EP1821366B1 - Mechanical scanning antenna scanning a broad spatial range with reduced bulk - Google Patents

Abstract

The antenna has a conical shaped stator (31) provided with an input (34) and an output (36), and a conical shaped rotor (32) rotating inside the stator, where the space between the rotor and the stator forms a waveguide. The rotor has a diametral waveguide e.g. path, that guides a microwave from the input to the output of the stator. A foster system has a focusing system (22) e.g. focusing radome, provided with a focusing lens (51) whose focal spot is situated at the output of the stator. The system (22) is integrated in an aerodynamic shaped radome of the antenna.

Description

The present invention relates to a mechanical scanning antenna scanning a wide space domain and reduced space. It is particularly applicable for airborne radar applications subject to space constraints and aerodynamic performance.

Radar detection is done within one or more transmit beams produced by the radar antenna. A beam covers a limited part of the space. The detection therefore requires scanning of a transmission beam to cover most of the space.
There are two basic modes for producing a transmit beam scan.
A first mode is mechanical scanning. In this mode, the beam remains fixed with respect to the mechanical structure of the antenna. It is the antenna movements, in azimuth and / or in site which then generate the scanning of the emission beam.
A second basic mode is electronic scanning. In this case, the antenna remains fixed and the scanning of the emission beam is generated electronically by playing on the phase shifts of radiating modules. A third mode can combine mechanical scanning and electronic scanning.
An electronic scanning antenna has very good scanning performance, in particular with regard to scanning speed and agility but also the spatial amplitude of the scanning. Moreover, its operation requires only a relatively small footprint since it is not necessary to reserve space for the movements of the antenna. This scanning mode nevertheless has certain drawbacks, especially as regards the costs and the complexity of implementation.
A mechanical scanning antenna, which does not always have the performance of an electronic scanning antenna, may be perfectly suitable for certain applications, particularly for economic aspects, where simplicity or low realization costs are required. The disadvantages of mechanical scanning are well known. In particular, the spatial amplitude of the scanning of the beam is limited by the movements of the antenna itself. In a mechanical configuration given, the antenna can move only inside a sphere of space limiting the spatial scanning range of the emission beam of the antenna. In addition, the space required for the operation of the antenna, in particular for performing its scanning movements in one or two planes, in fact its sphere of space itself, increases the overall bulk volume of the antenna. In fact, to obtain a sweep over a wide domain imposes a sphere of large size.

The document US 3,018,479 discloses a beam scanning antenna which has a conical shaped stator having an input and an output and a conically shaped rotor rotating inside the stator, the space between the rotor and the waveguide stator.

• un stator de forme conique comportant une entrée et une sortie ;
• un rotor de forme conique tournant à l'intérieur du stator, l'espace entre le rotor et le stator formant guide d'onde, le rotor comportant un guide d'onde diamétral, une onde hyperfréquence à émettre étant guidée de l'entrée jusqu'à la sortie via le guide diamétral du rotor ;
• un système focalisateur comportant au moins une lentille focalisante dont le foyer est situé sensiblement à la sortie du stator.

An object of the invention is to overcome this disadvantage, in particular by allowing the realization of a mechanical scanning antenna scanning a wide range of space. For this purpose, the subject of the invention is a beam scanning antenna comprising at least:

• a conical shaped stator having an inlet and an outlet;
• a conical rotor rotating inside the stator, the space between the rotor and the waveguide stator, the rotor comprising a diametrical waveguide, a microwave wave to be emitted being guided from the input to the at the outlet via the diametral guide of the rotor;
• a focusing system comprising at least one focusing lens whose focus is located substantially at the output of the stator.

The focusing system is for example integrated in a radome.
The stator comprises, for example upstream of the inlet and downstream of the outlet, a protuberance blocking the passage of the microwave wave.
The rotor comprises, for example downstream of the inlet and upstream of the output of the diametral waveguide, a microwave trap blocking the passage of the microwave wave.
The focusing system comprises a second focusing lens whose focus is located substantially at the entrance of the stator.

The main advantages of the invention are that it makes it possible to reduce the space required for the operation of the antenna, that it makes it possible to obtain a strong misalignment of the antenna, that it avoids the use of rotating joints, that it makes it possible to obtain an antenna radome of aerodynamic shape and that it is economical.

• la figure 1, un exemple d'antenne selon l'art antérieur nécessitant une sphère d'encombrement ;
• la figure 2, une illustration d'un système de Foster comportant d'un stator et d'un rotor et un système focalisateur formant une antenne selon l'invention ;
• la figure 3, les composantes d'un système de Foster ;
• les figures 4a et 4b, une illustration du fonctionnement d'un système de Foster tel qu'utilisé dans une antenne selon l'invention ;
• la figure 5, un exemple de réalisation d'une antenne selon l'invention.

Other characteristics and advantages of the invention will become apparent with the aid of the following description made with reference to appended drawings which represent:

• the figure 1 an example of an antenna according to the prior art requiring a sphere of space;
• the figure 2 , an illustration of a Foster system comprising a stator and a rotor and a focusing system forming an antenna according to the invention;
• the figure 3 , the components of a Foster system;
• the Figures 4a and 4b an illustration of the operation of a Foster system as used in an antenna according to the invention;
• the figure 5 , an exemplary embodiment of an antenna according to the invention.

The figure 1 shows by way of example the case of an antenna for SAR applications ("Synthetic Aperture Radar" in the English terminology). This antenna 3, slots, is placed in a pod 1 integrated under a plane 2. The deflections of the antenna are limited by the pod defining a sphere of space. In the case of a mechanical scanning antenna, due to the movements of the antenna, this sphere of space limits the scanning range of the antenna. Since the dimensions of a pod or radome inside which an antenna is placed can not be increased indefinitely, especially in airborne applications, it follows that the mechanical scanning of an antenna is systematically limited, more or more important depending on the case. In other words, the small volume of space available limits the misalignment of an antenna.

The figure 2 illustrates in perspective view a mechanical scanning antenna according to the invention. The antenna comprises a Foster 21 system which is associated with a focusing system 22 allowing in particular the emission of a high directivity antenna lobe. The focusing system is for example a tapered focusing radome. The Foster 21 system provides mechanical misalignment of an antenna beam.

The figure 3 presents the components of a Foster system. A Foster system 21 is thus composed of two parts 31, 32. A first part is a stator 31 and a second part is a rotor 32. The rotor and the stator are both of conical shape, the rotor rotating inside. of the stator. The wave 33 to be transmitted first enters an input 34 of the stator 31, continues its way in the space between the stator and the rotor and then enters the inside of the rotor in a diametrical passage 35. diametrical waveguide inside the rotor. At the output of the rotor the wave continues its path between the stator and the rotor to an output 36 of the stator.
The openings, at the inlet 34 and at the outlet 36, are made longitudinally along the entire length of the stator or in a portion. The diametral waveguide 35 is made in the rotor on the same length as the openings of the inlet and the outlet.
The incoming wave 33 is out of phase with an electrical path as described later.

θ _r étant exprimé en radians dans cette relation (1).The Figures 4a and 4b allows to describe the functioning of a Foster system. The figure 4a presents a perspective view of the stator and the rotor. The stator 31 of conical shape is fixed. The rotor 32, also conical in shape, rotates inside the stator. These two parts 31, 32 have a common axis of symmetry 41.
The figure 4b presents a sectional view of the figure 4a in a plane 42 perpendicular to the axis of symmetry 41 of the stator. Note the position of this plane 42 along the axis of symmetry 41. The wave 33 enters the stator and then into the rotor as indicated above. The input 34 and the output 36 of the stator are for example symmetrical with respect to the axis of symmetry 41 of the stator. The passage of the wave 33 through the rotor is inside a path 35 hollowed out diametrically in the rotor. At a given instant, this diametrical path 35 makes an angle θ _r with the entry point 34 of the wave in the stator. This angle θ _r is in fact the angle of rotation of the stator with respect to the stator having its origin at the point of entry 34 of the wave 33 in the stator. The angle θ _r is a function of time.
The wave 33 enters the stator at the input 34 then enters the diametrical path 35 and finally leaves the stator at the output 36. path traveled within the system formed by the stator and the rotor is thus composed of three portions. A first portion 43, located between the stator and the rotor, begins at the point of entry 34 to the point of entry into the rotor. The distance traveled in this first portion 43 is R (z) θ _r . R (z) is the radius of the path traveled, it depends on the position z of the plane 42 along the axis of symmetry 41 due to the conical shape of the stator and the rotor. The origin of the positions z on the axis of symmetry can for example be taken at the top of the cone S.
The second path portion consists of the entire transverse path within the rotor. The distance traveled in this second portion is substantially equal to 2R (z). Finally, the third portion 44 is symmetrically opposed to the first portion 43, it goes from the output of the rotor to the output point 36 of the stator. It is therefore equal to R (z) θ _r . Thus, at a point z of the axis of symmetry 41 of the stator, the electrical path δ (z) traversed by the incoming wave 33 is given by the following relation: $$\mathrm{\delta }\left(\mathrm{z}\right)\mathrm{=}\mathrm{2}\mathrm{R}\left(\mathrm{z}\right)\mathrm{+}\mathrm{2}\mathrm{R}\left(\mathrm{z}\right){\mathrm{\theta }}_{\mathrm{r}}\mathrm{=}\mathrm{2}\mathrm{R}\left(\mathrm{z}\right)\left(\mathrm{1}\mathrm{+}{\mathrm{\theta }}_{\mathrm{r}}\right)$$

θ _r being expressed in radians in this relation (1).

The interior of the stator 31 and the outside of the rotor 32 are for example covered with a metal layer so that the space between the stator and the rotor can guide the incoming microwave wave 33. In particular the width of this space can be defined according to the wavelength. Similarly, the passage 35 provided inside the rotor 32 forms a waveguide. Its inner faces are for example covered with a metal layer, the width of this passage depending in particular on the wavelength.
A protrusion 45 projecting inside the stator is located near the inlet 34 to block the passage of the incoming wave in one direction. For this purpose the protuberance 45 extends as far as possible from the surface of the rotor. It therefore prevents the incoming wave from following the wrong direction by forcing it to follow the intended path 43. To reinforce the locking of the passage a second protrusion 46 is, for example, provided next to the previous 45.

After following the path 43 inside the stator, the microwave must then enter the diametral path 35 of the rotor. To prevent the wave from continuing farther its travel inside the stator, a microwave trap 47 is placed downstream of the rotor inlet, and close to the latter. This trap 47 is, for example, formed of a microwave short circuit at λ / 4. This trap 47 is placed on the surface of the rotor but on the inside so as not to come into contact with a protuberance 45, 46 of the stator during the rotation of the rotor. Another microwave trap 48, for example of the same structure, is diametrically opposed to the previous 47 to force the microwave to follow the path 44 to the output 36 of the stator. The stator has another protrusion 49 diametrically opposed to the first 45 to force the microwave to continue to the output 36. The protuberances 45, 49 could be replaced by microwave traps.

où λ est la longueur de l'onde entrante 33.
En référence aux relations (1) et (2), le déphasage peut encore s'écrire : $$\varphi \left(z\right)=\frac{2\mathit{R}\left(z\right)\left(1+{\theta }_r\right)\mathit{.2}\mathit{\pi }}{\mathit{\lambda }}$$

Si α représente le demi-angle au sommet du cône, la relation entre le rayon R(z) en un point de l'axe de symétrie 41 du stator et la distance z de ce point au sommet est la suivante : $$\mathrm{R}\left(\mathrm{z}\right)\mathrm{=}\mathrm{z}\mathrm{.}\mathrm{tg}\left(\mathrm{\alpha }\right)$$

L'angle de dépointage θ_p de l'onde en regard d'un point z de l'axe 41 vérifie la relation suivante : $$\frac{2\mathit{\pi }\mathrm{.}z\mathrm{.}\sin {\theta }_p}{\mathit{\lambda }}=\frac{2\mathit{z}\mathit{.}\mathit{tg}\left(\alpha \right)\left(1+\theta \right).2\mathit{\pi }}{\mathit{\lambda }}$$

Il en résulte que : $$\sin \left({\mathrm{\theta }}_{\mathrm{p}}\right)\mathrm{=}\mathrm{2.}\left(\mathrm{1}\mathrm{+}{\mathrm{\theta }}_{\mathrm{r}}\right)\mathrm{.}\mathrm{tg}\left(\mathrm{\alpha }\right)$$

The incoming wave is out of phase with the electrical path δ (z) on the portions 43, 35, 44. The phase shift is therefore a function of the angular position of the rotor and the distance z with respect to the tip S of the cone.
The phase shift φ (z), a function of the position z along the axis of symmetry 41 is given by the following relation: $$\phi \left(z\right)=\frac{2\mathit{\pi }}{\mathit{\lambda }}\delta \left(z\right)$$

where λ is the length of the incoming wave 33.
With reference to relations (1) and (2), the phase difference can still be written: $$\phi \left(z\right)=\frac{2\mathit{R}\left(z\right)\left(1+{\theta }_r\right)\mathit{.2}\mathit{\pi }}{\mathit{\lambda }}$$

If α represents the half-angle at the apex of the cone, the relationship between the radius R (z) at a point of the axis of symmetry 41 of the stator and the distance z from this point to the vertex is as follows: $$\mathrm{R}\left(\mathrm{z}\right)\mathrm{=}\mathrm{z}\mathrm{.}\mathrm{tg}\left(\mathrm{\alpha }\right)$$

The misalignment angle θ _p of the wave opposite a point z of the axis 41 satisfies the following relation: $$\frac{2\mathit{\pi }\mathrm{.}z\mathrm{.}\sin {\theta }_p}{\mathit{\lambda }}=\frac{2\mathit{z}\mathit{.}\mathit{tg}\left(\alpha \right)\left(1+\theta \right).2\mathit{\pi }}{\mathit{\lambda }}$$

It follows that : $$\sin \left({\mathrm{\theta }}_{\mathrm{p}}\right)\mathrm{=}\mathrm{2.}\left(\mathrm{1}\mathrm{+}{\mathrm{\theta }}_{\mathrm{r}}\right)\mathrm{.}\mathrm{tg}\left(\mathrm{\alpha }\right)$$

The relation (6) gives the angle of misalignment of the microwave beam at the output 36 of the stator. This angle θ _p is the angle made by the direction 50 of the beam with the perpendicular. It is a function of the rotation angle θ _r of the rotor, itself a function of time. Rotation of the rotor thus induces a misalignment of the output microwave beam 36 of the stator, the misalignment angle being a function of the rotation of the rotor according to relation (6). The misalignment angle is independent of the microwave wavelength λ. At the output 36 of the stator, the microwave beam is very directional, due in particular to the small opening width of the stator at the outlet 36.

The figure 5 shows an embodiment of an antenna according to the invention where the assembly consisting of the stator and the rotor of the Figures 4a and 4b is equipped with a focusing system 22. The focusing system comprises at least one focusing lens 51. The focus of the lens 51 is substantially at the output 36 of the stator. In this way, the lens 51 makes it possible to focus the microwave beam and to obtain a very directional microwave beam. This beam forms the antenna beam 53 of an antenna according to the invention. This beam sweeps the space according to the misalignment angle θ _p itself a function of the rotation θ _r of the rotor.
The focusing system may be completed by a second lens 52, for example symmetrical with respect to the first. This second lens is useful when the input 34 of the stator becomes the output of the microwave wave.
A focusing lens 51, 52 is for example of type D glass to which is added a binder of polyester type.

Unrepresented waveguides are provided to guide the microwave to the input of the stator.
The focusing system 22 may also act as a radome. Its shape is imposed by the shape of the stator on which the lenses must focus. This results in a form of substantially conical radome so very aerodynamic.

Advantageously, a mechanical antenna beam scan according to the invention, although of mechanical type, does not require rotating joints.
In the solution proposed by an antenna according to the invention, no space sphere is necessary. This solution is, therefore, well suited for applications requiring high misalignments, for example, of ± 120 °.

Claims (5)

1. Beam-scanning antenna, the antenna comprising at least:

   - a stator (31) of conical shape comprising longitudinally over its length an input (34) and an output (36);

   - a rotor (32) of conical shape rotating inside the stator (31), the space between the rotor and the stator forming a waveguide, characterized in that the rotor comprises a diametral waveguide (35), a microwave (33) to be emitted being guided from the input (34) to the output (36) via the diametral guide of the rotor; and

   - a focusing system (22) comprising at least one focusing lens (51) whose focus is situated substantially at the output (36) of the stator.
2. Antenna according to Claim 1, characterized in that the focusing system (22) is integrated into a radome.
3. Antenna according to any one of the preceding claims, characterized in that the stator (31) comprises upstream of the input (34) and downstream of the output (36) a protuberance (45, 49) blocking the passage of the microwave (33).
4. Antenna according to any one of the preceding claims, characterized in that the rotor (32) comprises downstream of the input and upstream of the output of the diametral waveguide (35) a microwave trap (47, 48) blocking the passage of the microwave (33).
5. Antenna according to any one of the preceding claims, characterized in that the focusing system (22) comprises a second focusing lens (52) whose focus is situated substantially at the input (34) of the stator.

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