DESCRIPTION

The present invention refers to a novel process for obtaining reflectors of electromagnetic waves and to the reflectors obtainable by said process. Such reflectors are particularly suitable to be installed on mobile satellite antennas for terrestrial, aerial and naval use, dedicated to high frequencies (20-300 GHz). The present invention refers in particular to the attainment of the required reflectivity features on composite materials with a surface composition, and optional internal structure, having an epoxy resin matrix, and with adequate features of accuracy and structural rigidity.

STATE OF THE PRIOR ART

In the telecommunications field, by "parabolic antenna" it is meant an antenna having an opening fitted with a reflector, commonly of parabolic shape, and that can be used both in transmitting and receiving. An antenna is a device designed for the purpose of emitting or receiving electromagnetic waves towards open space, by converting an electric signal into an electromagnetic field, and vice versa.

In the case of antennas with a reflector, the transmitting and receiving is done through one or more reflective surfaces, referred to as reflectors.

In many applications, instead of using metallic materials to construct the reflective surfaces, composite materials formed by a conductive fiber (e.g., carbon fibers) and a resin matrix (e.g., polyester or epoxy resins), which compared to metallic materials entail a better compromise among lightness of weight, mechanical performance (e.g., rigidity, dimensional stability) and corrosion strength.

Some of these materials however, like carbon fiber composites, though having some degree of conductivity, have a reflectivity to electromagnetic waves which at high frequencies is influenced both by the asperities of the conductive reinforcement, which depend on both the geometry proper of the same fibers and the fabric, and on the degree of surface finish (roughness Ra in μηι).

Therefore, as frequency values rise and the wavelength shortens, reflectivity to the electromagnetic field degrades to unacceptable values.

One of the processes reported in the state of the prior art for the improvement of the reflectivity of antenna reflectors of composite material (Patents US 2010/288433 A1 and US 8317960 B2, of 2012), consists in incorporating in them a thin metallic layer, by a metallized plastic medium, suitably subdivided into strips, on the inside or on the external surface thereof. This process entails drawbacks, such as the forming of microgaps in the bonding lines between a metallic strip and the contiguous one, the scarce accuracy of the surface obtained, and does not ensure adequate dimensional stability, as the inserted layers have expansion modules different from the remainder of the structure. Moreover, if the reflective surface remains internal to the material, there are remarkable losses of efficiency due to crossing by the electromagnetic waves of the most superficial layers.

Patent DE2061840B describes another method for making antennas of composite material reflective by the inserting of an internal metallic layer, and not on the external surface, deposited by thermal wire spray during an intermediate stage of the lamination process; antennas thus obtained can operate well at low frequencies, which can cross without being significantly absorbed by layers overlapped to the metallic surface, but entail the problem of not being effective at higher frequencies, both due to significant crossing losses in crossing the above-mentioned layers, and to an intrinsic limit of reflective surface accuracy. In fact, the internal layer does not ensure the required tolerances, as it is applied on a surface with overly high geometric and dimensional irregularities. Moreover, during the reflector curing (hardening) stages, the metallic layer (furthermore irregular) can induce permanent setting of the piece, due to different thermal expansion coefficients of the layers, making the reflector inefficient at high frequencies.

Patent US4188358 may be interpreted as a variant of DE2061840B, with the sole substantial difference that metal deposition occurs on a substrate that is only partially hardened, i.e. with a catalysis process not completed upstream of the metallization stage. Antennas thus obtained, due to the hereto-disclosed reasons, cannot be efficient above 20 GHz.

Patent WO 1999028988 A2 describes a reflective mat-type material, that can be suitably included in the composite lamination sequence. This solution entails the drawback of a high resistivity of the reflective surface, not suitable to high frequencies. Object of the present invention is to provide a novel process for obtaining reflectors of composite material (composite) with the required electrical, geometrical and mechanical features for receiving and transmitting at Ka-band frequencies higher than 30 GHz, capable of solving the above-mentioned drawbacks.

SUMMARY OF THE INVENTION

The Inventor has set himself and solved the problem of functionalizing by metallization the surface layer of epoxy resin of a reflector for high frequencies (>20 GHz), made of composite material having a high structural rigidity for mobile (terrestrial and aeronaval) applications.

The RF features at the above-mentioned high frequencies of the reflector thus obtained are equivalent to those of a metal one, concomitantly ensuring the electrical conductivity, accuracy, surface finish, adhesiveness of the metal coating and dimensional stability suitable and required to use on mobile terrestrial and aeronaval systems, therefore subjected to mechanical and vibrational stresses under oft-times very hostile environmental conditions, like e.g. operating temperatures of between - 30°C and +60°C, 100% relative humidity, saline atmospheres, etc.

The reflectors designed on the basis of good engineering norms and on what known by the prior art, when mounted on terrestrial, aerial and naval mobile means of transport with the wire spraying technique inside the composite structure, are not capable of establishing an effective, stable and reliable satellite communication in transmitting and receiving at high frequencies (from 20 GHz up), or of obtaining an electromagnetic radiation diagram compliant with International Standards in Ka-band or higher (ref. ITU-R S.527-7, see Fig. 1). The Inventors set themselves and solved the problem, first of all by bringing the metallized surface from inside the composite structure to the outer surface of the material. However, this solution, though increasing antenna gain, and preventing signal loss or dispersion, like e.g. a gain loss and/or polarization change, etc. - due to the crossing of fibrous-structure dielectric layers interposed to the reflective surface, alone is not capable of giving an adequate answer to the problem. It has surprisingly been found that in order to reach the above- mentioned required performance it was necessary to converge and numerically determine various physical, mechanical and electromechanical parameters strictly interdependent thereamong. Seven parameters of the reflectors obtained by the process of the present invention were identified and measured by the Inventors:

1 . ELECTRICAL RESISTIVITY: R < 0.1 ohm per meter [Ω-rn] between any 2 points of the reflective surface.

2. RELATIVE ACCURACY, or dimensional and geometric tolerance of the outer surface of the coating relative to the nominal profile, expressed as the deviation 'RMS', in relation to the diameter of an opening 'D _A ' of the reflector, such that: RMS/D _A < 2.5x10 ^"4 . The RMS (root mean square) value equals the standard deviation or mean quadratic deviation of the real points of the reflector relative to its ideal mathematical profile. The diameter 'D _A ' [m] equals the diameter of the circle having the same opening (effective area) of the reflective surface.

3. FINISH of the outer surface of the coating: roughness Ra < 14 [micron]

4. Surface HARDNESS, in Shore-D, of the epoxy matrix layer concerned by the metallization coating, between 69 and 98 (ISO 868)

5. nominal THICKNESS Ύ of the metal coating: 50 < t < 300 [micron]

6. Flexural rigidity 'D' of the equivalent flat plate to the structural panel of the reflector, relative to the square of the diameter of the aperture 'D _A ' of the antenna (as defined at step 2.): 10 < (D / D _A ² ) < 1500 [N/m]. The flexural rigidity of a plate is a measure of the strength of the same plate to a bending deformation. In the isotropic case, for a plate of thickness s, Young's modulus of elasticity E and Poisson's ratio v, D = E s ³ / 12 (1-v ² ) [N m].

7. ADHESIVENESS of the metal coating according to ASTM D4541 (Pull-off): >5 [Mpa]

The above-mentioned parameters, and in particular parameters 4 and 7, have surprisingly represented a novelty compared to the prior art, since they proved essential, as reported in examples 1 , 2 and 3, in order to ensure a radiation diagram in accordance with Standard ITU-R S.527-7 (FIG. 1) suitable for high frequencies in Ka- band or higher. Moreover, it has to be stressed that all the abovecited parameters not only are not scientifically and numerically defined by the prior art, but are even omitted, and when cited are defined on a semi-quantitative or even simply qualitative basis. Finally, parameters 6 and 7 have proven essential for reflectors fitted on terrestrial, naval and aerospace mobile vehicles; parameter 6 in particular prevents the onset of the phenomenon of resonances generated by mobile vehicle vibrations, which can compromise the operation peculiar to the reflector regardless of the mechanical bearing system on which it is fitted, a problem not reported in the prior art which makes reference to stationary reflectors used for low and medium frequencies (< 20GHz). Parameter 7 above reaches the above-mentioned values on epoxy resin when the surface is treated, prior to metallization, with a sandblasting or shot-peening process; this process ensures a superficial micro-hammering, performed cold, which makes the surface itself uniform and homogeneous, producing a residual straining state of compression in the surface and subsurface layer of the material. Moreover, this allows optimum mechanical gripping of the metal coating, ensuring a greater tolerance to thermomechanical stresses and a longer duration in time of the coating.

The making of the reflector and of the antenna object of the present invention was possible by utilizing peculiar design and constructive parameters, very different from the initial ones provided by the prior art.

Upon quantifying and confirming the seven above-mentioned parameters, the Inventors have set themselves the problem of optimizing the processes correlated to the subjects involved in the present invention, among which materials science and technology, in particular of composites, and the receiving and transmitting of electromagnetic waves at high frequencies, higher than 20GHz, to obtain a reflector certifiable on the basis of International Standards of communication in Ka-band and higher.

The following Table 1 reports the maximum emission limits allowed for the Ku-band (10-15 GHz), expressed as EIRP (Effective Isotropic Radiated Power) and referred to a 40-Khz band. The reference Standard is ITU-R S.524-7. Radiated power limits are of course referred to the angle of displacement relative to the main axis of radiation, and must be complied with for all polarization planes of the antenna.

Table 2 reports the maximum limits of emission allowed for the Ka-band (15-40 GHz), expressed as EIRP and referred to a 40-Khz band - reference standard ITU-R S.524- 7. Here, parameter N is introduced to take into account the coexistence of plural stations simultaneously transmitting to the same satellite, in the case of spread spectrum/CDMA-type modulation technologies. Typically, for time division multiple access (TDMA) systems said parameter is equal to 1 and can be ignored.

Table 1: Acceptability limits for Ku-band emission according to ITU-R S.524-7, expressed as

EIRP on a 40 Khz band

IS

ft 4 es * <

Table 2: Acceptability limits for Ka-band emission according to ITU-R S.524-7, expressed as

EIRP on a 40 Khz band

2S -- S$ ¾C M ^; : : ··· * ^ί φ:¾;

■ :'■! ·· : S. )···«· H: ¾sk¾ * -·¾ isi ieg

As it may be noted, limits set for the Ka-band are much stricter (20 dB lower) since antenna gains are much greater, thereby increasing the risk of interference on adjacent satellites.

As shown in Figures 2 and 5, antennas with a reflector lacking the features of the reflectors of the present invention do not efficiently receive and transmit at 30 GHz frequencies (Figures 2 and 3), whereas antennas with the reflectors according to the present invention receive and transmit efficiently even at 30 GHz frequencies (Figures 4 and 5).

Figure 6 shows a graph of the results of the dimensional relief of a reflector. With modern three-dimensional scanning techniques it was possible to check the actual shape of the reflective surfaces, also assessing their deviations from nominal shape, to be able to characterize accuracy parameter effects on antenna performance.

Therefore, object of the present invention are: A reflector for mobile parabolic antennas or in the form a paraboloid of rotation, in particular for antennas for terrestrial, aerial, or naval use, suitable to receive and transmit at frequencies higher than 20 GHz, in particular of between 20 and 300 GHz, comprising:

-a surface layer of epoxy resin or composite material consisting of a fiber in epoxy resin matrix having a thickness between 10 and 250 μηι and wherein said surface layer has an outer metal coating having a thickness between 50 and 300 μηι;

and wherein said reflector is characterized by the following parameters:

-an electrical resistivity R of less than 0.1 ohms per meter [Ω-rn];

-a relative accuracy expressed as the deviation 'RMS', in relation to the diameter of an opening 'D _A ' of the reflector (RMS/DA ) of less than 2.5 x10 ^"4 ;

-a roughness Ra of the outer surface of the metallic coating of less than 14 μηι;

-a surface hardness of the metal-coated epoxy layer, between 69 and 98 Shore-D, measured according to the standard procedure ISO 868;

-a flexural rigidity 'D' of the flat plate equivalent to the structural panel of the reflector relative to the square of the diameter of the aperture 'D _A ' of the antenna (D/D _A ² ) between 10 and 1500 [N/m];

-an adhesiveness of the metal coating on said epoxy layer greater than 5 [Mpa], measured according to the standard procedure ASTM D4541 (Pull-off).

A parabolic antenna comprising a reflector according to the present invention.

A process for the manufacturing of a reflector according to the present invention, comprising the following steps:

a. subjecting to a step of curing in a mold a reflector of epoxy resin or composite material consisting of a fiber in epoxy resin matrix, wherein the outer surface layer has a thickness between 10 and 250 μηι;

b. performing a step of abrasion of said surface layer so as to remove a layer of at least 3 μηι;

c. performing a step of deposition of a metal coating on the outer surface of the reflector prepared at step b) by thermal wire spray technique with one or more phases of cooling on the acceleration stream of the metal particles and/or on the surface of the parabolic dish of said antenna, whereby the reflector obtained by said process has a metal outer coating with a thickness between 50 and 300 μηι and a surface hardness of the metal-coated epoxy layer between 69 and 98 Shore-D.

Still further advantages, as well as the features and the modes of employ of the present invention, will be made apparent in the following detailed description of some preferred embodiments thereof, given by way of example and not for limitative purposes. Reference will be made to the figures of the annexed drawings. BRIEF DESCRIPTION OF THE FIGURES

Figure 1 reports an example of radiation pattern (diagram) taken from ITU-R S.527-7 standards.

Figures 2 and 3 show the azimuth radiation pattern (diagram), for angles comprised between -30 and +30 degrees (Fig. 2); and the detail between -10° and +10° (Fig. 3), at 30 GHz frequency, of a parabolic antenna constructed of carbon fiber composite with epoxy resin matrix. The antenna proved to be not compliant with operation at 30 GHz according to ITU-R S.527-7 regulations.

Figures 4 and 5 show the 30 GHz-frequency azimuth radiation pattern (diagram) in a frequency of the same antenna of Figures 2 and 3 (for angles comprised between -30 and +30 degrees (Fig. 4); and the detail between -10° and +10° (Fig. 5) after the reflectors were manufactured in accordance with the process of the present invention, according to the embodiment described in the examples. The antenna proves to be perfectly compliant with standards at 30 GHz (+/-30 GHz and +/- 10 GHz scale) for angles comprised between -30 ° and +30° (Fig. 4); and the detail between -10° and +10° (Fig. 5).

Figure 6 shows a graph of the results of the dimensional relief of a reflector obtained with the processes described in the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The process for the manufacturing of reflectors according to the present invention comprises various steps that will hereinafter be described in detail according to various embodiments:

The process comprises a step a) wherein the mold of a reflector of epoxy resin or composite material consisting of a fiber in epoxy resin matrix is subjected to a step of curing (hardening), the outer surface layer will have a thickness between 10 and 250 μηι, preferably between 10 and 50 μηι. According to one embodiment the pre- impregnated (prepreg) fabrics, laid onto the mold according to a suitable sequence of layers, are subjected to the step of curing in a vacuum bag in an autoclave or alternatively by direct infusion of the epoxy resin. In the embodiment in a vacuum bag in an autoclave the conditions used could be, e.g., with an external pressure between 3 and 6 bars, holding a temperature between 90 and 120°C, preferably for a time of at least 60 minutes, and even more preferably with ascent and descent ramps not greater than 2°C/min. E.g., carbon or glass fibers reinforced with epoxy resin could be used, with a surface layer between 10 and 250 micrometers (μηι), preferably between 10 and 50 μηι. Examples of typologies of suitable epoxy resins are, for instance of DT121 R-47 and AX002-240-600F type, commercially available from DeltaPreg. The properties of the carbon fabric impregnated with epoxy resin will preferably be in accordance with the directions of UNI and ASTM Standards. Preferably, antenna surfaces wherein the carbon fabric has an areal weight comprised between 190 and 630 g/m ² , preferably between 190 and 240 g/m ² (measured according to Standards UNI EN 12127) could be used. Preferably, the carbon fibers of said antennas will have a tensile strength of at least 1700 MPa (measured by ASTM D-3039 testing). For the glass fibers, fibers with an areal weight e.g. of about 300 g/m ² could be used.

The process also comprises a step b) of abrasion of said surface layer so as to remove a layer of at least 3 μηι. To improve mechanical adhesiveness of metal particles on the epoxy resin, the process of the present invention will comprise a step, preceding the metallization (metal deposition) step, consisting in the removal from all the surface of the reflector of an epoxy resin of at least 3 micrometers, preferably a layer comprised between 3 and 6 micrometers. For instance, in the case of satellite dishes of composite material whose surface is comprised of a pure epoxy resin layer of a thickness between 10 and 50 μηι, a layer of epoxy resin equal to 3-4 μηι could be removed with a sandblasting process. Abrasion could be performed, e.g., with a sandblasting process having average grain size of 100 μηι, capable of ensuring an end surface roughness equal to 3-4 μηι and a surface penetration of the metal nano- and microparticles suitable to create, on the same surface of the paraboloid, a perfect mechanical adhesiveness greater than about 5 MPa (Standard ASTM D4541 procedures), preferably comprised between 5 and 10 MPa.

The process also comprises a step c) of deposition of a metal coating on the outer surface of the reflector prepared at step b) by thermal wire spray technique, so as to obtain a metal coating on the surface layer of epoxy resin or of composite material of epoxy resin comprised between 50 and 300 μηι. Thermal wire spray generates molten or semi-molten metal microparticles in a plastic state, from a metal wire of Al, of Cu or other metal which is partially melt by the flame and then atomized in the form of microparticles, accelerated in the same nozzle of the system with an air stream or an inert gas stream until impacting on the surface to be coated.

The step of metal deposition by thermal wire spray is characterized by one or more phases of cooling. The phase of cooling could be obtained on the acceleration stream of the metal particles and/or on the surface of the parabolic dish of the antenna; for instance, by blowing compressed air and/or compressed air mixed with crystals of solid C0 ₂ on the paraboloid surface not concerned by the coating, or in a refrigerated chamber (e.g., between -5 °C and 5°C). With this system it is possible to keep low the surface temperature of the satellite dish where it is present the epoxy resin having a glass transition temperature Tg « Tf at the melting temperature Tf of the metals to be deposited to cover the surface of the reflector. At the same time, a uniform coating of metal nano- and microparticles is obtained which, by impacting on the same surface without altering it, allows perfect maintaining of the parabolic geometric form, enabling optimum receiving/transmitting. In the step of deposition by thermal wire spray, oxide- acetylenic nozzles, or nozzles for oxygen mixed with combustible gases (such as, e.g. GPL, methane, propane) will preferably be used. Moreover, the acceleration fluid comprised of compressed air, nitrogen or argon, and of metal particles, is kept at a temperature lower than the <Tg of epoxy resin in order to prevent even very tiny deformations of the reflector which might nullify the parabolic geometry of the antenna and therefore its performance in the Ka- or higher band. The coating rate is a combination of rate vectors between the motions of the anthropomorphic robot, which could be, e.g., in a range comprised between 10-60 mm/sec and the rotation rate of the reflector, which could be comprised, e.g., between 1000-5000 mm/sec with a distance from the surface to be coated preferably of about 5-20 cm. In the process of the present invention various metals or metal alloys could be used, like e.g. Al, Zn, steel (for instance of the AISI 304, 316 and 316L series) Cu, Ag, Au, Ni/AI and SbSn alloys.

According to one embodiment of the process, to further improve the surface of the metal coating and to make it suitable to use for >150 GHz frequencies, the process could provide a (manual or automatic) final finishing process of the metal coating in order to obtain a mean surface roughness (Ra) comprised between 0.3 and 1 micrometers (μηι), to be carried out, for instance, with 3M abrasive papers for grinding having a decreasing grain size in the P200-P1200 range, giving to the same surface of the paraboloid a mirror finishing, i.e. a mirror-like surface having a mean surface roughness (Ra) equal to 1/1000 with respect to the 1-mm wavelength when operating at 300 GHz. Instead, a 7-15 μηι surface roughness (Ra) is obtained directly at the output from the process of metal deposition, without further finishing operations, it also with a value equal to about 1/1000 of 1 cm, referred to the 1-cm wavelength for 30 GHz frequencies. Hence, with the process provided by the present invention it is possible to cover a very wide range of frequencies (up to 300 GHz). Therefore, the process described herein enables to obtain a reflective surface (reflector) for an antenna with a resistivity in ohms near to zero in any point of the satellite dish. At the same time, the parabolic antenna will have physical and mechanical features of rigidity, accuracy, etc. described in the preceding steps. The process of coating of the present invention can be applied to parabolic antennas of different diameter, preferably from 30 cm to 10 m, for instance with parabolic antennas of composite material in epoxy resin matrix of 1 ,2,3,4,5,6,7,8 m. Comparative experiments showed that other processes for the manufacturing of reflectors do not afford reflective surfaces of antennas with the advantages had by the surfaces obtained.

Therefore, a further object of the present invention are reflective surfaces (reflectors) of parabolic antennas with a metal coating obtainable according to the above- described process and the parabolic antennas comprising them. The reflector for mobile parabolic antennas obtainable by said process is apt to transmit at frequencies higher than 20 GHz and comprises;

-a surface layer of epoxy resin or composite material consisting of a fiber in epoxy resin matrix having a thickness between 10 and 50 μηι and wherein said surface layer has an outer metal coating, obtained by thermal wire spray, having a thickness between 50 and 300 μηι

and is characterized by the following parameters:

-an electrical resistivity R of less than 0.1 ohms per meter [Ω-rn];

-a relative accuracy expressed as the deviation 'RMS', in relation to the diameter of an opening 'D _A ' of the reflector (RMS/DA ) of less than 2.5 x10 ^"4 ;

-a roughness Ra of the outer surface of the metallic coating of less than 14 μηι;

-a surface hardness of the metal-coated epoxy layer, between 69 and 98 Shore-D, measured according to the standard procedure ISO 868;

-a flexural rigidity 'D' of the flat plate equivalent to the structural panel of the reflector relative to the square of the diameter of the aperture 'D _A ' of the antenna (D/D _A ² ) between 10 and 1500 [N/m];

-an adhesiveness of the metal coating on said epoxy layer greater than 5 [Mpa], measured according to the standard procedure ASTM D4541 (Pull-off). Preferably, the conductive fiber of the composite material is a glass fiber or carbon fiber and the metal coating is of a metal or metal alloy selected from the group of Al, Zn, steel, Cu, SbSn Ag, Au, Ti, Cr, Ni molybdenum, Inconel, Monel, nickel/aluminum, bronze.

Parabolic antennas comprising the reflectors according to the present invention could advantageously be used for aboard communications at sea, radars, radiometers, radio telescopes and Earth observation satellites, as well as for other applications.

EXAM PLES

The present description will now be further detailed by the following examples, yet in no way being limited thereto.

EXAM PLE 1

Reflector materials and structure The parabolic reflector used had a circular shape and a diameter equal to 100 cm, and was constructed of GFRP (Glass Fiber Reinforced Polymer) composite material with a reinforcement in glass fiber fabric and epoxy resin matrix, obtained on a mold of composite material with vacuum bag and cure cycle in autoclave. Specifically, the lamination sequence was comprised of two pre-impregnated (epoxy prepreg) fabrics by DeltaPreg manufacturer, VV320P-DT120-32 and VV770-DT120-34, or equivalent ones by other manufacturers. The reinforcement was comprised of two glass fiber fabrics, with an aeral weight respectively equal to 320 g/m ² and 770g/m ² . The epoxy resin was of thermosetting type, with a reference cure cycle equal to 60 min at a temperature of 120°C, and specifically corresponding to the resin system DT120, marketed by DeltaPreg, or equivalent by other manufacturer. The pre-impregnated fabrics, laid onto the mold according to an appropriate sequence of layers, were subjected to the cure process in a vacuum bag in an autoclave, with external pressure between 3 and 6 bars, holding a temperature of 120°C for 60 min, with ascent and descent ramps not greater than 2°C/min.

The reflector thus obtained had a nominal thickness of 3 mm, and the flexural rigidity 'D' of the flat plate equivalent to the constructed structure was equal to about 75 N m . The main features of the matrix and of the reinforcement of the VV320P-DT120-32 material used as surface layer are reported in the following table:

Surface preparation and deposition of the metal layer on the reflector

Pretreatment of the epoxy resin-impregnated surface to be coated, of surface hardness equal to 87 Shore-D: average removal of 5-6 μηι of resin, with end roughness of the obtained surface (sandblasting with glass beads) of about 3 μηι; the processing was performed in a specially provided soundproofed chamber with closed- circuit ventilation. Steerable nozzles provide sending on the surface of the satellite dish an air stream with glass microspheres having an average diameter of 0.1 mm dispersed therein. A reference sample allows to check that the removed thickness and the surface roughness Ra be correct at the end of the process. The treatment just described consists, at a microscopic level, in a surface hammering, performed cold, which makes the surface uniform and homogeneous and produces a residual compression straining state in the surface and subsurface layer of the material; this allows optimum mechanical gripping of the metal coating, ensuring greater tolerance to thermomechanical stresses and longer duration of the coating over time.

Then, the reflective surface was coated by thermal wire spray technique.

Material utilized for the surface coating: Aluminum wire.

Process fluids: acetylene, oxygen and compressed air.

Cooling: cold compressed air, uniformly conveyed on the outer surface of the reflector.

Workpiece rotational speed: 180-250 rev/min.

Average speed of coating feed with robotized head: 30 mm/sec .

Head distance from surface of the satellite dish: constant in the range of 5-20 cm.

The above-mentioned parameters enable to avoid localized overheating.

Alternatively, an additional thermal shielding can be created with an initial layer of

SbSn alloy of a thickness of≥80 μηι.

The nominal thickness of the aluminum coating at the end of the coating process is of 150 μηι, with ±15 μηι tolerance, without polishing. The end-of-process roughness Ra of the coating surface is of about 11 μηι.

It is possible to perform a polishing process to bring the roughness to lower values (Ra = 0.3 - 1 μΓπ), to increase antenna performance at frequencies greater than 30 GHz; in this case, the nominal thickness of the coating before the polishing must be increased. The aluminum coating has a mean adhesiveness on the epoxy resin surface of about 7 Mpa (according to ASTM D4541 , Pull-off Test), and an electrical resistivity R near to zero, R < 0.1 ohm per meter [Ω-rn] between any two points of the reflective surface. The reflector, inspected by accurate contactless optical measuring systems, has a standard deviation (RMS) of the points of the metal-coated surface, with respect to ideal mathematics, of 0.18mm . EXAMPLE 2

Reflector materials and structure

The parabolic reflector used had a circular shape and a diameter equal to 100 cm, and was constructed of CFRP (Carbon Fiber Reinforced Polymer) composite material with a reinforcement in carbon fiber fabric and epoxy resin matrix, obtained on a mold of composite material with vacuum bag and cure cycle in autoclave. Specifically, the lamination sequence was comprised of two pre-impregnated (epoxy prepreg) fabrics by DeltaPreg manufacturer, GG200P-DT121 H-47 and GG630T-DT120-37, or equivalent ones by other manufacturers. The reinforcement was comprised of two carbon fiber fabrics, with an aeral weight respectively equal to 200 g/m ² and 630 g/m ² . The epoxy resin was of thermosetting type, with a reference cure cycle equal to 60 min at a temperature of 120°C, and specifically corresponding to the resin system DT120 or DT121 , marketed by DeltaPreg, or equivalent by other manufacturer. The pre- impregnated fabrics, laid onto the mold according to an appropriate sequence of layers, were subjected to the cure process in a vacuum bag in an autoclave, with external pressure of 5 bar, holding a temperature of 120°C for 60 min, with ascent and descent ramps not greater than 2°C/min.

The reflector thus obtained had a nominal thickness of 1.8 mm, and the flexural rigidity 'D' of the flat plate equivalent to the constructed structure was equal to about 39 N m. The main features of the matrix and of the reinforcement of the GG200P-DT121 H-47 material, used as surface layer, are reported in the following table:

Surface preparation and deposition of the metal layer on the reflector

Pretreatment of the epoxy resin-impregnated surface to be coated, of surface hardness equal to 90 Shore-D: average removal of 5-6 μηι of resin, with end roughness of the obtained surface (sandblasting with glass beads) of about 3 μηι; the processing was performed in a specially provided soundproofed chamber with closed- circuit ventilation, by steerable nozzles that provided sending on the surface of the satellite dish an air stream with glass microspheres having an average diameter of 0.1 mm dispersed therein. A reference sample allowed to check that the removed thickness and the surface roughness Ra were correct at the end of the process.

The treatment just described consists, at a microscopic level, in a surface hammering, performed cold, which makes the surface uniform and homogeneous and produces a residual compression straining state in the surface and subsurface layer of the material; this allows optimum mechanical gripping of the metal coating, ensuring greater tolerance to thermomechanical stresses and longer duration of the coating over time.

Then, the reflective surface was coated by thermal wire spray technique. Material utilized for the surface coating: Aluminum wire.

Process fluids: acetylene, oxygen and compressed air.

Cooling: cold compressed air, uniformly conveyed on the outer surface of the reflector. Workpiece rotational speed: 180-250 rev/min.

Average speed of coating feed with robotized head: 30 mm/sec .

Head distance from surface of the satellite dish: constant in the range of 5-20 cm. The above-mentioned parameters enable to avoid localized overheating.

Alternatively, an additional thermal shielding can be created with an initial layer of SbSn alloy of a thickness of≥80 μηι.

The nominal thickness of the aluminum coating at the end of the coating process is of 150 μηι, with a ±15 μηι tolerance, without polishing. The end-of-process roughness Ra of the coating surface is of about 11 μηι.

It is possible to perform a polishing process to bring the roughness to lower values (Ra = 0.3 - 1 μΓπ), to increase antenna performance at frequencies greater than 30 GHz; in this case, the nominal thickness of the coating before the polishing must be increased. The aluminium coating has an adhesiveness on the epoxy resin fiber surface of about 7 Mpa (according to ASTM D4541 , Pull-off Test), and an electrical resistivity R near to zero, R < 0.1 ohm per meter [Ω-rn] between any two points of the reflective surface. The reflector, inspected by accurate contactless optical measuring systems, has a standard deviation (RMS) of the points of the metal-coated surface, with respect to ideal mathematics, of 0.13 mm.

Performance-measuring tests conducted on antennas for satellite communications before and after the metallization process described in Example 2

The tests were conducted at a 30 GHz frequency on two identical samples of antenna, before and after the process described in Example 2; both samples were made of CFRP with the same technique, with the surface layer of T300 3K fiber (200 g/m ² ) as described in detail above. Only one of the two samples was coated with the metallization technique according to the embodiment of Example 2.

The graphs of Figures 1-4 show the pattern of the normalized gain of the antenna as a function of the angle on the horizontal plane (azimuth) and therefore provide the diagrams of horizontal radiation, over an angle comprised between -30 and 30 degrees, with respect to the main axis of symmetry of the antenna.

Such diagrams provide a very important indication for checking antenna performance, both in terms of efficiency and of limiting emissions in undesired directions; in fact, satellite antennas for bidirectional communications, by being transmitting, are subject to strict limitations set by International bodies in order to limit spurious emissions toward adjacent satellites, therefore the radiation pattern is a fundamental element (though not the sole one) for checking the compliance with such regulations.

The measurements clearly show a decided improvement in the radiation pattern in favor of the metallized antenna, as evidence of the fact that the performances of the mere carbon fiber tend to decrease with the increasing of the frequency. Such an antenna, in fact, is adapted to work at lower frequencies, having sufficient performances up to the ku-band (10-14 GHz). At 30 GHz the weft fabric of carbon fiber does not provide the required performances anymore, as it becomes partially transparent, and introduces signal diffraction phenomena which "foul" the radiation pattern with emission lobes in undesired directions. This is due to the fact that the wavelength becomes comparable with the dimensions of the weft of the fabric, which does not behave as an ideal surface anymore, by being not perfectly conductive. The metallization treatment described herein ensures perfect electrical conductivity and extremely accurate surface geometry, restoring the radiation pattern to its ideal shape, symmetrical and with extremely controlled and contained side lobes.

In the comparison between Fig. 2 and Fig. 4, in Fig. 2 it is evident the presence of 2 main lobes instead of only one, and the presence of relevant secondary lobes until above 5 degrees, whereas in Fig. 4 a first lobe is had at 0.8 degrees and a second lobe, already quite dampened, at 1.6 degrees, and then there are no further significant lobes.

EXAMPLE 3

Materials and construction

Reflector materials and structure

The parabolic reflector used had a circular shape and a diameter equal to 80 cm, and was constructed of GFRP (Glass Fiber Reinforced Polymer) composite material with a reinforcement in glass fiber fabric and epoxy resin matrix, obtained on a mold of composite material heatable with internal resistors, with VARTM (Vacuum Assisted Resin Transfer Molding) vacuum infusion technique. Specifically, the lamination sequence was comprised of non-impregnated E-glass fiber fabrics, with plain weave and aeral weight equal to 300 g/m ² . The epoxy resin was of thermosetting type, with a reference cure cycle equal to 240 min at a temperature of 65°C, post-360min cure at 80°C, and Tg=85°C. The fabrics, laid onto the mold according to an appropriate sequence of layers and compressed in a vacuum bag, were impregnated by resin sucked by the fluid driving force induced by the same vacuum.

The reflector thus obtained had a nominal thickness of 2.5 mm, and the flexural rigidity 'D' of the flat plate equivalent to the constructed structure was equal to about 20 N m. Surface preparation and deposition of the metal layer on the reflector

Pretreatment of the epoxy resin-impregnated surface to be coated, of surface hardness equal to 85 Shore-D: average removal of 5-6 μηι of resin, with end roughness of the obtained surface (sandblasting with glass beads) of about 3 μηι; the processing was performed in a specially provided soundproofed chamber with closed- circuit ventilation. Steerable nozzles provide sending on the surface of the satellite dish an air stream with glass microspheres having an average diameter of 0.1 mm dispersed therein. A reference sample allows to check that the removed thickness and the surface roughness Ra be correct at the end of the process.

The treatment just described consists, at a microscopic level, in a surface hammering, performed cold, which makes the surface uniform and homogeneous and produces a residual compression straining state in the surface and subsurface layer of the material; this allows optimum mechanical gripping of the metal coating, ensuring greater tolerance to thermomechanical stresses and longer duration of the coating over time.

Then, the reflective surface was coated by thermal wire spray technique.

Material utilized for the surface coating: Aluminum wire.

Process fluids: acetylene, oxygen and compressed air.

Cooling: cold compressed air, uniformly conveyed on the outer surface of the reflector. Workpiece rotational speed: 200 - 260 rev/min.

Average coating feed speed with robotized head: 30 mm/sec .

Distance of the head from the surface of the satellite dish: constant in the range of 5 - 20 cm .

The above-mentioned parameters enable to avoid localized overheating.

Alternatively, an additional thermal shielding can be created with an initial layer of SbSn alloy of≥80 μηι.

The nominal aluminum coating thickness at the end of the coating process is of 150 μηι, with a ±15 μηι tolerance, without polishing. The end-of-process roughness Ra of the coating surface is of about 11 μηι.

It is possible to perform a polishing process to bring the roughness to lower values (Ra = 0.3 - 1 μΓπ), to increase antenna performance at frequencies greater than 30 GHz; in this case, the nominal thickness of the coating before the polishing must be increased. The aluminum coating has an adhesiveness on the epoxy resin fiber surface of about 6 Mpa (according to ASTM D4541 , Pull-off Test), and an electrical resistivity R near to zero, R < 0.1 ohm per meter [Ω-rn] between any two points of the reflective surface. The reflector, inspected by accurate contactless optical measuring systems, has a standard deviation (RMS) of the points of the metal-coated surface, with respect to ideal mathematics, of 0.15mm.