element according to another embodiment of the invention, in a side sectional view;
• the figure 5 , a radiating element according to another embodiment of the invention, in a side sectional view;
• the Figures 6a and 6b , a radiating element according to another embodiment of the invention, respectively in a side sectional view, and in a perspective view.

The figure 1 presents a single air cavity radiator element, of the Perot-Fabry type, according to an embodiment itself known from the state of the art and described in the patent application FR2901062 supra.

A radiating element 10, shown in side sectional view in an XZ plane in the figure, may comprise an air resonant cavity 11 entirely delimited by a ground plane 110 at its bottom part situated in an XY plane, side walls 111 and a cover 112 at its upper part. The radiating element 10 comprises excitation means 12, which can be supplied with radiofrequency signals. The excitation means 12 may in particular comprise a supply port, for example formed by a metal waveguide 121 whose main axis is parallel to the Z axis, one of whose ends opens substantially at the level of the plane of Mass 110.

The air resonant cavity 11 has a transverse section, that is to say parallel to the XY plane, for example square, circular, hexagonal, or any other form that is compatible with the networking of the radiating element 10.

In the exemplary embodiment illustrated by the figure 1 , the side walls 111 may be of the "hard surface" type, that is to say for example made of a metallic material, in which longitudinal grooves are formed on either side of longitudinal ribs. The longitudinal grooves can be filled at least partially with a dielectric material. Longitudinal furrows and ribs can define periodic longitudinal structuring. As mentioned above, such a structuring is difficult to achieve in practice, and has a large footprint. In addition, the realization of such structuring is complicated by the need to load a dielectric material longitudinal grooves.

The cover 112 may for example be made of a thin or thick dielectric material. The dielectric material may for example comprise a face in which is formed a metal grid forming a semi-reflecting surface for increasing the excitation of the air resonant cavity 11 by the signals. The dielectric material may also comprise a face on which is formed a metal pad, said "patch", or a network of metal blocks, in order to induce a resonance complementary to that of the air resonant cavity 11. Also, the cover 112 can be made of a metallic material in which is formed a metal grid. The gate formed in the cover 112 may advantageously have a variable pitch in at least one selected direction.

The figure 2 has a stacking radiating element of two air cavities of the Perrot-Fabry type, according to an embodiment itself known from the state of the art and described in the patent application FR2901062 supra.

A radiating element 20 may comprise two concentric air resonant cavities 21 and 22 cascaded; an upper cavity 21 disposed above a lower cavity 22. This cascading is used to excite by the feed port a lower cavity 22 of reduced dimensions, and thus to limit the excitation of higher modes in this lower cavity 22, then by coupling in the upper cavity 21. The radiation can be better controlled, especially in the case of radiating elements 20 wide openings. It also makes it possible to reduce the reflectivities of the covers 212 and 222, and thus to more effectively couple the radiating element 20 to the supply access. The reflection losses in the access guide are reduced, and thus the adaptation of the input impedance of the radiating element 20 is facilitated.

The upper cavity 21 has substantially the same structure as the lower cavity 22. In a manner similar to the cavity structure described above with reference to the figure 1 , the radiating element 20 comprises excitation means 12, these being able to feed the lower cavity 22. The transverse section of the upper cavity 21 is greater than that of the lower cavity 22.

The upper cavity 21 is delimited in the XY plane by a first side wall 211, and covered at its upper part by a first cover 212. The first side wall 211 may be secured to a first ground plane 210, for example formed on the lower surface of a first SBT substrate. In the same way, the lower cavity 22 is delimited by a second lateral wall 221 and covered by a second cover 222. The second lateral wall 221 can be secured to a second ground plane 220, which can be formed on the lower surface of a second substrate SBT '. The first 212 and the first side wall 211 may be made according to the configuration described above with reference to the figure 1 . The first substrate SBT and the first ground plane 210 may include a through opening adapted to house the second cover 222 of the lower cavity 22. As illustrated by the figure 2 , the covers 212 and 222 may each comprise a metal grid 213, 223, more generally they may comprise partially reflecting surfaces.

The embodiments of the present invention described in detail hereinafter with reference to the following figures, apply to a structure comprising at least two stacked resonant cavities, however these can also be applied to structures comprising a stack of a plurality of air resonant cavities. The present invention proposes not to use the side walls of the resonant cavities to overcome the problems related to the upper electromagnetic modes.

The Figures 3a and 3b have a radiating element according to an exemplary embodiment of the invention, respectively in a sectional side view and a top view.

In the example illustrated by the figure 3a , a radiating element 30 presented in section in the XZ plane, may comprise an upper cavity 31 concentric with a lower cavity 32, the upper cavity 31 being stacked on the lower cavity 32, in a manner similar to the example described above with reference to the figure 2 . It is to highlight that the cavities 31, 32 are essentially cylindrical in the embodiments given by way of examples and described in the figures. Alternative embodiments may also include cavities 31, 32 of substantially conical shape. The lower cavity 32 may be powered by excitation means, for example a metallic waveguide 33, of cylindrical shape in the example illustrated by the figure. The upper cavity 31 may be delimited at its upper part by a first cover 312, in its lateral part by a first side wall 311, and in its lower part by a first ground plane 310. In the same way, the lower cavity 32 may be delimited at its upper part by a second cover 322, in its lateral part by a second side wall 312, and in its lower part by a second ground plane 320. The ground planes 310, 320 may for example be made in a metallic material. Also, the side walls 311, 321 may be made of a metallic material, and be free of dielectrics and / or reliefs. An opening may be made in the first ground plane 310, of surface substantially corresponding to the surface of the lower cavity 32 in the XY plane, said opening giving way to the second cover 322. The covers 312, 322 may be formed by surfaces partially reflective, for example by grids 313, 323. For example for applications requiring radiation in a single polarization, the grids 313, 323 may be one-dimensional grids, such as son networks, the son being aligned with the polarization excitation. In applications requiring double-polarization radiation, the grids 313, 323 must have identical reflectivity characteristics for the two excitation polarizations, so they are two-dimensional grids, the alignment of which does not have to correspond to that of the excitation polarizations.

For example, the waveguide 33 may emerge flush with the bottom of the lower cavity 32, or open into the lower cavity 32, slightly protruding from the bottom of the latter. Also, it may be envisaged to resort to means of adaptation, for example by iris.

In an alternative embodiment, not shown in the figures, it is also possible to form excitation means by double feeds from the side, respectively for applications requiring a simple polarization or multiple polarization. Also, double polarization excitation can be achieved by a bottom feed as described above, together with a double feed from the side. The dual power supplies open orthogonal to the lateral surface of the lower cavity 32, and opposite each other with respect to the main axis. In these various embodiments, each dual power supply is associated with a single access, for example by means of a suitable splitter, and all the power supplies are excited in a coherent manner, so that the excitations of the unwanted higher modes are filtered. Such structures make it possible to use the radiating element for applications requiring double polarization.

According to a feature of the present invention, corrugations 300 may be formed, substantially below the first ground plane 310. The corrugations 300 may be made of a metallic material, and may be of cylindrical shape, concentric of the resonant cavities 31, 32 In the example illustrated by the Figures 3a and 3b two cylindrical corrugations 300 are shown. In alternative embodiments, cylindrical corrugation may be contemplated. Also, more than two cylindrical corrugations may be disposed under the upper resonant cavity 31; it may be advantageous in such a case to use a plurality of corrugations 300 disposed periodically, that is to say that the spacing between two adjacent concentric corrugations remains constant.

In general, it is necessary to resort to a greater number of corrugations 300, if the lateral size of the upper resonant cavity 31 is larger. The position of a corrugation 300 may, for example, be characterized by its distance r _C with respect to the main axis of the radiating element 30. The dimensioning of the corrugations 300 may be characterized by their height I _C , their thickness d _C. In the case where several concentric corrugations 300 arranged periodically are used, the spacing between adjacent corrugations can be characterized by the period a _C.

The height I _C corrugations 300 allows control of the frequency band where the higher mode is deleted. It is for example advantageous to choose the height I _C of the order of a quarter of the nominal wavelength λ ₀ operating mode of the radiating element 30, this value allowing a deletion of the upper mode.

The position of the corrugations, that is to say the value r _C , makes it possible to optimize the axial symmetry of the radiation pattern of the radiating element 30, that is to say the desired similarity between the diagrams of radiation in the plane E and in the plane H of the radiated electromagnetic wave. It may be advantageous to choose the value r _C of the order of the nominal wavelength λ ₀ .

In a typical example, it is possible, for example, to produce a radiating element intended to operate in a frequency band ranging from 2.48 GHz to 2.5 GHz, the upper cavity 31 of which is cylindrical in shape with a circular cross-section. , with a diameter of the order of 2.5xλ ₀ , comprising a single corrugation 300 cylindrical circular section, disposed 118 mm from the main axis of the radiating element 30, a height of 31 mm and a width of 3.7 mm. The diameter of the lower cavity 32 may for example be less than half the diameter of the upper cavity 31. In this typical example, it is of the order of 1λ ₀ . Such a configuration makes it possible to achieve a perfectly axisymmetric radiation pattern, ie with a constant lobe width regardless of the observation plane, and also characterized by a secondary lobe or SLL level of less than -20 dB. . In addition, it has performances such that a directivity variation of between 16 dB and 16.2 dB, a variation of the surface efficiency of between 60% and 63%, a reflection coefficient | S ₁₁ | less than -25 dB. In comparison, a radiating element of similar structure without any corrugation is characterized by a non-axisymmetric radiation pattern, with a pinching of the lobe in the plane E associated with a rise in the secondary lobe or SLL, typically between -13 and -10 dB in the operating band.

As illustrated by the figure 3b the cavities 31, 32 and the corrugations 300 may be cylindrical with a circular section. Other embodiments of the invention, not shown in the figures, may for example comprise cavities 31, 32 and / or corrugations. 300 cylindrical non-circular section, for example square, rectangular, hexagonal, etc.

The reflectivities of the partially reflecting surfaces 313, 323 formed by the covers 312, 322 of the cavities 31, 32 may be adjusted to obtain concomitant matching and radiating bands. The lower cavity 32 may be chosen smaller in size than the upper cavity 31. For example, the partially reflecting surfaces 313, 323 may be formed by grids, and the reflectivity of the grid associated with the lower cavity 32 may be small. value, in order to obtain a good adaptation. The reflectivity of the upper cavity 31 may be of higher value, in order to spread the field over the opening of the radiating element, and achieve high directivities.

Values can be given here by way of non-limiting example of embodiment of the invention: it is for example possible to produce a linear polarization Ku band radiator element with corrugation 300 intended to operate in a frequency band. ranging from 11.8 to 13.2 GHz, whose opening is of the order of 1.85xλ ₀ , the thickness of which, that is to say the cumulative thickness of the two resonant cavities 31, 32, is of the order of λ ₀ , whose covers 312, 322 are respectively formed by semi-reflective gates respectively reflectivity coefficients (power) equal to 20% and 30%. Such a configuration makes it possible to achieve an axisymmetric radiation pattern characterized by a secondary lobe or SLL level of less than -18 dB. In addition, it has performances such that a directivity variation of between 14.59 dB and 15.39 dB, a variation of the surface efficiency of between 71.9% and 77.6%, as well as a reflection coefficient | S ₁₁ | less than -15.5 dB. In comparison, a radiating element of similar structure without corrugation is mainly different in that the radiation pattern is non-axisymmetric, and is characterized by a pinching of the lobe in the plane E associated with a rise of the secondary lobe or SLL, typically between -13 and -10 dB in the operating band.

The figure 4 presents a radiating element according to another embodiment of the invention, in a side sectional view. In the exemplary embodiment illustrated by the figure 4 , a radiating element 30 can be realized following a structure identical to the structure described above with reference to the Figures 3a and 3b but wherein the lower cavity 32 does not include a hood. Such a radiating element structure comprises only one gate 313, and hence is simpler and less expensive to produce. The removal of the grid in the lower cavity 32 is indeed possible because the only sudden transition between the lower cavity 32 and the upper cavity 31 generates a reflection phenomenon, a lower resonant cavity then being defined without a metal grid being necessary. Such a structure is for example suitable for apertures of the radiating element ranging from 1 to 3 λ ₀ , for example for S-band or Ku-band applications, the configuration being given previously by way of example corresponding to a band application. Ku.

As mentioned above, it is advantageously possible to confer on a radiating element according to the invention a greater compactness, by avoiding the additional bulk imposed by a polarization device or polarizer. The figure 5 presents an advantageous embodiment, in which a polarizer is integrated in the actual structure of the radiating element.

With reference to the figure 5 , a radiating element 50 shown in a side sectional view in an XZ plane, can be made in a structure similar to the structures of the radiating element 30 described above with reference to the Figures 3a, 3b and 4 . In the example illustrated by the figure 5 , a structure similar to the structure illustrated by the figure 4 is chosen. The radiating element 50 thus comprises in particular a lower cavity 32 fed by excitation means formed by a waveguide 33. The upper cavity 31 is covered by a cover formed by a grid 313 constituting a partially reflecting surface. In the example illustrated by the figure, a simple corrugation is performed substantially under the upper cavity 31. According to a feature of the embodiment illustrated by the figure 5 a polarizing radome 51 can be made in the upper part of the upper cavity 31. The polarizing radome 51 can be formed by the combination of at least two polarizing frequency selective surfaces, designated polarizing FSS according to the English terminology "Frequency Selective Surface". A polarizing radome is itself known from the state of the art, and makes it possible to induce a phase difference between the two components of the electric field E _x and E _y of the electromagnetic wave. When this phase difference is ± 90 °, the polarizing radome 51, excited in linear polarization in an oblique direction in the XY plane, that is to say at + 45 ° by means of the X axis, generates a right circular polarization, and excited in linear polarization in a direction of -45 °, generates a left circular polarization. It should be observed that the polarizing radome 51 transforms a linear dual-polarization type operation into circular double-polarization type operation.

In the non-limiting example illustrated by the figure 5 the polarizing radome 51 may be of the "double-FSS" type, and comprise two polarizing FSSs 511 and 512 arranged parallel to one another above, and separated by a distance D _FSS . The lower FSS 512 is arranged parallel to the gate 313 at a distance D ₃ from the latter. A double FSS type configuration makes it possible to obtain a wider bandwidth, and a signal transmission without loss, the signal transmission not inducing a return to the upper cavity 31. It is not possible to with a single layer polarizing radome, obtain a lossless transmission, and a phase shift of 90 ° according to the two components E _x and E _y of the incident signal.

Typically, the two polarizing FSSs 511 and 512 are identical and separated by a guided half-wavelength, in order to simultaneously obtain lossless transmission of the incident signal, and a phase quadrature delay. between the two orthogonal components of the transmitted signal. The polarizing radome 51 is positioned above the radiating element 50 designed to radiate in double linear polarization, at a distance typically of the order of a quarter of a guided wavelength. Thus, the polarizing radome 51 does not fundamentally disturb the operation of the radiating element 50. A slight modification of the dimensions of the patterns of the FSS can be adjusted in order to refine the radiation and the adaptation of the radiating element 50 .

The polarizing FSSs can be of the inductive or capacitive type: the polarizing FSS of the inductive type being essentially formed by metal surfaces in which slit-defined patterns are formed, the capacitive-type polarizing FSS being essentially formed by surfaces on which metallic patterns are made. The use of inductive type FSS can be advantageous because it does not require the use of a substrate, the FSS can then be directly made of a metallic material.

Each polarizing FSS 511, 512 may for example be made in the form of a metal plate provided with slots. For example, for applications requiring a bipolarization or circular polarization excitation, cross slot cells 520, designated "cross slots" cells according to the English terminology, may be arranged on the metal plate, for example in a periodic pattern. A cross slot cell 520 is shown in plan view on the figure 5 . The cross slot cell 520 is particularly characterized by the length of its side, or period a, by the length and the width respectively at _y and d _y of the horizontal slot (that is to say along the X axis ), as well as the length and width a _x and d _x of the vertical slot (along the Y axis). It is possible to obtain a phase difference between the two field components E _x and E _y by choosing horizontal and vertical slots of different sizes. The reflectivity according to a given polarization is adjusted by varying the length of the slot perpendicular to this polarization. Knowing that the reflectivity of the slot is zero at resonance, and that before its resonance the slot has a negative phase reflection coefficient and after the resonance a positive phase, the cross slots have different lengths according to each of the two polarizations of way to create a phase shift of 90 ° between the two polarizations, and thus generate a circular polarization. For example, the lengths at _x and _y slots can be determined so that one of the slots has an action on frequencies below the resonant frequency, and the other slots for higher frequencies. In this way, it is possible to obtain for the polarizing radome consisting of two separate FSS for example a distance D _FSS equal to λ _0/2 or close to this value, a phase difference of 90 ° in transmission between the components E _x and E _y . For example, it is possible to set the length a _x of the vertical slot to a value less than λ _0/2 , and the length _y of the horizontal slot to a value greater than λ ₀ / d2. It is of course reciprocally possible to set the length a _y of the horizontal slot to a value less than λ _0/2 , and the length a _x of the vertical slot to a value greater than λ _0/2 . Period a must be set to a value greater than a _x and a _y . Slit widths d _x and d _y are adjusted according to the thickness of the metal plate. Typically, the widths of the slots d _x and d _y are chosen well below the nominal wavelength λ ₀ . The aforementioned embodiment is based on cross-shaped slot cells 520 arranged in a square mesh, but it is also possible to use cells arranged in a different mesh, for example round, hexagonal, ...

Also, patterns other than crosses may be used, for example annular slots, or Jerusalem Cross type slits, etc.

It is advantageously possible to use a polarizing radome which is not directly integrated with the upper cavity, as in the embodiment described above with reference to FIG. figure 5 . The Figures 6a and 6b have a radiating element according to another embodiment of the invention, respectively in a side sectional view, and in a perspective view.

In the example illustrated by the Figures 6a and 6b a radiating element 60 may have a structure substantially similar to the structure of the radiating element 50 described above with reference to the figure 5 . Thus, the radiating element 60 comprises in particular an upper cavity 31, a lower cavity 32 fed by a waveguide 33. The upper cavity 31 is in this example covered by a cover formed by a grid 313. Corrugations 300 are made substantially below the upper cavity 31. In the example illustrated by the Figures 6a and 6b , the side walls of the upper and lower cavities 31, 32 are cylindrical in shape, circular section. A polarizing radome 61 is produced above the upper cavity 31. In this example, the polarizing radome 61 is also of cylindrical shape, but with a square section. As illustrated by the figure 6b , the polarizing radome 61 is delimited in its lateral part by side walls of substantially cylindrical shape, with a square section. The use of a square section allows here to have a larger number of square cross-shaped slot cells 620 on the surface of polarizing FSSs 611, 612 formed by two metal plates arranged parallel to each other.

In a typical example, it is possible to produce a radiating element intended to operate in a frequency band ranging from 2.48 GHz to 2.5 GHz, the polarizing radome 61 of which is square in shape, the side of which has a length of the order of 2.7xλ ₀ . Such a configuration makes it possible to achieve circular double polarization, that is to say right and left, by exciting the antenna by two linear polarizations + 45 ° to -45 °. In both cases, the radiation patterns are perfectly axisymmetric, that is to say that the lobe width is constant regardless of the observation plane, and also characterized by a secondary lobe level or SLL less than - 25 dB. In addition, in the frequency band mentioned above, for both polarizations, the directivity varies between 16.5 dB and 16.7 dB, and the surface efficiency is between 63% and 66%. The reflection coefficient | S ₁₁ | is less than -20 dB and the axial ratio is less than 1 dB on the band of interest.

Claims (20)

1. A radiating element (30) comprising at least two concentric resonant cavities (31, 32) formed by a lower cavity (32) fed by excitation means (12, 33) and an upper cavity (31) stacked on said lower cavity, each of said resonant cavities (31, 32) being delimited on its lower part by a ground plane (310, 320) and in its lateral part by a lateral wall (311, 321), with at least said upper cavity (31) being delimited in its upper part by a first substantially flat cap (313), said radiating element (30) being characterised in that substantially cylindrical corrugations (300) concentric to said resonant cavities (31, 32) are formed substantially below the first ground plane (310) of said upper resonant cavity (31).
2. The radiating element (30) according to claim 1, characterised in that said lateral wall (311, 321) is substantially cylindrical.
3. The radiating element (30) according to claim 1, characterised in that said lateral wall (311, 321) is substantially conical.
4. The radiating element (30) according to any one of the preceding claims, characterised in that said lower cavity (32) is also delimited in its upper part, substantially on the lower part of said upper cavity, by a second cap (323).
5. The radiating element (30) according to any one of the preceding claims, characterised in that said ground planes (310, 320), said caps (313, 323), said lateral walls (311, 321) and said corrugations (300) are substantially produced from a metal material.
6. The radiating element (30) according to any one of the preceding claims, characterised in that said caps (313, 323) are formed by a partially reflective surface.
7. The radiating element (30) according to any one of the preceding claims, characterised in that said caps (313, 323) are formed by a metal grid.
8. The radiating element (30) according to any one of the preceding claims, characterised in that said caps (313, 323) are formed by a dielectric material.
9. The radiating element (30, 50) according to any one of the preceding claims, characterised in that a polarising radome (51) is produced in the upper part of said upper cavity (31).
10. The radiating element (30, 50) according to claim 9, characterised in that said polarising radome (51) is formed by two substantially flat polarising frequency-selective surfaces (511, 512), referred to as polarising FSS, disposed parallel to each other and parallel to and substantially above said first cap (313).
11. The radiating element (30, 50) according to claim 10, characterised in that each polarising FSS (511, 512) is formed by a metal plate comprising a plurality of slots.
12. The radiating element (30, 50) according to claim 10, characterised in that each polarising FSS (511, 512) is formed by a metal plate comprising a plurality of cross-shaped slot cells (520).
13. The radiating element (30, 50) according to claim 10, characterised in that each polarising FSS (511, 512) is formed by a metal plate comprising a plurality of cross-shaped slot cells (520) arranged in a regular pattern on the surface of said metal plate.
14. The radiating element (30, 50) according to any one of claims 1 to 2 or 4 to 13, characterised in that said lateral walls (311, 321) and said corrugations (300) are cylindrical with a circular cross-section.
15. The radiating element (30, 50) according to any one of the preceding claims, characterised in that said excitation means (12, 33) comprise at least one feed guide (33) concentric to said resonant cavities (31, 32) and opening out directly, or via adaptation means, into said lower cavity (32).
16. The radiating element (30, 50) according to any one of claims 1 to 14, characterised in that said excitation means (12, 33) comprise at least one dual feed formed by two lateral waveguides opening out symmetrically relative to the main axis of said lower cavity (32), substantially on said lateral wall (321) of said lower cavity (32), with the signals carried by said excitation means being in phase so that the unwanted upper modes are filtered.
17. The radiating element (30, 50) according to any one of claims 1 to 14, characterised in that said excitation means (12, 33) comprise at least one feed guide (33) concentric to said resonant cavities (31, 32) and opening out directly, or via adaptation means, in said lower cavity (32), and at least one dual feed formed by two lateral waveguides opening out symmetrically relative to the main axis of said lower cavity (32), substantially on said lateral wall (321) of said lower cavity (32), with the signals carried by said excitation means being in phase so that the unwanted upper modes are filtered.
18. The radiating element (30, 60) according to any one of claims 1 to 8, characterised in that a polarising radome (61) is produced above said upper cavity (31), said polarising radome (61) being substantially cylindrical and concentric to said resonant cavities (31, 32).
19. The radiating element (30, 60) according to claim 18, characterised in that said polarising radome (61) is substantially cylindrical with a square cross-section.
20. An array antenna, characterised in that it comprises one or more radiating elements (30, 50, 60) according to any one of the preceding claims.

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