EP0978898A1 - Scanning offset reflector antenna with movable feed, in particular for the reception of multiple TV satellites, and operating method therefor - Google Patents

Abstract

The swept head antenna has an offset reflector (1) with a head receiver (2) receiving signals from a number of satellites. Movement between different satellites is achieved by movement of the head receiver in a perpendicular plane to the polar reflecting axis.

Description

The present invention relates to a scanning antenna receiving the type comprising a fixed eccentric parabolic reflector, in particular for the reception of several television satellites.

The subject of the invention is also a method of implementing such a antenna.

Re-orientable antennas are interesting, especially in applications for reception of television programs relayed by satellites geostationaries placed on different orbital positions.

This reception can be carried out, in particular, using antennas at eccentric parabolic reflector (or "offset" according to English terminology commonly used) in different ways: re-orientation of the complete antenna (reception head and reflector), switching between several reception heads offset, mechanical scanning of a single receiving head, electronic scanning a network antenna, etc.

It is clear that there is a need for individuals to be able to receive using a single antenna the television broadcasts relayed by geostationary satellites such as "EUTELSAT", "ASTRA", "TELECOM 2A" and "2B", "INTELSAT", as regards Western Europe.

In particular, with the introduction of Isotropic Power satellites High Effective Radius (EIRP typically greater than 50 dBW), as "ASTRA" and the "EUTELSAT - Hotbird" series, the European market is over 10 million users. Inexpensive "kits", including a fixed antenna and a decoder, are offered at around 1500 FF. However, although a market considerable global potential exists, there are no re-orientable antennas, domestic use, capable of receiving geostationary satellites distributed over an arc of 10 to 15 degrees.

In the prior art, a wide variety of configurations have been studied mechanical to meet the above technical requirements. Systems antennas use a conventional reception head of the horn type. Typically, the antenna structure must fit in a 90 cm cube side to meet certain building regulations.

The Applicant has conducted studies which show that the systems motorized polar mount antennas, meeting technical requirements mentioned above, entail a cost 1.5 to 2 times higher than the price of an antenna conventional static disc from 70 to 90 cm. These studies also conclude that this type of antenna will remain the preferred approach of isolated users, compared to a system adopting the reception head scanning, as long as the additional cost of eliminating the degradation of antenna performance will remain important and that the switching (or "zapping") between satellites will remain very slow.

As non-exhaustive examples, we can briefly indicate some antenna characteristics according to several solutions of the known art.

A first example of antenna (called solution "A", below) is constituted by the case of a motorized reflector antenna. This type of antenna includes generally a 70 to 90 cm disc acting as a simple eccentric reflector, pole mount, connected to a single receiving head and a low-power converter noise ("LNB" or "Low Noise Block downconverter", according to the Anglo-Saxon name commonly used). The polarization of the converter output signal is adjustable. The receiving head converts electromagnetic energy into signals usually transmitted by a coaxial cable. Typically, characteristics of such an antenna, compared to those of a fixed antenna, stand out of TABLE I placed at the end of this description.

A second example (called solution "B" below) consists of the case of an antenna with a motorized reception head. We choose again simple eccentric reflector connected to a single receiving head, provided with a low noise converter. In this case the scanning is accomplished by moving the reception head linearly in the plane of the satellites, with an amplitude of movement of about 80 mm. The reflector remains static. We only use one channel and an adjustable polarization. Such an antenna is described, by way of example, in the book by T. A. Milligan: "MODERN ANTENNA DESIGN", McGrawhill, 1985. The movement mechanism of the receiving head is more particularly described in European patent application EP-A1-0 655 796. Typically the characteristics of such an antenna, compared to those of a fixed antenna, stand out of TABLE II placed at the end of this description.

A third example (called solution "C" below) consists of the case of an antenna with multiple reception heads. It includes a reflector single offset with two or more receiving heads, each separate from the next at least 6 cm. Each head is connected by a coaxial cable to a channel single, with selectable polarization. One of the outputs is selected by switching of intermediate frequency in the decoder. Characteristics typically of such an antenna, compared to those of a fixed antenna, emerge from the TABLE III placed at the end of this description.

A fourth example (called solution "D" below) is constituted by the case of an antenna provided with a secondary reflector. The configuration of a antenna of this type is similar to that corresponding to solution number 2 above. Only the scanning system is different. Such an antenna is suitable especially for PLURAL EUTELSAT. The scan is obtained by doing rotate the secondary reflector, made up of a disc, which moves the feed image. As is to be expected, this antenna configuration limits scanning at about one beamwidth. It follows that only 6 ° of orbit geostationary are covered, three satellites, with performance (typically around a second). However, switching between satellites ("zapping") can be very fast since the secondary reflector is weak dimensions compared to the main reflector, therefore also light in weight and low inertia. The additional costs and prices are not precisely known, because this type antenna is not widely distributed in the market. Typically, characteristics of such an antenna appear from TABLE IV placed at the end of the present description:

As will be shown below, none of these solutions satisfies simultaneously all the technical and financial requirements set out previously.

We also proposed the theoretical study of a reflector antenna symmetrical parabolic dish having a single movable receiving head. Movement of this head is characterized in that it takes place on a sphere passing through the point focuser, the center of which is located near the top of the reflector symmetrical parabolic above. This type of antenna should allow gain maximum and minimum beamwidth. This study is disclosed by the article by H. A. Whale and E. Putz: "Beam Steering Using Feed Displacement with a Paraboloïdal Reflector ", published in" IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION ", Vol. AP-34, N ° 11, November 1986, pages 1368-1372.

However, if this solution seems attractive at first glance, we can imagine easily that the command and control of the movement of a receiving head on a sphere are complex operations, all the more so as the orientation of the head, cannot be any moment. Besides, this theoretical article does not bring no solution as to the means allowing to obtain practically the movement prescribed.

On the contrary, the invention sets itself the goal of an antenna system compact which, for a low additional cost (typically less than 50%) and a low additional complexity, allows rapid re-orientation of the direction of pointing, while maintaining sufficient reception quality for an orbit arc important geostationary. By rapidity, in the context of the invention, is meant a switching ("zapping") between satellites carried out on the order of one second. From this fact, it allows the reception of television broadcasts relayed by the main geostationary television satellites.

To do this, the invention takes advantage of certain characteristics techniques related to reception on the ground of the aforementioned emissions: polar coordinates, beam polarization, etc.

The subject of the invention is therefore a scanning antenna for receiving the type comprising a fixed eccentric parabolic reflector, the reflector being intended for capture the beams of polarized electromagnetic waves, emitted by at least two satellites in geostationary orbit, located in locations distinct from an arc of said orbit, and to focus these captured beams at focal points, the reflector being oriented towards satellites so that its polar axis is perpendicular in the plane of the geostationary orbit and parallel to the axis of the earth, the antenna further comprising at least one receiving head so that it is arranged in proximity relationship with said focal points so as to receive the energy carried by the focused beams and converting it into signals electric, characterized in that said receiving head, having a diagram of radiation having an axis of maximum sensitivity, this axis being oriented substantially towards the center (C) of the reflector, and in that this axis is in a plane perpendicular to the polar axis reflected by the reflective mirror effect and which includes the center (C).

Preferably, the reception antenna according to the invention has only one receiving head which is movable and which is arranged to describe a segment a circle around said center (C), in said plane perpendicular to the polar axis reflexive.

According to another characteristic of the invention, the reflector (1) is oriented in space so that its eccentricity plane (XZ) is aligned with one polarization modes of the received beam.

In another version of the invention, the antenna has a single head mobile reception and is willing to describe a straight line which is substantially in the plane of the images of the satellites of the reflector and in said plane perpendicular to the reflected polar axis.

According to another embodiment, the antenna has several heads of fixed reception, these heads are contiguous and arranged in a segment of a circle around the center (C), the circle lying in said plane perpendicular to the axis reflective fleece.

In another version of this embodiment, the antenna has several fixed reception heads, these heads are contiguous and arranged in a straight line which is substantially in the plane of the images of the satellites of the reflector, this line lying in said plane perpendicular to the reflected polar axis.

• une phase préliminaire consistant à orienter dans l'espace ledit réflecteur par rapport à un axe polaire parallèle à l'axe de rotation du globe terrestre et à l'axe de ladite orbite géostationnaire, de manière à ce qu'il soit pointé vers un premier satellite localisé sur ledit arc de l'orbite géostationnaire pour une réception optimale de ce satellite, cette orientation comprenant l'alignement du plan d'excentricité du réflecteur et de l'axe de polarisation vertical du faisceau reçu, en ce que, ladite tête de réception ayant un diagramme de rayonnement présentant un axe de sensibilité maximale, cet axe coupe la surface du réflecteur parabolique en son centre et en ce que la tête soit placée dans le plan d'excentricité du réflecteur en une première position de proximité desdits points focaux pour la réception de l'énergie véhiculée par le premier satellite, dite position de repos ;
• et au moins une étape de balayage dudit arc de l'orbite géostationnaire pour réaliser la commutation entre ce premier satellite et un deuxième satellite comprenant le déplacement de la tête de réception à partir de la dite position de repos vers une deuxième position, le déplacement s'effectuant dans un plan perpendiculaire à un axe, dit polaire réfléchi, représentant la réflexion dudit axe polaire par rapport au plan tangent à la surface du réflecteur parabolique en son centre.

The invention also relates to a method of implementing a reception antenna of the type comprising a fixed eccentric parabolic reflector, the reflector being intended to pick up the beams of polarized electromagnetic waves, emitted by at least two satellites in orbit geostationary, located in separate places of an arc of said orbit, and to focus these captured beams at focal points, the antenna further comprising at least one reception head so that it is arranged in relation to proximity to said focal points so as to receive the energy conveyed by the focused beams and to convert it into electrical signals, characterized in that it comprises:

• a preliminary phase consisting in orienting in space said reflector with respect to a polar axis parallel to the axis of rotation of the terrestrial globe and to the axis of said geostationary orbit, so that it is pointed towards a first satellite located on said arc of the geostationary orbit for optimal reception of this satellite, this orientation comprising the alignment of the eccentricity plane of the reflector and the axis of vertical polarization of the received beam, in that said head of reception having a radiation diagram having an axis of maximum sensitivity, this axis cuts the surface of the parabolic reflector in its center and in that the head is placed in the eccentric plane of the reflector in a first position of proximity of said focal points for receiving the energy carried by the first satellite, called the rest position;
• and at least one step of scanning said arc of the geostationary orbit to carry out the switching between this first satellite and a second satellite comprising the displacement of the reception head from said position of rest towards a second position, the displacement s 'effecting in a plane perpendicular to an axis, said reflected pole, representing the reflection of said pole axis with respect to the plane tangent to the surface of the parabolic reflector at its center.

• La figure 1 est un diagramme représentant les coordonnées locales d'une station de réception terrestre par rapport à six satellites particuliers ;
• La figure 2 est un diagramme représentant les coordonnées locales polaires d'une station de réception terrestre et de six satellites particuliers ;
• La figure 3 est une construction théorique représentant, dans l'espace, une antenne à réflecteur parabolique excentré 1 conforme à l'invention ;
• La figure 4 est un diagramme relatif aux écarts (en mm) pendant le déplacement de la tête de réception de l'antenne dans un plan focal, lors du balayage des différents satellites, par rapport à la focalisation nominale ;
• Les figures 5 et 6 illustre un exemple de réalisation pratique d'une antenne selon l'invention ;
• La figure 7 est un diagramme illustrant les résultats obtenus pour six pointages de faisceaux, de satellites par une antenne selon l'invention.

The invention will be better understood and other characteristics and advantages will appear on reading the description which follows with reference to the appended figures, and among which:

• FIG. 1 is a diagram representing the local coordinates of a terrestrial reception station with respect to six particular satellites;
• FIG. 2 is a diagram representing the local polar coordinates of a terrestrial reception station and six particular satellites;
• FIG. 3 is a theoretical construction representing, in space, an antenna with an eccentric parabolic reflector 1 according to the invention;
• FIG. 4 is a diagram relating to the deviations (in mm) during the displacement of the reception head of the antenna in a focal plane, during the scanning of the different satellites, with respect to the nominal focusing;
• Figures 5 and 6 illustrates a practical embodiment of an antenna according to the invention;
• FIG. 7 is a diagram illustrating the results obtained for six pointing of beams, of satellites by an antenna according to the invention.

Before describing an antenna embodiment in accordance with the invention, we will describe an example of a real environment that will allow highlight, in an encrypted manner, the most reception parameters significant of the emissions received from the main geostationary satellites. This example could not of course limit the scope of the invention in anything.

As a first step, an analysis of the antenna pointing requirements, i.e. its orientation in the space relative to the position of the different satellites to be received. In the example described, we consider the family of the following six satellites: the "ASTRA" satellite (below N ° 1), the satellites "EUTELSAT 2F3" (N ° 2), "HotBird" (N ° 3), "2F2" (N ° 4) and "2F4" (N ° 5) as well as the "SIRIUS" satellite (N ° 6). The geostationary arc has a amplitude ranging from 19.2 ° east (ASTRA) to 5 ° east (SIRIUS), for the place of the experiment, in this case the city of OEGSTGEEST in the Netherlands, which will called local station "Os" below.

TABLE V, arranged at the end of this description, indicates the terrestrial coordinates in degrees of this local station "Os", as well as the coordinates of four stations located at the four extreme cardinal points of the coverage area of these satellites, stations called "E" (east), "W" (west), "N" (north) and "S" (south). It can be seen from this TABLE V that the station "Os" is located substantially in the center of the satellite coverage area.

We have gathered in TABLE VI (also placed at the end of the present description) the coordinates of the six aforementioned satellites with respect to the local "Os" receiving station.

This table shows, for the local coordinates, a relatively large pointing excursion, whether for the azimuth, the site or the alignment in polarization. It is recalled that the electromagnetic wave beams emitted by the satellites are polarized and have vertical (main) and horizontal polarization axes. These coordinates have been plotted on FIG. 1. This is a diagram representing two curves: the curve C ₁ representative of the pairs of values "site angle - azimuth east" (represented by diamonds on the graph) for the different satellites (N ° 1 to 6) and the curve C ₂ representative of the pairs of values "polarization - azimuth east" (represented by solid stars on the graph) for these same satellites.

Even if you just reorient the antenna to scan a reduced arc, it is however necessary to carry out this reorientation according to the three axes of an orthonormal trihedron. This delicate operation requires organs of complex controls.

In a second step, the polar coordinates for the local station "Os" and the stations "" E "," W "," S "and" N ", were determined, with reference to a polar axis parallel to the axes of l geostationary orbit and the terrestrial globe. These coordinates are gathered in the graph of figure 2, which represents the "northern latitudes" (vertical axis) according to "east azimuths" (horizontal axis) .. For each of the six satellites and for each station ("Os", "E", "W", "S", "N"), we have plotted on the graph a point representing these coordinates: "site - azimuth is". The convention adopted on the graph Figure 2 is as follows: for each satellite, a white square for the station "Os", associated with the satellite name and its number (see TABLE IV), and black squares associated with the stations and satellite numbers ( for example E ₁ for station "E" and satellite N ° 1, that is to say the satellite "ASTRA"). We note, for all the statistics we have a negligible excursion with regard to the north latitude parameter. In other words, these stations are substantially distributed on straight lines parallel to the axis of longitudes East, whatever the satellites considered (for each station).

It follows that the alignment of the "vertical" reference polarization of the beam received (i.e. perpendicular to the plane of the orbit) with the plane containing the polar and pointing axes of the station on the satellite also remains substantially constant. Also, if we now consider the four stations extremes of satellite coverage, "E", "W", "S" and "N", variations in latitude are equal to ± 1 °. It is therefore only necessary to carry out very low adjustments.

This great simplification in the pointing process is exploited by solution "A" of known art. However, the fact remains that the complete antenna structure must be re-oriented to switch between two satellites. It is indeed necessary to carry out a pointing movement along the edge of a cone having for axis the local polar axis. This results in a very slow scan as shown in TABLE I (typical scanning time equal to 10 seconds). In in addition, the motorization member must be very powerful, because it must take into account the significant inertia of the reflector and its wind resistance.

The third step is to analyze the scanning possibilities of the beam in a "parallel plane", that is to say in a plane perpendicular to the axis polar above, without moving the reflector 1, but only the receiving head.

FIG. 3 is a theoretical construction representing, in space, an antenna with an eccentric parabolic reflector 1. This figure shows 3 the main parameters to take into account: axes, planes, angles, etc., parameters which will be detailed below.

In particular, the line of sight A _sat of a particular satellite S _{sat is shown} on which the reflector 1 of the antenna is momentarily oriented. This axis A _sat passes through the center C of the reflector 1. Due to the remoteness of the satellite S _sat (in geostationary orbit), that is to say approximately 39,000 km (see TABLE VI), the beam portion f picked up by the dish 1 is substantially cylindrical. The rays composing this beam f are reflected in a focused beam f _foc and converge at the focal point P _foc . The focal length of the dish is defined equal to d _jib . The aperture angle of the focused beam f _foc is defined equal to 2ψ ₀ . Also shown in this figure 3 is the polar axis A _P or "north" of the antenna mount, the tangent plane P _T at the top of the reflector 1 and the direction of the "reflected polar axis N *", by mirror effect, on the P _T plane. The polar axis A _P is defined as being the common axis of rotation of the earth and symmetry of the orbital plane of the satellites.

At the focal point P _foc , an orthonormal axis XYZ ₁ has been represented, oriented in space so that the plane YZ ₁ is orthogonal to the polar axis N *.

To fix the ideas, we choose as reference the satellite "EUTELSAT-Hot Bird" (N ° 3), located substantially in the center of the geostationary arc. It is assumed that the reflector 1 is a conventional type reflector constituted by a portion of a dish 80 cm in diameter. TABLE VII, placed at the end of this description, gathers the main data (in relative values) characterizing the other satellites by reference to the above-mentioned "EUTELSAT-Hot Bird" satellite (N ° 4): latitude (in °), longitude ( in °), scanning in beamwidths, displacements, in the meridian plane P _M (XZ ₁ ), along X (in mm), and in the parallel plane, along Y (in mm), better focusing and radial displacement along Z (in mm). This axis of symmetry Z is coincident with that of the focused beam f _foc .

We could see that with a sweep of amplitude less than three beam openings in the plane of symmetry Y-Z perpendicular to the plane eccentricity X-Z ("off-set"), loss of gain, cross polarization and the amplitude of the side lobes remain acceptable for eccentric reflectors with an F / D ratio (focal length over reflector diameter) greater than or equal to 0.7; this provided that the radiation pattern of the receiving head remains centered on reflector 1.

It can be seen that this is not the case with the above-mentioned solutions "B" or "C" of the known art. They do not maintain the rotation around the pointing direction of satellite Z, required to align the offset plane XY with the vertical "reference" polarization. The reception head 2 is also not kept pointed at the center C of the reflector 1. For solution "B", the reception head is linearly movable (scanning) in the horizontal plane XZ ₁ . For solution "C", the image of this head performs a rotational movement around the axis of the secondary reflector.

The fourth step consists in determining an antenna geometry (reflector 1 and movable support of the reception head) and means allowing the head to move in space around an axis of rotation A _rot passing through the center C of the reflector 1 and so as to scan the beam f inside a cone, at constant polar latitude, in order to be able to pick up the different satellites.

• focalisation du faisceau f du fait de sa forme parabolique ;
• et réflexion de l'orbite et des images de satellites avec les angles d'excentricité et de balayage de faisceau.

In a simplified way, the eccentric reflector accomplishes two functions:

• focusing of the beam f due to its parabolic shape;
• and reflection of orbit and satellite images with eccentricity and beam scanning angles.

It should be clear that, in a dual manner, the reception head "sees" the reflected orbit and a "reflected polar axis N *" (by mirror effect with respect to the tangent plane P _T at the center C of the reflector 1).

According to the most important characteristic of the invention, the reception head 2 performs a rotation scan around an axis A _rot passing through the reflector 1 at its center C and parallel to the "reflected polar axis" N *.

In practice, the receiving head 2 is supported by an arm, symbolized, in FIG. 3, by a line segment 3 intersecting the axis A _rot and passing through the focal point P _foc . More precisely, this straight line makes an angle equal to ψ ₀ (half-opening angle) with the axis of symmetry A _jib of the focused beam f _jib .

a/ la polarisation verticale de référence et le plan de symétrie d'excentricité du réflecteur 1 sont alignés ;

b/ l'illumination (diagramme de rayonnement) de la tête de réception reste centrée sur le réflecteur 1 ;

c/ et le balayage de faisceau est réalisable à "latitude constante" dans le cône, d'axe polaire A_P, traversant l'arc géostationnaire.

Naturally, in order to use the antenna, it is first of all necessary to carry out initial adjustments aimed at orienting the reflector 1 properly with respect to all of the satellites that it is desired to receive. Advantageously, the initial adjustment is carried out on the satellite closest to the center of the geostationary arc targeted, namely the "EUTELSAT-Hot Bird" satellite (No. 3) in the example described. Once an optimal initial pointing has been carried out on this satellite, the reflector is rotated around the pointing axis Z so as to align the planes of vertical polarization of the beam f and of eccentricity symmetry of the reflector. The three conditions required to obtain an optimized scan are then met, namely:

a / the vertical reference polarization and the eccentricity plane of symmetry of the reflector 1 are aligned;

b / the illumination (radiation diagram) of the reception head remains centered on the reflector 1;

c / and the beam scanning is possible at "constant latitude" in the cone, of polar axis A _P , crossing the geostationary arc.

In terms of quadratic phase errors and minimal aberrations, we can show that the best displacement surface for the head, in terms of concerns an eccentric reflector, is a parabola with the same axis of symmetry as the reflector 1, with focal length equal to half that of this reflector and passing by the focal point.

The scanning movement carried out by the receiving head 2 in the case of the invention, around the axis A _rot , parallel to the axis N *, is actually carried out on a cylinder "oscillating" the parabola of the reflector 1 (Z axis).

Although, according to the configuration of the invention, the losses are very weak, to optimize focusing at the extreme limits of the scan of the receiving head 2, according to a preferred variant of the invention, it is possible to move this radially (a few millimeters) towards the reflector 1.

FIG. 4 is a diagram relating to the deviations (in mm) during the displacement of the reception head 2 in the focal plane, when scanning the different satellites, compared to the nominal focus. We have represented on the same diagram three curves: the displacement along X, in the meridian plane, the radial displacement Z and the best focus along Z. According to the curves, the axis vertical represents displacements along X or Z and the horizontal axis of displacements along Y, in the parallel plane.

The scanning losses can be further reduced, in particular if this scanning exceeds an amplitude of ± 3 beam openings. In this case, the section of the reflective surface of the reflector 1 in a parallel plane XZ ₁ is modified so as to approach a circular section of radius equal to twice the focal distance of the parabola.

Finally, by appropriately choosing the different eccentricity angles, it is possible to modify the angle of the tangent plane P _T with the pointing direction of satellite A _sat and the polar axis A _P. The reflected polar axis N * will rotate symmetrically. For a given latitude of the receiving station, and the corresponding angle of inclination of the polar axis A _P relative to the local vertical, it becomes possible to make vertical, either the reflector 1, or the axis of rotation of the receiving head 2, or else to place it in an area very close to the edge of the disc forming the reflector 1, which allows a very compact overall antenna structure.

We will now describe a practical example of a antenna incorporating the teachings of the invention. The antenna was made in modifying the configuration of a conventional 80 cm commercial antenna diameter, with eccentric reflector. More specifically, the support of the receiving head has has been fitted with a movement mechanism for this head, as will be shown below. The head consists of a horn illuminator, 54 mm in diameter. The nominal gain is 37.9 dBi, beamwidth 2.4 °, eccentricity ratio ("off-set"): F / D = 0.8 and the eccentricity angle of 44 °. The receiving head 2 is directed towards the top of the parabolic reflector 1.

Figures 5 and 6 illustrate this exemplary embodiment. More particularly, Figure 6 illustrates a detail of the arm drive mechanism support 3 of the receiving head 2 (end 31).

The aforementioned modifications are moreover very limited. It provides the base (end 30) of the arm 3, a drive mechanism 4 allowing the rotation of the arm 3 about the axis of rotation A _rot . This axis, according to one of the main characteristics of the invention passes through the center C of the parabola 10 of the reflector 1. It is also parallel to the axis N * ("reflected polar axis": see FIG. 3).

First of all, conventionally, the reflector 1 comprises, on its rear face, fixing means 5, allowing adjustment of the orientation of the antenna in elevation and in azimuth. A first sub-assembly, 11, is integral with the parabola 10, or at least fixed on its rear by any suitable conventional means, for example a set of "screws - nuts", 110 to 112. A second sub-assembly, 51, is fixed to a vertical mast 50 or a similar member, for example using screwable flanges and nuts 510. This second sub-assembly 51 has the general shape of a "U" whose side walls , 51a and 51b, are perforated, each by a pair of grooves in an arc, three of which are visible in FIG. 6: 511 and 512, on the wall 51a, and 520, on the wall 51b. These grooves have graduations, 5110 and 5120, the usefulness of which will be explained below. The aforementioned first part, 11, is also provided with side walls, 113 and 14, cooperating with the walls 51a and 51b, respectively. Using a set of screws, nuts or butterfly screws, 513 to 515, the reflector 1 can be fixed; more precisely the first fixing means 11, on the second fixing means 51. In addition, the aforementioned grooves, 511, 512 and 520, make it possible to adjust the orientation of the antenna in a plane parallel to the axis of the mast 50. To do this, the abovementioned gradations, 5110 and 5120 can be used. Advantageously, a first coarse adjustment is made and the nut 514 is tightened, then a fine adjustment and the butterfly screw 513 is tightened. Finally, the flanges 510 allow the antenna to be oriented by rotation around the mast 50, that is to say, in the example described, relative to the local vertical (axis A _V ). In itself, these means for fixing and adjusting the orientation of the antenna are well known, in particular in the case of fixed antennas.

In the context of the example described, the reflector 1 is oriented very precisely to obtain optimal reception of the "EUTELSAT-Hot Bird" satellite, in azimuth (13 ° East), then in the plane parallel to the axis of symmetry A _V of the mast 50 (vertical, in the example described). It is of course possible to use a compass and / or instruments for measuring the electromagnetic field, in the range of frequencies emitted.

So that the receiving head 2 can sweep the entire arc geostationary desired, that is to say so that the antenna can pick up the different aforementioned satellites, the first fixing means 11 are provided, that is to say the fastening means integral with the dish 10, means 4 for driving the arm 3.

It is first of all necessary to provide an axis of rotation 6 of the arm 3, support of the head 2, whose axis of symmetry A _rot passes through the center C of the parabola portion 10 and is parallel to the axis reflective polar N * (see Figure 3). The first condition is obtained by construction. The second condition is satisfied by initial adjustment of the orientation of the antenna, according to the process recalled above.

Next, provision is made, in a zone 30 of the arm 3, close to the opposite end with the fixing means, 11 - 51, for means of rotation about the axis 6. It is possible, for example , use a tensioning cable 410, under sheath 41, causing rotation of the arm 3 in a first direction (arrow f ₁ , to the left in FIG. 6) when a tensile force is exerted on it. A retaining tongue 115 is provided for a first end 411 of the sheath 41, placed in abutment on this tongue. The latter is advantageously made integral with the first fixing means 11, themselves integral with the parabola 10. Naturally, the arm 3 cannot be left "floating". Additional means are provided for obtaining a rotation in the opposite direction (arrow f ₂ , to the right in FIG. 6) when the tensile force exerted on the cable 411 is released. To do this, a spring can be used with pre-tensioned flange 42, fixed, on the one hand, to the end 30 of the arm 3 and, on the other hand, to one of the attachment points of the parabola, in this case the nut 110 in the example described. At all times the spring 42 exerts on the end 30 of the arm 3 a restoring force (in the direction of the arrow f ₂ ) which balances the tensile force exerted on the cable. It therefore suffices to block the cable so that the arm 3 remains locked in a desired desired position in space and that the antenna "points" at the desired satellite. It should be understood that the dish 1 itself remains fixed. It is the receiving head 2 which describes an arc of a circle of center coincident with the axis of rotation 6, that is to say around the axis of symmetry A _rot passing through C and parallel to the axis reflective polar N * (see Figure 3). The orientation of the head 2 (fixed relative to the end 31 of the arm 3) is such that the axis of symmetry of its radiation diagram (or at least, the axis of greatest sensitivity if the diagram does not does not have a symmetrical section) passes through the center C of the reflector 1. The movement of the head 2 necessary to scan all the satellites being limited, the amplitude of the movement of the end 30 of the arm 3 is even more limited, because this end is very close to the axis 6. It follows that the space to be provided between the two walls 51a and 51b is compatible with conventional antenna fixing systems. In the rest state, that is to say in the initial state, the support arm 3 is preferably in the intermediate position, halfway between the two walls 51a and 51b. The arm 3 is left in this so-called rest position during the initial pointing of the satellite located closest to the center of the geostationary arc to be explored, in this case the above-mentioned "EUTELSAT-Hot Bird" satellite. During subsequent scans, the arm 3 oscillates (in rotation about the axis of symmetry A _rot ) on either side of this rest position, so that the antenna can pick up the other satellites, including satellites in extreme positions, that is to say the "ASTRA" and "SIRIUS" satellites in the example described.

In an economical version, the means of pulling the cable 410, under the unique reference 42 (FIG. 5), are purely manual: notch lever, screw wheel, etc. In a preferred variant of the invention, these traction means 42 are constituted by a “gear - rotary motor” assembly, a linear motor or a stepping motor. The latter can be located near the end 30 of the arm 3 or, on the contrary, placed at a distance. In the case of motors, it is no longer necessary to provide a return spring. These are advantageously provided with a drive piston at the end of the arm, which can move forward or backward (arrows f ₁ and f ₂ ). The control circuits of this motor (not shown) can be autonomous or, advantageously, be integrated in the decoder (not shown) receiving the signal antenna carried by a coaxial cable 20. In most cases, these are programmable circuits allowing automatic positioning on the various satellites of the arc to be explored These control circuits are known per se and there is no need to describe them p They can also be associated, in a manner also known, with feedback circuits allowing fine control of the best reception, for each satellite.

Such an antenna has been tested in real reception conditions. The experimental results confirm the theoretical predictions. Figure 7 is a diagram illustrating the results obtained for six beam pointing, F _P1 to F _P6 . The directivity remains greater than 37.5 dBi for the main vertical polarization and remains less than 15 dBi for the crossed polarizations. These two categories of curves have been represented on the same graph. These characteristics allow overall "signal / interference" ratios greater than 20 dB.

As indicated, it is still possible to improve reception by adjusting the radial position of the reception head 2 so that it best merges with the focal point P _foc (FIG. 3). To do this, in a variant not shown, it is possible to provide the head with motorized means allowing a slight radial movement (FIG. 3: Z axis) thereof relative to a rest position. These means are arranged on the end 31 of the support arm 3 and make it possible to compensate for focusing defects. If we refer again to FIG. 4, a linear translation of the reception head 2 of only a few millimeters is necessary (less than 10 mm at most for extreme satellites). One can use, for example a step motor. The control signals of this motor can be transmitted directly by the coaxial cable 20 in multiplexing with the output signals from the reception head 2.

We can estimate the additional cost involved in a motorization system simple, according to the invention, at around 650 FF. In addition, the windward catch of the moving structure (arm 3 and receiving head 2) is weak (twenty times lower than that of solution "A") and low inertia (15% compared to a structure according to solution "A"), it follows that the invention allows switching between satellites ("zapping") very fast.

On reading the above, it can easily be seen that the invention achieves the goals it has set for itself. In particular, it allows both simple and inexpensive antenna structure, compatible with consumer applications (indeed, it requires only few modifications compared to an antenna classic with fixed eccentric reflector) and allows rapid scanning of an arc important geostationary, on which are distributed the main satellites of television, while maintaining good performance, whatever the satellite point.

It should be clear, however, that the invention is not limited to only examples of embodiments precisely described, in particular with regard to FIGS. 3 to 6.

Indeed, although the structure described in relation to these figures or any similar structure is particularly interesting, we can apply also teachings of the invention to the antennas according to the known art, in particular in the case of the above-mentioned solutions "B" and "D".

With regard to solution "B" (linear scanning of the head of reception), it is however necessary to adopt special provisions. In indeed, if the scanning is carried out in a plane perpendicular to the axis N *, passing through the center C of the reflector, this leads to an eccentricity angle equal to zero. This value is not acceptable, as it would result, due to the particular configuration of drive mechanism subject to the request for above-mentioned European patent EP-A1-0 655 796, a blocking thereof.

To avoid this blockage, the eccentricity plane is made perpendicular to the "vertical reference polarization" plane of the satellite by rotating 90 ° from the complete antenna around the pointing axis. We place the mechanism drive, in accordance with the aforementioned patent application on the edge of the disc antenna reflector and the scanning is carried out in the eccentric plane, this plane being perpendicular to the axis N * and passing through the center of the reflector.

To comply with these requirements, it is naturally necessary make minor modifications to the reflector mounting system a fixed structure (mast or other), as well as on its initial adjustment system of the orientation of the antenna (pointing to one of the satellites of the arc to be scanned).

In the case of the above-mentioned solution "D" (antenna comprising a auxiliary reflector), the auxiliary reflector is scanned around an axis parallel to the axis N *.

The Applicant has been able to observe that, in both cases, the antenna scanning performance was improved.

The invention is also not limited to the reception of the six satellites explicitly described. It also applies equally well to the reception of other geostationary satellites, in Europe or in other regions of the terrestrial globe. KNOWN ART: SOLUTION N ° A Antenna diameter 70 cm Antenna gain 36.5 dBi Signal / interference ratio ≅ 20 dB Geostationary orbit scanning range ± 70 degrees Scan time > 10 seconds Estimated additional cost 1000 FF approx. Total price (price range) 2400 - 2700 FF KNOWN ART: SOLUTION N ° B Antenna diameter 80 cm Antenna gain <37.5 dBi Signal / interference ratio <20 dB Geostationary orbit scanning range ± 6 degrees Scan time 20 seconds Estimated additional cost 1550 FF Total price (price range) 2900 - 3300 FF KNOWN ART: SOLUTION N ° C Antenna diameter 80 cm Antenna gain ≅ 37.5 dBi Signal / interference ratio ≅ 20 dB Geostationary orbit scanning range 6 degrees Scan time <1 second Estimated additional cost 850 FF Total price (price range) 2200 - 2600 FF KNOWN ART: SOLUTION N ° D Antenna diameter 80 cm Antenna gain <37.5 dBi Signal / interference ratio <20 dB Geostationary orbit scanning range ± 3 degrees Scan time ≅ 5 seconds Station coordinates (in °) Station OEGSTGEEST E W NOT S North Latitude 52.14 50.00 50.00 60.00 40.00 East longitude 4.48 20.00 0.00 10.00 10.00 Satellite coordinates (in ° or km) SATELLITE ASTRA N ° 1 EUT-2F3 N ° 2 EUT-HB N ° 3 EUT-2F2 N ° 4 EUT-2F4 N ° 5 SIRIUS N ° 6 SITE 28.76 29.30 29.75 30.06 30.24 30.30 AZIMUTH -18.22 -14.52 -10.81 -7.05 -3.26 -0.60 POLARISAT. -11.16 -8.92 -6.67 -4.36 -2.02 0.00 Range km 38809 38755 38716 38689 38673 38669 Latit. polar -7.45 -7.46 -7.46 -7.47 -7.47 -7.47 Longi. polar -20.68 -17.16 -13.86 -10.56 -7.26 -5.05 Relative satellite data compared to EUT-HB (N ° 3) SATELLITE ASTRA N ° 1 EUT-2F3 N ° 2 EUT-HB N ° 3 EUT-2F2 N ° 4 EUT-2F4 N ° 5 SIRIUS N ° 6 Latitude 0.02 0.01 0.00 -0.01 -0.01 -0.01 Longitude -6.81 -3.30 0.00 3.30 6.61 8.81 Scan / beam -2.92 -1.41 0.00 1.41 2.83 3.77 Move. X -0.2 -0.1 0.0 0.1 0.1 0.1 Move. Y 81.2 39.3 0.0 -39.3 -78.7 -104.9 Meil. jib. Z 10.3 2.4 0.0 2.4 9.7 17.2 Pos. radial Z 5.1 1.2 0.0 1.2 4.8 8.5

Claims (10)

Scanning reception antenna of the type comprising a fixed eccentric parabolic reflector (1), the reflector (1) being intended to receive the beams (f) of polarized electromagnetic waves, emitted by at least two satellites (N ° 1 to N ° 6) in geostationary orbit, located in distinct locations of an arc of said orbit, and to focus these captured beams (f) at focal points (P _foc ), the reflector being oriented towards the satellites so that its polar axis is perpendicular to the plane of the geostationary orbit and parallel to the axis of the earth, the antenna further comprising at least one receiving head (2) so that it is arranged in proximity relationship with said focal points (P _foc ) so as to receive the energy conveyed by the focused beams (f _foc ) and to convert it into electrical signals, characterized in that each reception head (2) has a radiation diagram having u n axis of maximum sensitivity, this axis being oriented substantially towards the center (C) of the reflector, and in that this axis is in a plane perpendicular to the polar axis reflected (N *) by the mirror effect of the reflector and which includes the center (C). Antenna according to claim 1, having a single reception head, characterized in that this head is movable and is arranged to describe a segment of a circle around the center (C) in said plane perpendicular to the reflected polar axis. Antenna according to claim 1, having a single reception head, characterized in that this head is movable and is arranged to describe a straight line which is substantially in the plane of the images of the satellites of the reflector and in said plane perpendicular to the reflected polar axis. An antenna according to claim 1, having a plurality of reception heads fixed, characterized in that these heads are contiguous and arranged in a segment of circle around the center (C), the circle lying in said plane perpendicular to the reflected polar axis. An antenna according to claim 1, having a plurality of reception heads fixed, characterized in that the heads are contiguous and arranged in a line line which is substantially in the plane of the images of the satellites of the reflector, this line lying in said plane perpendicular to the reflected polar axis. Antenna according to claim 1, characterized in that the reflector (1) is oriented in space so that its eccentricity plane (XZ) is aligned with one of the polarization modes of the received beam. Method for implementing a reception antenna of the type comprising a mobile reception head (1) and a fixed eccentric parabolic reflector (1), the reflector (1) being intended to pick up the beams (f) of electromagnetic waves polarized, emitted by at least two satellites (N ° 1 to N ° 6) in geostationary orbit, located in distinct places of an arc of said orbit, and to focus these captured beams (f) in focal points (P _foc ), the antenna further comprising a support (3) of the reception head (2) so that it is arranged in proximity relation with said focal points (P _foc ) so as to receive the energy conveyed by focused beams (f _foc ) and converting it into electrical signals, characterized in that it comprises: a preliminary phase consisting in orienting in space at the focal points said reflector (1) with respect to a polar axis (A _P ) parallel to the axis of rotation of the terrestrial globe and of geostationary orbits, so that it either pointed to a first satellite (No. 3) located on said arc of the geostationary orbit for optimal reception of this satellite, this determined orientation of the eccentricity plane (XZ) of the reflector (1) and the plane of geostationary orbits (f), in that, said receiving head (2) having a radiation diagram having an axis of maximum sensitivity, this axis intersects the surface of the parabolic reflector (1) at its center and in that the head (2) is placed in the eccentricity plane (XZ) of the reflector (1) in a first position of proximity of said focal points (P _foc ) for the reception of the energy conveyed by the first satellite (No. 3), said position of rest; and at least one step of scanning said arc of the geostationary orbit to effect the switching between this first satellite (No. 3) and a second satellite comprising the displacement of the reception head (2) from said rest position towards a second position, the displacement taking place in a plane (YZ ₁ ) perpendicular to an axis, called reflected pole (N *), representing the reflection of said polar axis (A _P ) relative to the tangent plane (P _T ) at the surface of the parabolic reflector (1) at its center (C). Method according to Claim 7, characterized in that the said displacement during the scanning step is carried out by rotation around an axis (A _rot ) passing through the center (C) of the said parabolic reflector (1) and parallel to the said polar axis reflected (N *) Method according to claim 8, characterized in that during said displacement of said axis of maximum sensitivity of the reception head (2) traverses, at at all times, the parabolic reflector (1) at its center (C). Method according to any of claims 7 to 9, characterized in that the satellites being at least three in number (N ° 1 to N ° 6), the phase preliminary orientation in the space of the reflector (1) consists in pointing it on the satellite (No. 3) closest to the center of the arc of the geostationary orbit at sweep away.

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