EP0202979B1 - Polarization-selective reflecting device and method of making such a device - Google Patents

Description

The main object of the invention is a device reflecting the electromagnetic waves of a polarization and its production method.

Specialized speed cameras are being added to modern speed cameras to collect additional information. It is essential to be able to separate electromagnetic waves belonging to various channels. It is known in Cassegrain type antennas to use crossed polarizations for the various channels. For example, a main channel works in horizontal rectilinear polarization while an additional channel works in vertical rectilinear polarization. Separation at reception takes place at an auxiliary reflector of the Cassegrain antenna, reflecting for one polarization and transparent for the other. Such reflectors were made with parallel metal wires. The parallel metal wires are embedded in a low loss dielectric, or form a screen. The known type of auxiliary reflector was not perfectly transparent for one channel without being perfectly reflective for the other. In theory, the system only works in the main planes, that is to say the horizontal plane and the vertical plane.

US Patent 4,144,535 describes a system which attempts to optimize simultaneously the transmission of electromagnetic waves having a first polarization and the reflection of waves having a second polarization orthogonal to the first. Thus, neither the reflection nor the transmission are completely optimized.

The device according to the present invention is made perfectly reflective for one of the polarizations. So the addition of an additional lane to an existing radar does not in any way disturb its operation. However, it should be noted that the reflector according to the invention is not perfectly transparent for the electromagnetic waves belonging to the additional channel.

où y,z sont les coordonnées dans le repère O, x, y, z, a est la distance entre le sommet du réflecteur et l'intersection des asymptotes au réflecteur, c la demi-distance focale du réflecteur,

α est un paramètre.The main object of the invention is a microwave wave reflector comprising metallic surfaces and non-metallic surfaces, mainly characterized by the fact that it is perfectly reflective for waves of a first polarization and partially transparent for waves of a second polarization orthogonal to the first polarization, the metal surfaces are at the intersections of the surface of the reflector with a beam of straight lines passing through a focal point 0 of said reflector and through a horizontal circle included in a sphere of center 0 in such a way that said metal surfaces are placed at the intersection of the reflector surface and the equation planes: $$\text{ay ± cos α (cz-b²) = 0}$$

where y, z are the coordinates in the coordinate system O, x , y , z , a is the distance between the top of the reflector and the intersection of the asymptotes at the reflector, c the half focal length of the reflector,

α is a parameter.

• _ la figure 1 est un schéma explicatif;
• _ la figure 2 est un schéma explicatif;
• _ la figure 3 est un schéma d'un exemple de réalisation du dispositif selon l'invention.

The invention will be better understood by means of the description below and the appended figures given as nonlimiting examples among which:

• _ Figure 1 is an explanatory diagram;
• _ Figure 2 is an explanatory diagram;
• _ Figure 3 is a diagram of an exemplary embodiment of the device according to the invention.

In FIGS. 1 to 3, the same references designate the same elements.

In Figure 1, the principle of embodiment of a reflector 1 according to the invention is illustrated. In Figure 1 we can see an orthonormal reference frame 2 O, x , y , z . The axis of z is the axis of propagation of electromagnetic energy. The origin O of the reference corresponds for example to the opening of a primary source. The primary source not shown is for example a rectangular monopolarization horn. The opening of the primary source is the seat of uniform illumination in polarization, the electric field being, for example, parallel to the axis Ox. The field radiated in space by such a source is at any point parallel to the axis Ox. On a sphere 3, of center O, the lines of electric fields are circles 4 included in horizontal planes. In Figure 1, there is shown in solid lines the part 41 of the circle 4 corresponding to an actual illumination; this corresponds to the directivity of the radiation sources.

In FIG. 1, a straight line 5 passes through the origin O and through a point 42 of the part 41 of the circle 4. Let us note α the angle formed between the line 5 and the axis Oy. By making describe all the part 41 at point 42 of circle 4 we obtain a portion of cone with axis Oy. The intersection 6 of this cone with the reflector 1 corresponds to the metallizations to be carried out on the reflector 1 to make it perfectly reflecting radiation of horizontal polarization, the electric field parallel to Ox.

The metallizations are carried out successively for a series of horizontal circles 4 belonging to the sphere 3. The metallizations are for example wires of round section. In an alternative embodiment of the device according to the invention, the wires are included in a dielectric material. The diameter of the wires, their spacing and the thickness of the dielectric are chosen in a manner known to those skilled in the art. This choice is the same as for the known type of reflector with parallel metallic wires.

In another alternative embodiment of the device according to the invention, metallic surfaces are produced by depositing metallic layers on a dielectric. This realization implements the same techniques as those of the realization of printed circuits.

For clarity of the figure only one metal surface is illustrated.

In FIG. 2, one can see an explanatory diagram making it possible to determine the lines of current induced in the auxiliary reflector 1. In the example illustrated the auxiliary reflector 1 is a hyperbolic reflector. The current lines obtained on the reflector 1 are sections of this reflector by the apex cones O resting on the circles C. In the example illustrated in FIG. 1, these cones are cones of revolution whose apex is located at the focal point O of the hyperboloid of revolution 1. The lines 6 of current induced in the auxiliary reflector 1 are the planar sections of this reflector. The cone of revolutions of vertex O, of axis Oy and of half-angle at vertex α has the equation: $$\text{(x² + z²) cos²α - y²sin²α = 0}$$

où a désigne le demi-axe, la méridienne du réflecteur auxiliaire 1, c la demi-distance focale de l'hyperboloïde 1 et

The equation for hyperbolic auxiliary reflector 1 is: $$\text{b² (zc) ² - a² (x² + y²) - a²b² = 0}$$

where a denotes the half-axis, the meridian of the auxiliary reflector 1, c the half-focal length of the hyperboloid 1 and

By eliminating x between equation (1) and (2) we get: $$\text{ay ± cos α (cz-b²) = 0}$$

$$\text{z = b²/c}$$

Equation (3) is the equation of a linear bundle of planes parameterized by α passing through the straight line 73 fixed with equations: $$\text{y = 0}$$

$$\text{z = b² / c}$$

Let 11 be the hyperboloid of revolution corresponding to the auxiliary reflector 1. Let 7 and 70 be the asymptotes with the hyperboloid of revolution 11 included in the plane of equation x = 0. The point 8 intersection of the lines 7 and 70 is the center of symmetry of the hyperboloid of revolution 11. Let J be the projection of the focal point 0 of the hyperboloid of revolution 11 on the line 70. I is the projection of the point J on the axis Oz and the intersection of the line 73 with the equation plane x = 0. The current lines 6 on the auxiliary reflector 1 are defined as the intersections of the reflector 1 with the equation planes (3). By replacing the surface of the auxiliary reflector 1 with a layer of metallic wires which follows the current lines 6, a device is made which is strictly equivalent to the auxiliary reflector 1 for the radiation having induced the current line 6.

In FIG. 3, an exemplary embodiment of a radar antenna according to the invention can be seen. The antenna of FIG. 3 comprises a first source 9 and a second source 15 of electromagnetic radiation. The sources 9 and 15 are for example horns with rectangular opening. Sources 9 and 15 work in orthogonal polarizations between them. The antenna also includes an auxiliary reflector 1 and a main reflector 74.

Source 9 corresponds to the main channel of the antenna illustrated. For the source 9 the antenna is of the Cassegrain type, the waves 19 emitted for example in horizontal polarization are first of all reflected on the auxiliary reflector 1 then on the main reflector 74. The replacement of a fully metallized reflector by the reflector auxiliary 1 according to the invention in no way disturbs the operation of the channel corresponding to the source 9. The reflector 1 is perfectly reflective for the waves emitted by the source 9.

The radiation source 15 corresponds to an additional channel added to the antenna. For this source, the antenna behaves like a monoreflective antenna. The radiation 115 emitted by the source 15, for example in vertical polarization, passes through the auxiliary reflector 1 before being reflected by the main mirror 74. The auxiliary reflector 1 according to the invention does not constitute an obstacle and does not present any shadow effect for the radiation emitted by the source 15.

It should be noted that the optimization of the auxiliary reflector 1 for the channel corresponding to the source 9 does not make it perfectly transparent for the channel corresponding to the source 15. However, this is not very troublesome for an auxiliary channel.

The invention applies mainly to radars with several channels.

Claims (9)

1. A microwave reflector (1) including metallic surfaces (6) and non-metallic surfaces, characterized in that it is per­fectly reflecting for waves (19) of a first polarity and par­tially transparent for waves (115) of a second polarity dis­posed orthogonally to the first polarity, the metallic sur­faces being located in the intersections of the surface of the reflector (1) with a bundle of straight lines (5) passing through a focus 0 of said reflector and through a horizontal circle 4 included in a sphere (3) having a center at 0, in such a manner that said metallic surfaces (6) are located in the intersection of the surface of the reflector (1) and of the planes of the following equations: $$\text{ay ± cos α (cz-b₂) = 0}$$

wherein y, z are the coordinates arising from the reference system (2), O, x, y, z, a is the distance between the summit of the reflector (1) and the intersection of the asymptotes (7, 70) to the reflector (1), c is half of the focal distance of the reflector (1),

and α is a parameter.

2. A reflector according to claim 1, characterized in that said reflector (1) has a hyperbolic shape of revolution.

3. A reflector (1) according to claim 1 or 2, characterized in that the metallic surfaces (6) include metal wires following the lines of current induced into the reflector (1).

4. A reflector (1) according to claim 1 or 2, characterized in that the metallic surfaces (6) include metallizations deposit­ed on a dielectric support.

5. A radar antenna, characterized in that it comprises a re­flector (1) in accordance with any one of the preceding claims.

6. An antenna according to claim 5, characterized in that it comprises two rectangular horns (9, 15) operating in the cross polarisation mode.

7. An antenna according to claim 5 or 6, characterized in that said antenna is a Cassegrain antenna for one polarisation and a monoreflecting antenna for the second polarisation.

8. A method of implementation of the microwave reflector (19, 115) which has reflecting properties for a first polarisation and transparent properties for a second polarisation, charac­terized in that metallic surfaces are formed on a rotary sym­metrical hyperboloid (11) in the intersection of said hyper­boloid with the planes corresponding to the following equa­tion: $$\text{ay ± cos α (cz-b₂) = 0}$$

wherein y, z are the coordinates arising from the reference system (2) O, x, y, z, a is the distance between the summit of the reflector (1) and the intersection of the asymptotes (7, 70) to the reflector (1), c is half of the focal distance of the reflector (1),

and α is a parameter.

9. A method according to claim 8, characterized in that the metallic surfaces (8) are made of metal wires.

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