FR2787244A1 - ELECTROMAGNETIC WAVE REFLECTOR FOR TELECOMMUNICATION ANTENNA - Google Patents

Abstract

The invention relates to an electromagnetic wave reflector for a telecommunications antenna, in particular for an antenna on board a satellite comprising a substrate covered with a reflective metallic layer (16) on its front face, this reflective layer having a coefficient of thermal expansion different from the coefficient of thermal expansion of the substrate. The reflector has, in its rear zone, another layer (18) having a coefficient of thermal expansion different from that of the substrate (10, 12, 14). (18) of the rear zone compensates for the variations, as a function of temperature, in the shape of the reflector which result from the different values of the expansion coefficients of the substrate and of the front layer.

Description

TITLE: ELECTROMAGNETIC WAVE REFLECTOR FOR ANTENNA

TELECOMMUNICATION

  The invention relates to an electromagnetic wave reflector and to its

manufacturing process.

  It relates more particularly to a reflector which can be used under conditions

  variable temperature conditions, especially in a space environment.

  An electromagnetic wave reflector performs a function similar to

  that of an optical mirror, that is to say that the waves received in a certain direction

  tion must be reflected in another well-defined direction. These reflec-

  ers demand that the reflected direction be defined precisely, which implies,

  during manufacturing, small tolerances on the shape of the reflector and, during use

  sation, great shape stability, especially when the temperature varies.

  An electromagnetic wave reflector for space applications including

  most often carries a core or spacer comprising a honeycomb type structure and, on each side of this core, mechanically load-bearing skins constituted by carbon fibers impregnated with epoxy resin or resin

cyanate ester.

  These composite laminates based on carbon fibers and resin have a

  very high temperature stability and can be used directly as a

  reflective faces. But at high frequencies, generally above 30 GHz, they generate significant losses, greater than 0.2 dB. This is why, at these high frequencies, the reflecting surface is usually covered by

  a metal deposit to reduce losses.

  The invention results from the observation that this metallic deposit alters the

  shape of the reflector, in particular its curvature, and, in particular, it induces a varia-

  tion of shape caused by a bimetallic strip effect, both during manufacture and during use, especially in space where temperature variations are significant. Indeed, we know that a material formed by the superposition of two layers

  in different materials having different coefficients of thermal expansion pre-

  feels a curvature that varies when the temperature varies.

  This bimetallic strip effect can be particularly troublesome for space applications in which, as indicated above, the temperatures vary significantly. For example, for mobile satellites in low (or medium) orbit, the temperature can vary between -60 C and + 60 C approximately. For a geostationary satellite the temperature can vary between -180 C (in case of solar eclipse)

and +150 "C.

  In addition, the manufacture of the reflector is carried out at a temperature which depends on the kinetics of polymerization of the resin. This temperature is for example close to 175 C. Thus, even in the absence of temperature variation during use, it is difficult to obtain the desired curvature due to the effect of

  bimetallic strip resulting from the difference between manufacturing and operating temperatures.

  The invention makes it possible to significantly reduce the variations in

  bure of the reflector if it is used at variable temperatures,

  and / or to modify the curvature of a reflector.

  To this end, the reflector according to the invention is characterized in that its zone

  rear - preferably its rear side - has an additional layer, front

  metallic tagging, which, with the laminate, forms another bimetallic strip opposing the effect of the bimetallic strip formed by the substrate and the metallic layer on the active face

  and / or making it possible to impart a determined curvature to the reflector.

  It has been found that with this arrangement, it could significantly improve

  ficative the quality of the shape of the reflector without increasing the cost.

  The present invention provides an electromagnetic wave reflector for telecommunication antenna, in particular for an on-board antenna with

  edge of a satellite comprising a substrate covered with a reflective metallic layer

  trice on its front face, this reflective layer having a coefficient of dilata-

  different thermal coefficient of thermal expansion of the substrate. This reflec-

  in its rear area, there is another layer having a coefficient of

  thermal expansion different from that of the substrate.

  According to one embodiment, the layer of the rear zone compensates for the variations, as a function of the temperature, of the shape of the reflector which result from the

  different values of the expansion coefficients of the substrate and the front layer.

  According to one embodiment, the layer of the rear zone modifies, at a determined temperature, the curvature presented by the reflector in the absence of

this layer.

  According to one embodiment, the layer of the rear zone is deposited on

the back side of the substrate.

  According to one embodiment, the layer of the rear zone is metallic.

  According to one embodiment, the layer of the rear zone is made of a metal

  of the same nature as that constituting the reflective layer of the front face.

  According to one embodiment, the coupling membrane / bending of the assembly

  bonding of the substrate layers with the reflective layer and the zone layer

rear is minimized.

  Other characteristics and advantages of the invention will become apparent with the

  description of some of its embodiments, this being carried out in

  referring to the attached drawings on which

  Figure 1 is a partial sectional view of a reflector according to the invention.

  tion, Figure 2 is a diagram for explaining how the shape quality of a surface is defined, and Figures 3 and 4 are diagrams showing some effects and some

  properties of the reflector according to the invention.

  The reflector shown in FIG. 1 is intended to be integrated into a telecommunication antenna on board a satellite in order to reflect Ka-band waves from 20 GHz to 44 GHz, or even at higher frequencies,

up to 80 GHz or more.

  This reflector comprises, in a conventional manner, a core 10 in the form of an aluminum or aramid honeycomb, stiffened on each of its sides by laminates of carbon fibers impregnated with epoxy resin. In this example, there is provided, on each side of the core 10, a stack 12, 14 of four elementary carbon / epoxy layers, each elementary layer being unidirectional. In each one

  laminates of the same stack, the carbon fibers have different directions

  annuities. For example, the fibers of the outermost ply have a first orientation, called the reference to 0, the immediately lower ply has fibers oriented at -45 relative to the direction of the fibers of the first ply, the third ply has

  fibers oriented at +45 compared to fibers of the first ply and fibers of the qua-

  third fold, closest to the core 10, are oriented at 90 relative to the fibers

  of the first fold. The back laminate 14 of the sandwich has elementary layers

  hides or folds whose fibers have the same relative orientations, the outermost fold

  laughing having fibers oriented parallel to those of the outermost fold of the

laminated before 12.

  On the outer surface of the front laminate 12, an aluminum layer 16 is deposited, this aluminum layer being for example deposited in the vapor phase. It allows electromagnetic waves to be reflected with a minimum of

radio losses.

  - TI According to the invention, on the rear face of the reflector another is deposited

  layer 18, for example metallic, and in the same material, i.e. aluminum

  nium, than layer 16 of the active face.

  The purpose of this layer 18 is to compensate for the bimetallic strip effect resulting from the differences in expansion coefficients between, on the one hand, the metal layer 16 and, on the other hand, the whole, or sandwich, formed of the core 10 and laminates 12 and 14 whose coefficient of thermal expansion is very low. The effect of bimetallic strip results in the fact that, when the temperature of the reflector varies, its curvature changes, this

which alters the accuracy of shape.

  In other words, the layer 18 forms with the sandwich 10, 12, 14 another

  bimetallic strip counteracting the effects of the first bimetallic strip.

  Furthermore, if, at a determined temperature, the curvature of the reflector devoid of the layer 18 does not exactly correspond to the desired curvature,

  the deposition of layer 18 can make it possible to modify this curvature at this temperature.

  determined ture. This change in curvature depends on the temperature at

  which the metal layer is deposited and the thickness of this deposit.

  It has been found that with the reflector according to the invention, if its diameter is of the order of a meter, an RMS error can be obtained on a shape of the order of 15 pm in manufacture and of the order of 30 pm when used on board a satellite. These

  RMS errors are about three times lower than those obtained when the

che 18 is not present.

  We recall here that the RMS factor is defined as follows: we start from a desired perfect surface 20 (FIG. 2) and we determine, at n points of the real surface 22, the deviations dzl, dz2 ..., dzn between the real surface 22 and the desired surface 20. The RMS error then has the value: n dz. RMS = i = 1 n In the case of a reflector of one meter in diameter, provision is generally made

  Around 500 points distributed evenly.

  The curvature compensation effect is obtained by depositing the layer at a determined temperature, for example of the order of 50 C. At this temperature, the stress which is exerted is zero; on the other hand, at a different temperature, for example at an ambient temperature of 20 ° C., there will be a constraint which may compensate for a defect in curvature present before this layer 18 is deposited. The thickness and, possibly, the nature of the coating 18 can be determined using conventional material strength calculations. In fact, account is taken of the fact that the membrane / flexion coupling of the assembly is canceled.

  formed by the various layers of the reflector.

  The nature and thickness of the layer 18 can also be determined.

born empirically.

  To obtain the desired effect, that is to say a minimization of the deformation of the active surface of the reflector during temperature variations, the following parameters can be chosen: the thickness of the front and rear deposits, the nature of the front and rear deposits and the temperatures at which the front layers are deposited 16

and rear 18.

  FIG. 3 is a comparative diagram showing RMS defects obtained for several types of material and, in each case, with and without the rear layer 18. In this example, a reflector with a diameter of 600 mm is used

  with prepreg type M55J / 914 skins (M55J is a charac-

  verifying the nature of the fibers and 914 is a reference characterizing the epoxy resin

  used), and a 15 mm thick aluminum honeycomb core.

  The thickness e in micrometers is plotted on the abscissa, of the metal deposit on the front face and, if appropriate, on the rear face and, on the ordinate, the RMS error, also in micrometers. The straight line 30 corresponds to the variation curve as a function of the thickness of the RMS error for an aluminum deposit only on the front face while the straight line 32 corresponds to an aluminum deposit on the

  front and rear sides of the reflector.

  Figure 4 is a diagram corresponding to a diameter reflector

  600 mm with M55J skins and a 15 mm thick aluminum honeycomb core

  minimum. In this case, the metal deposit on the front face has a thickness of

  4 µm aluminum. On the rear face of this reflector, metal deposits have been provided.

  various thicknesses and the resulting RMS error was analyzed. On this slide

  gram, we have plotted on the abscissa, the thickness e of the metal deposit on the rear face

  and, on the ordinate, the RMS error, both being expressed in micrometers.

  In FIG. 4, curve 34 shows that the RMS error is minimal when the thickness of the back layer is of the order of 9 μm, while the thickness of the deposit on the front face is 4 μm. In other words, the optimum is not necessarily

  obtained with thicknesses of deposits 16 and 18 which are equal.

  It is not essential that the metal coating 18, intended to com-

  think of the bimetallic strip effect resulting from the differences in expansion coefficient between the layer 16 and the core and the laminates, that is to say constituted by an external deposit. As a variant

  (not shown), this covering 18 is arranged between two plies of the rear laminate 14.

Claims (7)

  1. Electromagnetic wave reflector for telecommunication antenna, in particular for an antenna on board a satellite comprising a substrate covered with a metallic layer (16) reflecting on its front face, this reflecting layer having a coefficient of thermal expansion different from the coefficient of thermal expansion of the substrate, characterized in that it has, in its rear region, another layer (18) having a coefficient   thermal expansion different from that of the substrate (10, 12, 14).   2. Reflector according to claim 1, characterized in that the layer (18) of the rear zone compensates for the variations, as a function of the temperature, of the shape of the reflector which result from different values of the expansion coefficients of the substrate and the front layer.   3. Reflector according to claim 1 or 2, characterized in that the layer (18) of the rear zone modifies, at a determined temperature, the curvature presented   by the reflector in the absence of this layer.   4. Reflector according to any one of the preceding claims, characterized   in that the layer of the rear zone is deposited on the rear face of the substrate.   5. Reflector according to any one of the preceding claims, characterized   in that the rear area layer is metallic.   6. Reflector according to claim 5, characterized in that the layer of the zone   back is of a metal of the same kind as that constituting the reflective layer front panel.   7. Reflector according to any one of the preceding claims, characterized   in that the membrane / bending coupling of the assembly of the layers of the sub-   strat with the reflective layer and the layer of the rear area is minimized.