EP2081258B1 - Secondary reflector of an antenna with double reflector - Google Patents

Abstract

The sub-reflector (2) has an end coupled to an end of a waveguide (3) by a junction, and another end with diameter larger than diameter of the junction. A convex reflective inner surface having a revolution axis is placed at the latter end of the sub-reflector. An outer surface connects the ends of the sub-reflector. A dielectric body extends between the ends of the sub-reflector, and is limited by the inner surface and the outer surface. The outer surface has a convex profile described by a specific polynomial equation of sixth degree. An independent claim is also included for a dual-reflector antenna comprising a primary reflector and a sub-reflector.

Description

The present invention relates to Radio Frequency (RF) antennas with dual reflectors. These antennas generally comprise a large-diameter concave primary reflector having a surface of revolution, and a convex smaller-diameter secondary reflector ("sub-reflector") located near the focus of the primary reflector. These antennas operate indifferently in transmitter mode or in receiver mode, corresponding to two opposite directions of RF wave propagation. In what follows, the description is given either in transmission mode or in reception mode of the antenna, according to which allows to better illustrate the described phenomena. It should be noted that all the reasonings apply to the antennas as well in reception as in emission.

The first antennas had only one reflector, most often parabolic. The end of the radiofrequency waveguide is at the focus of the reflector. The waveguide is inserted into an orifice on the axis of the reflector, and its end is bent 180 ° to face the reflector. The maximum half-angle of radiation at the folded end of the waveguide to illuminate the reflector is small, of the order of 70 °. The distance between the reflector and the end of the waveguide must be large enough to illuminate the entire surface of the reflector. For these shallow reflector antennas ("shallow reflector" in English), the F / D ratio is of the order of 0.36. In this report, F is the focal length of the reflector (distance between the top of the reflector and its focus) and D is the diameter of the reflector.

In these antennas, the value of the diameter D is determined by the central working frequency of the antenna. The lower the working frequency of the antenna (for example 7.1 GHz or 10 GHz) and the greater the diameter of the reflector is important at equivalent antenna gain: it is then necessary that the end of the waveguide is very far from the reflector to illuminate it well (emission mode), and the antenna becomes more cumbersome as the working frequency is low. For these shallow reflector antennas, it is essential to add an absorbing shield to minimize over-flow loss and improve radio performance.

In order to produce more compact systems, dual reflector antennas are used, in particular those referred to as Cassegrain type. The double reflectors comprise a concave primary reflector, frequently parabolic, and a convex secondary reflector having a much smaller diameter and placed in the vicinity of the focus on the same axis of revolution as the primary reflector. A secondary reflector of dual reflector antenna is for example described in the document GB-973,583 . The primary reflector is pierced at its top and the waveguide is inserted on the axis of the primary reflector. The end of the waveguide is no longer folded, but faces the secondary reflector. In transmission mode, the RF waves transmitted by the waveguide are reflected by the secondary reflector to the primary reflector.

It is possible to make secondary reflectors having a half-angle of illumination of the primary reflector much greater than 70 °. For example, an illumination half-angle of 105 ° can be used. In a double reflector antenna, the secondary reflector can thus be axially very close to the primary reflector. In practice, the secondary reflector may be located within the volume defined by the primary reflector which reduces the size of the antenna.

In these dual reflector antennas, the F / D ratio used is often less than or equal to 0.25. These antenna are said to deep reflector ("deep reflector" in English). An F / D ratio of the order of 0.25 corresponds, for the same value of the working center frequency D, to a shorter focal length than in the case where the F / D ratio is close to 0.36. The size of a double reflector antenna can therefore be smaller than that of a single reflector antenna by eliminating the absorbing screen which is no longer essential.

Although the dual reflector antennas are well suited to the production of compact antennas, for example by using double reflectors whose F / D ratio is close to 0.2, it may be preferable to use F / D values other than to optimize also other characteristics that congestion, such as the radiation pattern of the antenna for example.

In a dual reflector antenna, the secondary reflector must be maintained in the vicinity of the focus of the primary reflector. One of the possible ways is to fix the secondary reflector at the end of the waveguide. In this case, the secondary reflector usually comprises a dielectric body (frequently plastic) of generally conical shape and transparent to RF waves. The substantially conical outer surface of the secondary reflector faces the primary reflector. The convex inner surface of the secondary reflector is coated with a treatment to reflect the RF waves towards the primary reflector through the dielectric body. This coating is most often metal.

Multiple reflections of RF waves occur between the end of the waveguide and the primary reflector, involving the secondary reflector. In order to reduce these reflections, it has been proposed to introduce local disturbances on the outer surface of the secondary reflector facing the primary reflector. These disturbances have the form of reliefs forming rings around the dielectric body. These corrugated reliefs are reliefs of revolution around the axis of the secondary reflector. The profile of these corrugated reliefs consists of ridges and projections of different heights and depths. These reliefs may be distributed periodically over the entire external surface of the secondary reflector. However, non-periodic corrugated reliefs can be used to modify the reflection characteristics of the secondary reflector, in order to further reduce the multiple reflections of the RF waves for the two polarization planes of the electromagnetic wave.

The introduction of annular reliefs on the outer surface of the dielectric body reduces the multiple reflections of RF waves that occur between the waveguide and the primary reflector via the metallized inner surface of the secondary reflector. On the other hand, these reliefs have a lesser effect on two other important characteristics of the double reflector: the antenna gain, expressed in dBi or isotropic decibel, and the spillover losses, expressed in dB.

In the antenna transmission mode, for example, the overflow losses correspond to the energy reflected by the secondary reflector towards the primary reflector, and whose path ends beyond the outer diameter of the primary reflector. These losses lead to pollution of the environment by RF waves. These overflow losses should be limited to levels defined by standards.

A conventional solution is to attach to the periphery of the primary reflector a skirt which has the shape of a cylinder, of diameter close to that of the primary reflector and of suitable height, lined internally with a layer absorbing RF radiation. In addition to the resulting congestion, this known solution has the inconvenient today inconvenient cost of the material of the skirt, as well as the cost of assembling this skirt on the primary reflector.

The present invention aims to provide a dual reflector antenna whose losses overflow are significantly reduced.

• une première extrémité ayant une jonction d'un premier diamètre, adaptée pour le couplage à l'extrémité d'un guide d'onde,
• une seconde extrémité, ayant un second diamètre plus grand que le premier diamètre,
• une surface interne convexe réfléchissante placée à la seconde extrémité ayant un axe de révolution,
• une surface externe de même axe, reliant les deux extrémités,
• un corps diélectrique s'étendant entre la première et la seconde extrémité et limité par la surface interne et la surface externe,

The subject of the present invention is a secondary reflector antenna with double reflector comprising

• a first end having a junction of a first diameter, adapted for coupling to the end of a waveguide,
• a second end, having a second diameter larger than the first diameter,
• a convex inner reflective surface placed at the second end having an axis of revolution,
• an outer surface of the same axis, connecting the two ends,
• a dielectric body extending between the first and second ends and bounded by the inner surface and the outer surface,

According to the invention, the outer surface has a convex profile described by a polynomial equation of the sixth degree of the form: y = ax ⁶ + bx ⁵ + cx ⁴ + dx ³ + ex ² + fx + g where a is not no.

The invention consists in proposing a secondary reflector whose outer surface has a profile according to a particular curve. The secondary reflector is a volume of axial symmetry having a surface whose generator is a curve described by a polynomial equation of degree 6. Numerical optimizations make it possible to adapt the coefficients of this polynomial equation of degree 6 according to the type of double reflector used. and the possible presence of a skirt.

In the equation y = ax ⁶ + bx ⁵ + cx ⁴ + dx ³ + ex ² + fx + g the coefficients b, c, d, e, f, and / or g can be zero.

In a variant of the invention, the outer surface of the secondary reflector further comprises a single ring-shaped relief surrounding the dielectric body.

The section of this relief may be a portion of a disk or a parallelogram (square or rectangle for example). Preferably the relief has a rectangular section.

More preferably the relief is projected in a direction perpendicular to the axis of revolution of the secondary reflector.

This unique raised ring is placed on the outer surface of the secondary reflector to reduce the multiple reflections of the RF wave. At the same time, a reduction in overflow losses and multiple reflections of the RF waves is achieved simultaneously. Preferably the relief is disposed on the half of the outer surface closest to the second end.

• une première extrémité ayant une jonction d'un premier diamètre, adaptée pour le couplage à l'extrémité d'un guide d'onde,
• une seconde extrémité, ayant un second diamètre plus grand que le premier diamètre,
• une surface interne convexe réfléchissante placée à la seconde extrémité ayant un axe de révolution,
• un corps diélectrique s'étendant entre la première et la seconde extrémité et limité par la surface interne et la surface externe,
• une surface externe de même axe, placée au plus près du réflecteur primaire, ayant un profil convexe décrit par une équation polynomiale du sixième degré de la forme : y = ax⁶ + bx⁵ + cx⁴ + dx³ + ex² + fx + g où a n'est pas nul.

The present invention also relates to a dual reflector antenna comprising a primary reflector and an associated secondary reflector. The secondary reflector comprises:

• a first end having a junction of a first diameter, adapted for coupling to the end of a waveguide,
• a second end, having a second diameter larger than the first diameter,
• a convex inner reflective surface placed at the second end having an axis of revolution,
• a dielectric body extending between the first and second ends and bounded by the inner surface and the outer surface,
• an outer surface of the same axis, placed closest to the primary reflector, having a convex profile described by a polynomial equation of the sixth degree of the form: y = ax ⁶ + bx ⁵ + cx ⁴ + dx ³ + ex ² + fx + where a is not zero.

Because of the reduction of overflow losses, the present invention makes it possible to dispense with the skirt, or at least to reduce the height of the skirt of the primary reflector, which provides a cost and space advantage.

The improvement afforded by the invention makes it possible to use a skirt of low height which can be made in one piece with the primary reflector, that is to say that a single mechanical part having a reflector is produced in the central part and a skirt in the peripheral part. This entails an additional cost reduction compared to the conventional solution of a skirt attached to a primary reflector by any known method such as welding, screwing, etc. This saves the cost of assembly.

The invention can be used in applications such as, for example, the production of terrestrial antennas for receiving a radiofrequency signal emitted by a satellite or the link between two terrestrial antennas, and more generally in any application concerning radiofrequency links. point-to-point in the frequency band from 7 GHz to 40 GHz. The typical central frequencies of operation of these systems are 7.1 GHz, 8.5 GHz, 10 GHz, etc. The bandwidth around each frequency is in general of the order of 5% to 20%. Each central frequency corresponds to a suitable secondary reflector diameter: the higher the frequency, the shorter the wavelength, and the smaller the diameter of the secondary reflector.

• la figure 1 représente une vue schématique en coupe axiale d'une antenne radiofréquence selon un premier mode de réalisation de l'invention,
• la figure 2 montre une vue schématique en coupe axiale du réflecteur secondaire d'une l'antenne RF selon un premier mode de réalisation de l'invention,
• la figure 3 montre une vue schématique en coupe axiale du réflecteur secondaire d'une antenne RF selon un deuxième mode de réalisation de l'invention,
• la figure 4 est une vue générale schématique des paramètres de rayonnement d'une antenne à double réflecteur analogue à celle de la figure 1,
• la figure 5 représente une vue schématique en coupe axiale d'une antenne RF dont le réflecteur primaire comprend une jupe selon un troisième mode de réalisation de l'invention,
• la figure 6 est un exemple de profil de la surface externe du réflecteur secondaire selon un mode particulier de réalisation de l'invention,
• la figure 7 est le diagramme de rayonnement du réflecteur secondaire dans le plan vertical en fonction du demi-angle d'éclairement θ pour trois profils différents de la surface externe du réflecteur secondaire,
• la figure 8, analogue à la figure 7, est le diagramme de rayonnement du réflecteur secondaire dans le plan horizontal en fonction du demi-angle d'éclairement θ pour trois profils différents de la surface externe du réflecteur secondaire.
• la figure 9 représente le diagramme de rayonnement du réflecteur primaire en fonction du demi-angle β, complémentaire du demi-angle de rayonnement θ, d'une antenne à double réflecteur selon l'art antérieur,
• la figure 10, analogue à la figure 9, représente le diagramme de rayonnement du réflecteur primaire en fonction du demi-angle β d'une antenne à double réflecteur selon le premier mode de réalisation de l'invention,
• la figure 11, analogue à la figure 9, représente le diagramme de rayonnement du réflecteur primaire en fonction du demi-angle β d'une antenne à double réflecteur selon le deuxième mode de réalisation de l'invention.

The invention will be better understood and other advantages and particularities will appear on reading the following description of embodiments, given by way of illustration and not limitation, accompanied by the appended drawings among which

• the figure 1 represents a schematic view in axial section of a radiofrequency antenna according to a first embodiment of the invention,
• the figure 2 shows a schematic view in axial section of the secondary reflector of an RF antenna according to a first embodiment of the invention,
• the figure 3 shows a schematic view in axial section of the secondary reflector of an RF antenna according to a second embodiment of the invention,
• the figure 4 is a schematic overview of the radiation parameters of a dual reflector antenna similar to that of the figure 1 ,
• the figure 5 represents a schematic view in axial section of an RF antenna whose primary reflector comprises a skirt according to a third embodiment of the invention,
• the figure 6 is an example of a profile of the external surface of the secondary reflector according to a particular embodiment of the invention,
• the figure 7 is the radiation pattern of the secondary reflector in the vertical plane as a function of the half illumination angle θ for three different profiles of the external surface of the secondary reflector,
• the figure 8 , analogous to the figure 7 , is the radiation pattern of the secondary reflector in the horizontal plane as a function of the illumination half-angle θ for three different profiles of the external surface of the secondary reflector.
• the figure 9 represents the radiation pattern of the primary reflector as a function of the half-angle β, complementary to the half-radiation angle θ, of a double-reflector antenna according to the prior art,
• the figure 10 , analogous to the figure 9 , represents the radiation pattern of the primary reflector as a function of the half-angle β of a double-reflector antenna according to the first embodiment of the invention,
• the figure 11 , analogous to the figure 9 , represents the radiation pattern of the primary reflector as a function of the half-angle β of a double-reflector antenna according to the second embodiment of the invention.

On the figures 7 and 8 , the amplitude in dBi of the radiation V in the vertical plane and the radiation H in the horizontal plane respectively of the secondary reflector are given on the ordinate, and on the abscissa the half-angle of illumination θ in degrees.

On the Figures 9 to 11 , the radiation T of the primary reflector is expressed in dB on the ordinate and on the abscissa the half-angle β expressed in degrees. The T radiation of the primary reflector is normalized to 0 dB for a half-angle β equal to zero degrees.

On the figure 1 , in axial section, an RF antenna is shown according to a first embodiment of the invention. This antenna comprises an assembly consisting of a concave primary reflector 1 and a secondary reflector 2 , as well as a waveguide 3 also serving as a mechanical support for the secondary reflector 2 . The set has a symmetry of revolution around the axis 4 .

The primary reflector 1 may be metal reflective surface, for example aluminum. The waveguide 3 may be, for example, a metal hollow tube, also of aluminum, of circular section having an outside diameter of 26 mm or 3.6 mm for transmission / reception frequencies of 7 GHz and 60 GHz, respectively. Of course the waveguide could have a different section, rectangular or square, for example.

The focal point 5 (also called the phase center) is represented on the axis of revolution 4 , and the focal length F 6 which separates the focus from the top of the primary reflector 1 . The primary reflector 1 is for example a paraboloid of revolution about the axis 4 with a depth P 7 and a diameter D 8 .

For such an antenna having an F / D ratio of the order of 0.2, the focal length F is for example 246 mm and the diameter D is 1230 mm (4 feet). In this case, the limiting illumination angle 2θ _p of the primary reflector is 210 °.

The figure 2 represents the secondary reflector 10 of an antenna according to the first embodiment of the invention. The dielectric body 11 of the secondary reflector may be of a dielectric material such as plastic. The inner surface 12 of the secondary reflector 10 may be a surface of revolution described by a polynomial equation around an axis of revolution 13 . The inner surface 12 may be covered with a reflective metal, such as silver.

The outer surface 14 of the secondary reflector 10 is the surface placed opposite the primary reflector. The outer surface 14 is a surface of revolution about the axis of revolution 13 .

According to the first embodiment of the invention, the outer surface 14 of the secondary reflector 10 has a profile which is a curve described by a polynomial equation of the sixth degree of the form: y = ax ⁶ + bx ⁵ + cx ⁴ + dx ³ + ex ² + fx + g. The Calculations make it possible to show that the choice of such a curved profile for the external surface 14 makes it possible to reduce the overflow losses of the double reflector.

The shape of the inner surface of the secondary reflector influences the intensity and phase of the electromagnetic wave from the waveguide and received by the primary reflector.

The figure 3 represents the secondary reflector 20 of an antenna according to a second embodiment of the invention. A relief 21 forming a ring is formed on the outer surface 22 of the reflector 20 . The profile of the outer surface 22 on either side of the relief 21 is a curve described by a polynomial equation of the sixth degree of the form: y = ax ⁶ + bx ⁵ + cx ⁴ + dx ³ + ex ² + fx + boy Wut

In the second embodiment of the invention, the outer surface 22 of the reflector 20 consists of three successive portions 22a , 21 , 22b . The portions 22a and 22b each have a profile described by a portion of the sixth degree curve. The parts 22a and 22b and the relief 21 have a symmetry of revolution about the axis of revolution 23.

The overflow losses for the transmission mode of an RF antenna according to the first embodiment of the invention are explained on the figure 4 . These losses correspond to values of the illumination angle 2θ of the primary reflector by the secondary reflector for which the RF waves coming from the waveguide 3 are reflected by the secondary reflector 2 in a direction which is outside the perimeter of the primary reflector 1 .

This figure shows the illumination half-angle θ (theta) 30 and the half-angle β (beta) 31 , which is the half-angle complementary to the half-angle 0. The two half-angles 0 and β are measured by relative to the axis of revolution 4 of the secondary reflector 2 , and they have the focal point 5 of the primary reflector 1. There is overflow loss for half-angle values θ greater than the limit value θ _p 32 for which the rays reflected 33 by the secondary reflector come to be tangent to the edge of the primary reflector.

The overflow losses are therefore due to all the rays 33 reflected by the secondary reflector 2 within the angular range 34 . The angular range 34 is defined by two radii 35 , coming from the focus 5 and symmetrical with respect to the axis of revolution 4 , which are tangent to the edges of the primary reflector 1 .

The figure 5 represents an axial sectional view of an RF antenna according to a variant of the first embodiment of the invention. The primary reflector 50 is provided with a skirt 51 in order to limit the losses by overflow. The skirt 51 is a screen covered with a material 52 absorbing RF waves. For example, the skirt 51 is made of aluminum and the absorbent layer 52 consists of a foam loaded with carbon oxides.

The skirt 51 is of less height than the skirts used in the prior art, because the overflow losses are significantly reduced by the use of a secondary reflector 53 provided with an external surface 54 having a profile according to a curve described by a polynomial equation of the sixth degree. The parameters of the sixth-degree equation describing the profile of the outer surface 54 can be optimized. This optimization makes it possible to reduce the height of the skirt 51 to allow the production of a single piece of the primary reflector 50 and the skirt 51 , as shown in FIG. figure 5 . The skirt 51 thus constitutes an extension of the primary reflector 50 . This can be achieved for example by stamping a single aluminum plate so as to define successively or simultaneously the shape, preferably paraboloid of revolution, of the primary reflector 50 and the shape, preferably cylindrical, of the skirt 51 .

The figure 6 represents an exemplary profile 60 of the outer surface of the secondary reflector according to a particular embodiment of the invention, which was obtained by digitizing the level of overflow losses. The position of the X and Y axes, used respectively in abscissa and ordinate, is represented on the figure 2 . The reference (X, Y) originates from a point of the axis of revolution 13 located at the second end of the secondary reflector 10. The X axis is aligned on the axis of revolution 13 and the Y axis has a direction perpendicular to the axis of revolution 13 . Distances are expressed in centimeters.

The example described in this figure corresponds to a double reflector antenna whose primary reflector is of parabolic type corresponding to the equation: P / D = D / (16F) in which P is the depth of the primary reflector, D is the diameter of the primary reflector, and F is the focal length of the primary reflector.

In this example, F / D = 0.25 and the limiting half-angle of illumination θ _p is such that 0 _p = 90 °, because in any parabola θ _p = 2 arc-tangent (D / 4F).

In this embodiment of the invention, the polynomial equation defining the profile of the external surface of the secondary reflector is as follows: $$\mathrm{there}\mathrm{=}\left({\mathrm{-}\mathrm{3}\mathrm{,}\mathrm{904.10}}^{\mathrm{-}\mathrm{7}}\right){\mathrm{x}}^{\mathrm{6}}\mathrm{+}\left({\mathrm{4}\mathrm{,}\mathrm{658.10}}^{\mathrm{-}\mathrm{5}}\right){\mathrm{x}}^{\mathrm{5}}\mathrm{+}\left({\mathrm{-}\mathrm{1}\mathrm{,}\mathrm{917.10}}^{\mathrm{-}\mathrm{3}}\right){\mathrm{x}}^{\mathrm{4}}\mathrm{+}\left({\mathrm{3}\mathrm{,}\mathrm{358.10}}^{\mathrm{-}\mathrm{2}}\right){\mathrm{x}}^{\mathrm{3}}\mathrm{+}\left({\mathrm{-}\mathrm{2}\mathrm{,}\mathrm{927.10}}^{\mathrm{-}\mathrm{1}}\right){\mathrm{x}}^{\mathrm{2}}\mathrm{+}\left({\mathrm{3}\mathrm{,}\mathrm{006.10}}^{\mathrm{-}\mathrm{1}}\right)\mathrm{x}\mathrm{+}\left(\mathrm{3},\mathrm{462.10}\right)$$

The numerical values given here for the parameters a, b, c, d, e, f, g of the sixth degree equation depend on the numerical values chosen for the focal length F, the depth P and the diameter D of the primary reflector. as well as the level of overflow losses that we allow ourselves. If we change these numerical values, we can find another set of values for the parameters a, b, c, d, e, t, g to minimize overflow losses. Thus the parameters a, b, c, d, e, f, g of the sixth degree equation can take different values.

• un profil conique connu de l'art antérieur (courbe de référence 70),
• un profil correspondant au premier mode de réalisation de l'invention (courbe 71), et
• un profil comprenant un relief annelé selon le deuxième mode de réalisation de l'invention (courbe 72).

The figure 7 shows the radiation pattern in the vertical plane of the secondary reflector of a dual-reflector antenna for three different profiles of the outer surface of the secondary reflector:

• a conical profile known from the prior art (reference curve 70 ),
• a profile corresponding to the first embodiment of the invention (curve 71 ), and
• a profile comprising a corrugated relief according to the second embodiment of the invention (curve 72 ).

The radiation pattern is represented by the amplitude of the radiation V expressed as a function of the illumination half-angle θ. This radiation pattern is relative to the antenna in transmission mode. The best antenna design is that which makes it possible to obtain a radiation, or emitted electric field, as small as possible for the illumination half-angle values θ greater than the limit value θ _p represented here by the vertical line 73 . The vertical line 73 represents the value θ _p of the half-angle θ which tangents the outer edge of the primary reflector as shown in FIG. figure 4 . For values of the half-angle θ greater than the value θ _p defined by the vertical line 73 , the rays are reflected in the angular range 34 and participate in overflow losses.

It is observed that the curve 71 , associated with the first embodiment according to the invention, shows a lower radiation for values of the angle θ greater than the value θ _p that the radiation given by the curve 70 associated with a profile of the prior art. The curve 72 associated with a second embodiment according to the invention further improves the result obtained with the curve 71 .

• un profil conique connu de l'art antérieur (courbe de référence 80),
• un profil correspondant au premier mode de réalisation de l'invention (courbe 81), et
• un profil comprenant un relief annelé selon le deuxième mode de réalisation de l'invention (courbe 82).

The figure 8 , analogous to the figure 7 , represents the radiation pattern of the secondary reflector, this time measured in the horizontal plane, for three different profiles of the external surface of the secondary reflector:

• a conical profile known from the prior art (reference curve 80 ),
• a profile corresponding to the first embodiment of the invention (curve 81 ), and
• a profile comprising a corrugated relief according to the second embodiment of the invention (curve 82 ).

In this figure, the vertical line 83 represents the value θ _p of the half-angle θ which tangents the outer edge of the primary reflector as shown in FIG. figure 4 .

As in the previous case, the best antenna design is that which makes it possible to obtain the lowest radiation for the half-angles 0, greater than the value θ _p , situated to the right of the vertical line 83 . It is observed that the curve 81 associated with the first embodiment according to the invention shows radiation values lower than the values given by the curve 80 associated with a profile of the prior art. Curve 82 associated with a second embodiment according to the invention further improves the result obtained with curve 81 .

The figure 9 shows the radiation pattern of the primary reflector as a function of the half-angle β of a double-reflector antenna according to the prior art. The power levels reflected in the vertical and horizontal planes of the antenna as a function of the half-angle β are represented on the ordinate. The curve 90 corresponds to the power reflected in the vertical plane, and the curve 91 corresponds to the power reflected in the horizontal plane.

A broken line 92 indicates for each value of the half-angle β the reflectivity limits allowed by the ETSI standard R1C3 Co. For a value of the half-angle β close to 65 °, which is the limit value corresponding to the diffraction of the RF wave on the edge of the primary reflector, the difference 93 between the value of the radiation of the primary reflector and the limit value imposed by the standard is here of the order of 5 dB.

The figure 10 relates to a dual reflector antenna using a secondary reflector according to a first embodiment of the invention. The outer surface of the antenna has a profile described by a polynomial equation of the sixth degree. The reflected power levels in the vertical and horizontal planes of the antenna are represented as a function of the half-angle β. The curve 100 corresponds to the power reflected in the vertical plane and the curve 101 corresponds to the power reflected in the horizontal plane. A broken line 102 indicates, for each value of the half-angle β the reflectivity limits authorized by the ETSI R1C3 Co. standard.

The difference 103 is here of the order of 7 dB, increasing with respect to the difference of 5 dB obtained for an antenna of the prior art.

The figure 11 relates to a double reflector antenna using a secondary reflector according to a second embodiment of the invention. The outer surface of the secondary reflector has a profile described by a polynomial equation of the sixth degree on which has been added an annular relief. The reflected power levels in the vertical and horizontal planes of the antenna are represented as a function of the half-angle β. The curve 110 corresponds to the power reflected in the vertical plane and the curve 111 corresponds to the power reflected in the horizontal plane. A broken line 112 indicates, for each value of the half-angle β the reflectivity limits allowed by the ETSI R1C3 Co. standard.

The difference 113 is of the order of 9 dB, much higher than the 93 dB difference of 5 dB obtained for a prior art antenna and improved with respect to the difference 103 of 7 dB obtained according to the first embodiment of FIG. embodiment of the invention.

The greater the difference between the value of the radiation of the primary reflector and the limit value imposed by the ETSI R1C3 Co standard, the lower the intensity of the radiation of the antenna in this angular zone. This quality of the antenna is important for the user because it ensures less electromagnetic pollution of neighboring antennas.

Claims (5)

1. A secondary reflector for a dual-reflector antenna, comprising:

   - a first end having a junction with a first diameter, suitable for coupling to the end of a waveguide (3),

   - a second end, having a second diameter greater than the first diameter,

   - an internal convex reflective surface (12) placed at the second end, having an axis of revolution (13),

   - an external surface (14) of the same axis (13), connecting the two ends,

   - a dielectric body (11) extending between the first and second end and limited by the internal surface (12) and external surface (14),

   characterized in that the external surface (14) has a convex profile described by a sixth-degree polynomial equation whose form is: y = ax⁶ + bx⁵ + cx⁴ + dx³ + ex² + fx + g, where a is non-zero and where x is the axis aligned on the axis of revolution (13) and y is the axis whose direction is perpendicular to the axis of revolution (13).
2. A secondary reflector according to claim 1, wherein the external surface (22) further comprises a single ring-shaped relief (21) surrounding the dielectric body (11).
3. A secondary reflector according to claim 2, wherein the relief (21) projects out in a direction perpendicular to said axis of revolution (23).
4. A dual-reflector antenna comprising a secondary reflector (2, 10), according to one of the preceding claims, combined with a primary reflector (1), characterized in that the external surface (14) of the secondary reflector (2, 10) is placed as close as possible to the primary reflector (1) and has a convex profile described by a sixth-degree polynomial equation whose form is: y = ax⁶ + bx⁵ + cx⁴ + dx³ + ex² + fx + g, where a is non-zero.
5. A dual-reflector antenna according to claim 4, comprising a primary reflector (50) comprising a shroud, the shroud (51) and the primary reflector (50) being constructed from a single piece.

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