EP0375542B1 - Reflector of electromagnetic waves for an antenna, and method for its manufacture - Google Patents

Description

The subject of the invention is a reflector of electromagnetic waves with a convex surface and its manufacturing process. This reflector more specifically constitutes the secondary reflector of a radio antenna with a configuration of the "Cassegrain" type, intended to work in a wavelength range up to 20 GHz.

These antennas are used in particular in the telecommunications field (see for example: GLOBECOM'85 - IEEE GLOBAL TELECOMMUNICATIONS CONFERENCE, New Orleans, Louisiana, December 2-5, 1985, vol. 1, pages 412-416, IEEE, New York, US; P. BIELLI et al .: "Dichroic subreflectors for multifrequency antennas"); they can be used on earth or in space. In the space sector, these antennas are intended to equip telecommunications satellites.

Although the reflector of the invention is more particularly intended to constitute the secondary reflector of an antenna of the "Cassegrain" type, it can also be used as a reflector in a conventional monoreflector antenna or as a main reflector in a bireflector antenna.

A conventional configuration antenna is made up of a radiofrequency source and a parabolic reflector, the concave face of which generally constitutes the active face. The source is placed at the focus of the reflector and it is intended to emit or receive electromagnetic radiation that the reflector focuses.

In certain fields and more particularly in the space field, it is preferable to use a secondary reflector antenna, having a so-called "Cassegrain" configuration in order to limit the size of the antenna for the same focal distance (generally from 1 to 3 m ). In FIG. 1, a “Cassegrain” type antenna is shown diagrammatically.

This antenna essentially comprises a main reflector or mirror 2 which is a paraboloid of focal point F₁, a secondary reflector or mirror 4 whose surface is of the hyperboloid type of focal point F₂ and a primary source 6 placed at the focal point F₂.

For an emission operation, the source 6 illuminates the secondary reflector 4 which reflects the radiation 7 on the main reflector 2, the latter ensuring the directivity of the emission of electromagnetic radiation.

In reception, the operation takes place in the opposite direction: reception of the electromagnetic waves by the main mirror 2 which reflects them towards the secondary mirror 4 where they are again reflected towards the source 6.

The configuration shown in FIG. 1 is of the "Offset" or "offset" type. The operation of a "centered" type antenna is quite similar.

In the space domain, the active face of the antenna reflectors, that is to say the reflecting faces 4a and 2a respectively of the main 4 and secondary 2 mirrors, are covered with a white paint generally based on silicone. The purpose of this paint is to protect the reflectors mounted on the satellites from cyclic thermal variations caused by the alternating passages of shadow zones and solar illumination zones.

This thermal protection makes it possible to minimize the thermoelastic deformations resulting from the reflector, by maintaining the active faces 4a and 2a in a range of profiles which retains the desired radioelectric performance of the antenna.

Although this paint provides generally satisfactory thermal insulation, it has, in certain cases, drawbacks. This is due to the fact that the incident radiation passes through the layer of paint before being reflected on the conductive surface 4a or 2a of the reflector.

In the case of an electromagnetic wave with circular polarization, the paint layer causes a phase shift between the components of the vertical and horizontal electric field. This phase shift destroys the purity of the circular polarization and the reflected radiation then has an elliptical polarization corresponding to a loss of energy. This phenomenon is all the more important as the angle of incidence i (FIG. 1) made by the radiation relative to the normal to the active surface, is high.

For low incidences, which is generally the case in antennas with a single reflector, the effect of this phase shift may not be taken into account. On the other hand, these disturbances are by no means negligible in the case of secondary antenna reflectors of the "Cassegrain" type, and more especially in an "eccentric" configuration, where the angles of incidence of the radiation can reach high values (of the order 60 °) on the secondary reflector.

Furthermore, in the space domain, the antenna reflectors must be as light as possible in order to facilitate the putting into orbit of the satellite equipped with these reflectors.

To overcome these drawbacks, the antenna reflector with a convex active face has been designed very recently, shown diagrammatically in FIG. 2. This antenna reflector 4 comprises a rigid support 10 whose active face 10a is completely covered with the layer of thermal insulation paint 12. This insulation layer 12 is itself covered with a metallized coating 14. This coating 14 is in particular a polyimide film such as Kapton ^R , 25 micrometers thick, covered with a layer of aluminum of 30 to 40 nm.

This coating 14 is relatively light and ensures the reflection of the electromagnetic waves 7, as is clear from FIG. 2, and thus prevents the electromagnetic radiation from passing through the layer of paint 12 and therefore its modification of polarization.

In order to ensure a minimum weight of the reflector, the rigid support 10 is formed of a rigid honeycomb structure 16 sandwiched between two carbon coatings 18 and 20.

The reflector of FIG. 2 makes it possible to remedy the preceding drawbacks.

Unfortunately, the use of a coating 14 in aluminized Kapton ^R has a certain number of drawbacks. Indeed, this type of material is difficult to manufacture since it must be formed with a precise mechanical tension in order to absorb the expansions of the support 10 in a temperature cycle typically from -160 ° C to + 100 ° C in the case of a satellite antenna placed in orbit, while ensuring good reflection of the waves.

Furthermore, its implementation is costly and this type of coating risks tearing. Finally, this coating is not very deformable, which limits its use. In particular, this material cannot be used for reflectors with very high convexity.

The subject of the present invention is precisely an electromagnetic wave reflector constituting in particular the secondary reflector of a radio antenna with two reflectors, making it possible to remedy the drawbacks given above. In particular, this reflector comprises a wave reflective material, solid usable whatever the convexity of the reflector, absorbing all thermal expansions of the reflector support while preventing the polarization of the electromagnetic radiation from being modified, when using a thermally insulating paint.

Furthermore, the reflector of the invention can be used in the space field, given its low weight.

The subject of the invention is therefore a convex reflector of an electromagnetic wave of wavelength λ, comprising a curved rigid support, provided with a convex front face constituting the active face of the reflector and a rear face, a paint thermally insulating and dielectric coating the active face, a stretched electrically conductive knitted fabric capable of reflecting said wave and covering the insulating paint, the knitted fabrics of the knitted fabric having a diameter less than λ / 8, and means for fixing the knitted fabric on the support.

The conductive knit, according to the invention, easily adapts to non-developable shapes and high convexity, unlike the aluminized polyimide of the prior art.

In addition, with a thermal insulator such as a silicone-based paint, completely covering the active face of the reflector, this knitted fabric, ensuring the reflection of the electromagnetic waves, prevents them from passing through the underlying paint layer and therefore their change of polarization.

According to the invention, the knitting can be made of any material which is good conductor of electricity and has a low coefficient of expansion. This knit can be platinum, silver, titanium, gold, molybdenum, tungsten or a metal alloy. Preferably, molybdenum covered with a gold film is used; molybdenum is the metal with the best coefficient of expansion (5.10⁻⁶m / m ° C) and one of the lowest electrical resistivities (5.2.10⁻⁶Ω.cm. In addition, it has a low specific mass (9 g / cm³) which is very advantageous for a application in the space field: the gold film covering the molybdenum improves the metal contacts.

In addition, a knit is extremely light, which is the aim sought for a reflector intended to equip an antenna placed on a satellite. In this particular case, it is advantageous to use a rigid support constituted by a honeycomb structure sandwiched between a first coating constituting the front face of the reflector and a second coating constituting the rear face of said reflector.

The honeycomb structure can be metallic, glass, Kevlar ^R or carbon. Furthermore, the coatings located on either side of the honeycomb can be made of carbon, Kevlar ^R or glass.

In order to improve the thermal insulation of the reflector, additional means of thermal insulation are provided on the whole of the rear face of the reflector. These means may consist of a single layer of insulating paint or of a stack of metallized layers and of insulating layers. Preferably, a stack of metallized polyimide layers and fabric gauzes is used.

Although any means of attachment can be envisaged to make the conductive knit and the rigid support integral, preferably one or more adhesive tapes mounted integrally on the rear face, or on the edge of the support or on both at the same time are used. . Preferably, an adhesive strip mounted integrally on the rear face of the reflector is used, consisting of a first part provided with pins or hooks and a second part intended to adhere to the first part, generally called felt part, the rim of the knitted fabric being inserted between these two parts.

The invention also relates to an antenna with a convex secondary reflector produced as described above. This antenna is in particular a “Cassegrain” type antenna with a “centered” or “eccentric” configuration.

• monter sur la face arrière du réflecteur les moyens de fixation du tricot sur le support,
• placer sur la face active peinte du réflecteur une pièce de tricot de dimension supérieure à celle de la face active,
• tendre ledit tricot à la tension souhaitée,
• implanter des aiguilles dans le tricot tendu, à la périphérie du support,
• surfiler le tricot tendu à une distance déterminée des aiguilles et en dehors du support,
• rabattre la partie du tricot non-surfilée sur la face arrière du support,
• fixer ladite partie non-surfilée sur la face arrière du réflecteur à l'aide desdits moyens de fixation, et
• enlever lesdites aiguilles.

The subject of the invention is also a method of manufacturing an electromagnetic wave reflector as described above, consisting in:

• mount the means for fixing the knitted fabric on the support on the rear face of the reflector,
• place on the painted active face of the reflector a piece of knit larger than that of the active face,
• stretch said knitted fabric to the desired tension,
• implant needles in the stretched knit, on the periphery of the support,
• overcast the stretched knit at a determined distance from the needles and outside the support,
• fold down the part of the non-overcast knitted fabric on the back of the support,
• fixing said non-overcast portion on the rear face of the reflector using said fixing means, and
• remove said needles.

Other characteristics and advantages of the invention will emerge more clearly from the description which follows, given by way of illustration and not limitation with reference to Figures 3 to 8 appended, Figures 1 and 2 having already been described.

FIG. 3 schematically represents an overall view of the reflector according to the invention.

FIG. 4 represents an enlarged part of the reflector of the invention further showing the fixing knitting on the active side.

FIG. 5 illustrates the additional thermal insulation of a reflector according to the invention and FIG. 6 illustrates the maintenance of the fixing of this insulation.

Figures 7 and 8 schematically illustrate the mounting of the knitted fabric on the support of the reflector according to the invention.

The description below refers to a convex secondary reflector of a "Cassegrain" type antenna (see FIG. 1) although, as we have seen previously, the invention is of much more general application. In addition, the elements of the reflector, common to those of the prior art, have the same references.

With reference to FIGS. 3, 4 and 5, the electromagnetic wave reflector of the invention comprises a rigid convex support 10, of elliptical contour, consisting of a honeycomb structure 16 of aluminum, sandwiched between a coating upper 18 and a lower carbon coating 20. The support 10 has a total thickness of approximately 25 mm for an elliptical shaped reflector of 500 mm of major axis and 350 mm of minor axis.

The upper face 10a of the support, constituting the active face of the reflector, is equipped with a layer of paint 12 based on silicone such as the paint PSG 120 FD manufactured by Astral. This paint has the advantage of having thermo-optical thermal protection characteristics of the support 10 that are entirely satisfactory. Indeed, the solar absorbance (or absorption coefficient) of this paint is less than 0.2.

This layer of paint 12 completely covers the upper face 10a of the rigid structure; it has a thickness of about 0.1 mm, which corresponds at a weight of 260 g / m².

According to the invention, a metallic knitted fabric 22 completely covers the insulating paint 12. The stitches of this knitted fabric are a function of the frequency of the radioelectric radiation to be reflected. In order for the knitted fabric to reflect a wave of wavelength λ, the size or "diameter" of the stitches 23 (FIG. 5) must be <λ / 8. For example, one uses a mesh of 2 mm in diameter for a radio frequency ≦ 2 GHz and a mesh of 1 mm for a radio frequency ≦ 15 GHz.

This knitting is in particular made up of 25 micrometer thick son of golden molybdenum and is sold by the company Brochier (France).

As shown in FIG. 3, this knitted fabric 22 ensures the reflection of the electromagnetic waves 7 coming in particular from a radiofrequency source 6. The reflection of the waves on the knitted fabric 22 does not in any way modify the properties (and in particular the polarization) of the received wave.

The knitting 22 is fixed to the support 10 in particular by an adhesive strip 24 of the Velcro ^R strip type, located on the rear face 20a of the reflector and at its periphery. For this purpose, the knitted fabric 22 must have dimensions greater than those of the surface 10a of the reflector so as to be folded down under the rear face 20a of the reflector.

As shown in FIG. 4, a Velcro strip ^R consists in a known manner of a part 26 fitted with pins or hooks 28 and a felt part 30 intended to adhere to the pins of part 26, maintaining the knitted fabric 22 is ensured by placing the end of the latter between the two parts 26 and 30; the pins 28 ensuring the fixing of the felt part 30 pass through the knitted fabric 22.

The back of part 26 of Velcro ^R is rendered secured to the lower face 20a of the reflector using an adhesive of the cold glue type of the epoxy-modified type known under the brand REDUX 408.

The strip 24 velcro ^R is located in particular 10 mm from the periphery of the support 10 of the reflector.

When the Velcro strip ^R used is in the form of a continuous rectilinear strip, it is necessary to cut it regularly according to the radius of curvature of the reflector (every 30 to 60 mm approximately) in order to align it as precisely as possible with respect to the periphery of the reflector.

To improve the finish of the support 10 and protect it from surrounding pollution, an adhesive polyimide film 32 is positioned by bonding over the entire edge 33 of the support 10 and on the periphery of the rear face 20a of the reflector. This adhesive film 32 is placed between the support 10 and the knitted fabric 22 and is arranged so as to be flush with the layer of paint 12.

To improve the thermal insulation of the reflector, additional thermal insulation 35 can be provided over the entire rear face 20a of the reflector, as shown in FIGS. 3, 5 and 6. This thermal super-insulation consists in particular of a stack of layers aluminized or gilded polyimide and gauze of glass or nylon fabric. This insulation is extremely light. Its precise structure and its manufacture are well known to those skilled in the art. the polyimide used is in particular Kapton ^R.

As shown in FIGS. 5 and 6, strips of adhesive polyimide 34, for example made of adhesive Kapton ^R , maintain the thermal insulation 35. These tongues are spaced 20 mm apart, for example, and have a width of 10 mm. They are glued to the knitted fabric 22 and the thermal superinsulation so as to cover the edge 33 of the reflector and the periphery of the rear face 20a.

Unlike the prior art (Figure 2), the Kapton ^R used does not need to be stretched; the requirements of the Kapton ^R in the invention are not at all the same as those of the prior art since it is in no way used for the reflection of electromagnetic waves, this function being provided by knitting.

To avoid possible detachment of the tabs 34 glued to the knitted fabric, an adhesive banding 36 can be placed on the edge of the reflector so as to surround the entire reflector (FIG. 6). This strapping is an adhesive polyimide and in particular Kapton ^R adhesive.

With reference to FIGS. 7 and 8, the placement of the knitted fabric 22 on the rigid painted support 10 is described below.

The knitting 22 is mounted on the support 10 after gluing the part 26 of the Velcro ^R , fitted with its pins, to the periphery of the lower face 20a of the support as well as the adhesive Kapton ^R 32 on the edge of the support. The reflector is then centered on the movable board 37 of a tensioning table 38 by means of a cylindrical support 39. This positioning is carried out so that the surface tangent to the surface 10a of the reflector passes over tensioning rollers 40.

After placing the knitted fabric 22 on the painted active face 10a of the reflector, the latter is stretched by hanging masses 42 of approximately 40 g distributed over the entire periphery of the reflector (FIG. 8) every 40 mm approximately in order to obtain in the chain and width direction, noted x and y respectively, a tension of 10 Newton per meter.

After charging, the whole is vibrated to distribute the tensions evenly. In order to immobilize the knitted fabric thus stretched, needles 44 are arranged at the periphery of the reflector 10. As shown in FIG. 4, these needles are threaded between the edge 33 of the support 10 and the adhesive Kapton ^R 32. These needles only pass through knitting 22. They are arranged at a pitch of approximately 4 mm.

Using a curved needle and a thread of composition different from that of knitting 22 (cotton or Kevlar ^R ), overcasting 46 of the knitting is carried out at a distance e from the periphery of the support 10 and therefore of the needles 44, which is equal to the thickness of the support 10 (in particular 25 mm).

The tensioning masses are then released, the needles 44 ensuring the maintenance of the knitting on the support 10 and the overcasting 46 allowing the tension of the knitting 22 to be recovered when the knitting is fixed on the Velcro ^R.

The knitting 22 is then folded down on the edge 33 of the reflector (in other words on the adhesive Kapton ^R 32) and then on the periphery of the lower face 20a of the reflector; the non-overcast portion of the knitted fabric is then hung on the pins 28 of the portion 26 of the Velcro strip ^R. Then, we apply the felt part 30 of Velcro ^R to part 26.

The assembly obtained can no longer be dismantled, the knitting being held permanently by the Velcro ^R band thanks to the pins 28.

The entire knitting is then cut flush with the Velcro ^R strip (Figure 4) in order to avoid any overshooting of the knitting of the Velcro ^R strip. The retaining needles 44 can then be removed. The superinsulation 35 is then fixed on the rear face of the reflector. The reflector is then finished.

Tests of temperature rise and fall between -160 ° C and + 100 ° C in a solar vacuum box, for prolonged periods, confirmed the good thermal resistance of the reflector. In addition, radio tests on flat test pieces representative of the reflector confirmed the desired radioelectric properties of the reflector.

Claims (11)

1. Convex electromagnetic wave reflector for a wavelength λ and comprising a rigid curved support (10), provided with a convex front face (10a) constituting the active face of the reflector and a rear face (20a), a dielectric and heat-insulating paint (12) coating the active face, the reflector being characterized by a taut electrically conductive fabric (22) able to reflect said wave and covering the insulating paint (12), the stitches (23) of the fabric having a diameter of less than λ/8, and means (24) for securing the fabric to the support.
2. Reflector according to claim 1, characterized in that the fabric (22) is made of molybdenum coated with gold.
3. Reflector according to claim 1 or 2, characterized in that the support (10) is constituted by a honeycomb structure (16) sandwiched between a first coating (18) constituting the front face (10a) and a second coating (20) constituting the rear face (20a).
4. Reflector according to any one of the claims 1 to 3, characterized in that addtional heat-insulating means (35) are provided on the rear face (20a) of the support (10).
5. Reflector according to claim 4, characterized in that the additional insulation means (35) are constituted by a stack of metallized layers and insulating layers.
6. Reflector according to claim 4 or 5, characterized in that the additional insulation means (35) are constituted by a stack of metallized polyimide layers and fabric gauzes.
7. Reflector according to any one of the claims 1 to 6, characterized in that the fixing means (24) include an adhesive strip mounted integral on the rear face (20a) comprising a first section (26) provided with picots (28) and a second section (30) for adhering to the first section, the edge (22a) of the fabric being inserted between the first and second sections.
8. Reflector according to claim 7, characterized in that adhesive fixing means (34) are provided so as to ensure the holding in place of the additional heat-insulating means (33) on the support (10).
9. Convex secondary reflective antenna, characterized in that the reflector is according to any one of the claims 1 to 8.
10. Method for producing an electromagnetic wave reflector having a rear face (20a) and an active front face (10a) coated with a thermally insulating paint according to any one of the claims 1 to 8, the method being characterized by:

    - mounting on the rear face (20a) means (24) for securing an electrically conductive fabric (22) able to reflect said waves onto a support (10),

    - placing on the active painted face (10a) a fabric section (22) whose size is larger than that of the active face,

    - stretching said fabric (22) to the desired tension,

    - implanting needles (44) in the taut fabric (22) at the periphery of the support (10),

    - overcasting (46) the taut fabric at a specific distance from the needles (44) and outside the support (10),

    - folding down the non-overcast fabric section onto the rear face (20a) of the support,

    - securing said non-overcast section (22a) to the rear face (20a) with the aid of said fixing means (24), and

    - removing the needles (44).
11. Method according to claim 10, characterized in that the distance (e) separating the overcasting (46) and the needles (44) is equal to the thickness of the painted support (10).

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