GB2270421A

GB2270421A - Telecommunications apparatus

Info

Publication number: GB2270421A
Application number: GB9317836A
Authority: GB
Inventors: Donald Franklin Foust; Bradley Ross Karas
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1992-09-08
Filing date: 1993-08-27
Publication date: 1994-03-09
Also published as: GB9317836D0

Abstract

Lightweight, distortion-resistant signal-carrying units, including feed horns, waveguides and multiplexers, particularly for satellite communication systems, comprise a body 10 of fiber-reinforced thermoplastic polymer, preferably polyetherimide or fluorinated polyimide, or cyanate ester polymer. A transmissive layer 22 is disposed on adhesion-promoted surfaces of the body. The transmissive layer comprises a primer layer of nickel deposited on the surfaces of the body and an outer, corrosion-resistant layer, preferably of silver, deposited on top of the primer layer. The outer layer may be matched with a mating component to reduce passive intermodulation. <IMAGE>

Description

TELECOMMUNICATIONS APPARATUS The present invention is directed to feed horns and other signal-carrying units used in telecommunication systems and is particularly directed to lightweight, distortion resistant units used in severe thermal stress environments, e.g., space-deployed structures.

The successful operation of satellite communication systems requires the use of components that are capable of withstanding severe, often rapid, changes in the environmental conditions, e.g., rapid thermal transients in response to changes in solar exposure (full sun versus eclipse). Because of these demands, structural units that are acceptable for terrestrial use are often unsuited for spaceborne applications without substantial modifications of hardware design.

This problem is particularly acute with respect to the electrical components of telecommunication systems which employ metallic surfaces for signal coupling functions, e.g., waveguide interfaces, and the radiating components, e.g., surface signal-carrying units. Because of the substantial magnitudes of their coefficients of thermal expansion, metallic structures suffer from inherent dimensional instability. The resulting physical distortion, e.g., warping and bowing of the feed horn and waveguide interfaces, changes the field pattern characteristics of the telecommunication system, thereby adversely affecting its performance. Moreover, repeated thermal cycling of the structures leads to structural fatigue and to eventual separation of components of the system.

One approach for dealing with this problem has been to provide error tolerance performance through the use of a large number of radiator and intercoupled feed units for which a complex support framework is required. This approach is, in effect, a brute force solution, adding weight to the system and considerably increasing its size, both of which significantly increase earth to space transportation costs.

U.S. Patent 4,929,962 discloses the fabrication of a less expensive feed horn from a metal coated epoxy material. The horn body is provided with a progressively varying waveguide sections to achieve broad-band source (10.7-12.75 GHz).

U.S. Patent 4,757,324 discloses an arrangement by which an array of flared feed horns, including closely packed hexagonally shaped flared horns, may be arranged in close proximity.

U.S. Patent 4,700,195 teaches a constraining mechanism for a feed horn that provides a high degree of dimensional stability over a wide thermal range. The constraining mechanism consists of a graphite composite honeycomb structure covered with an epoxy laminate.

U.S. Patent 4,398,200 discloses compensation for cross-polarization of the telecommunication signal by providing feed horns with septa.

Another important aspect of a space-based telecommunications system is its working life. It can be significantly extended by increasing the amount of onboard fuel carried by the satellite for its attitude adjustments.

The attitude of a satellite is defined as its spatial orientation and position with respect to a land- or spacebased receiving and/or transmitting station.

However, due to extremely high payload launching costs there is a limit to how much fuel can be carried by a satellite compared to its gross payload weight, i.e., satellite dead weight plus fuel weight. Therefore it makes economic sense to significantly reduce the deadweight of the satellite without compromising its structural strength or operation. For every gram of deadweight reduced, an equivalent quantity of weight in fuel can be added to the satellite, thereby extending its working life. The present invention is expected to significantly extend the life of a satellite by replacing the conventional metal feed horn array and ancillary signal carrying units with lightweight elements made of fiber reinforced polymer composite.

In one of its aspects, therefore, the present invention is a lightweight, distortion resistant signalcarrying unit for a telecommunication system, said unit comprising a polymer selected from the group consisting of thermoplastic and cyanate ester polymers and an adherent transmissive layer disposed on surfaces thereof, said transmissive layer comprising a nickel layer contiguously disposed on said surfaces and a non-lossy, corrosion resistant, electrically conductive metal layer on top of said nickel layer.

In the drawings, FIGURE 1 is a block diagram showing the units involved in the reception, processing and transmission of signals via a telecommunications satellite.

FIGURE 2 is a frontal plan view, FIGURE 3 a posterior plan view and FIGURE 4 a cross-sectional view in elevation of a feed horn incorporating the preferred embodiment of the invention.

FIGURES 5 and 6 are enlarged cross-sectional views of the walls of the preferred embodiment and another embodiment, respectively, of feed horns of the invention.

FIGURES 7-10 are frontal plan views of various embodiments of feed horns of the invention.

The present invention is adaptable to essentially any signal-transmitting unit of a telecommunication system. Illustrative of such elements are feed horns, waveguides and multiplexers. It is particularly adaptable to feed horns and will be described hereinafter with particular reference to them; however, those skilled in the art will recognize that waveguides, multiplexers and the like can be constructed similarly except for differences in shape.

A highly preferred embodiment of the invention, therefore, is a feed horn for a telecommunication system, said horn being fabricated from a polymer selected from the group consisting of thermoplastic and cyanate ester polymers, comprising a hollow shaped feed horn body having a first opening at one end and a second opening at the other end, the hollow shape being uniformly convergent from the first opening to the second opening, a flange surrounding the second opening of the body, securing means disposed on the flange for securing the feed horn to a waveguide interface and an adherent transmissive layer disposed on surfaces of said feed horn.

Reference is now made to FIGURE 1 which is a block diagram of typical units on a telecommunication satellite directly involved in the collection, processing and transmission of signals. One or more incoming reflectors pick up the signals and reflect them (through space, as indicated by the dashed line) to an array of incoming feed horns. They are then passed via waveguide means, designated W, to receiving means, thence to amplifying means and finally to a multiplexing system generally comprising a plurality of incoming and outgoing multiplexing units separated by a further amplifier. The output of the multiplexing system is sent, again via waveguides W, to an array of outgoing feed horns which send it, again through space, to one or more outgoing reflectors which reflect it to a receiving station on earth or to another telecommunication satellite.

Thus, another aspect of the invention is a telecommunication system of a telecommunication satellite, comprising: at least one incoming or outgoing reflector for reflecting the incoming telecommunication signals, said signals generally being in the microwave frequency range; at least one lightweight, distortion resistant incoming or outgoing feed horn located in the focal region of said reflector and having its axis pointed at said reflector; at least one waveguide comprising transmissive metal at least in part, any such waveguide which is secured to said feed horn having said transmissive metal matched to the metal on said feed horn to reduce passive intermodulation, said waveguide being effective to transmit signals received by said telecommunication system; receiving means for the incoming signal; transmitting means for the outgoing signal; and signal processing means including at least one multiplexer comprising metal at least in part; wherein at least one feed horn, waveguide or multiplexer comprises a polymer selected from the group consisting of thermoplastic and cyanate ester polymers and an adherent transmissive layer disposed on surfaces thereof.

Most often, said telecommunication system comprises both incoming and outgoing reflectors and feed horns, and all feed horns, waveguides and multiplexers comprise said polymer and adherent transmissive layer.

The feed horn(s); preferably an array thereof, are generally positioned adjacent to each other with the axis of each feed horn being pointed at the incoming or outgoing reflector. The waveguide interface(s) is generally located on a tower structure of the satellite. Usually one waveguide is used per feed horn. Separate telecommunication systems may be employed for bland (about 3.65 GHz) and Ku-band (about 14.5 GHz) microwave frequencies that are generally used by the telecommunication satellites.

Referring now to FIGURES 2-4, there are shown in various views a feed horn, generally indicated by numeral 1, with the cross-sectional view of FIGURE 4 being taken along the line 3-3 of FIGURE 3. Feed horn 1 comprises a hollow shaped feed horn body 10 having a first opening 12 at one end and a second opening 14 at the other end. Body 10 uniformly converges from first opening 12 to second opening 14, and has the shape of a multilateral frustum, preferably pyramidal. The dimensions of body 10 may be adjusted to optimize the land area covered by the telecommunication satellite.

As shown in FIGURES 2 and 4, body 10 may be provided with one or more septa 20 on an inner side thereof.

Septum 20, preferably shaped as a rib, is integrally connected to the inner side of body 10. The length, shape, thickness and location of septum 20 may be adjusted to compensate for cross-polarization that can occur during reception and transmission of the telecommunication signal through body 10.

Body 10 is further provided with a flange 16 that surrounds second opening 14. Flange 16 is preferably an integral part of body 10. The shape and size of flange 16 are preferably matched to those of a waveguide interface (not shown) for securing horn 1 thereto. Securing means, disposed on flange 16, preferably comprise a plurality of openings 18, as shown in FIGURE 3, for the entry of bolt means. Said securing means facilitate attachment and detachment of body 1 to and from the waveguide interface.

Other securing means, such as threads on openings 18 in combination with screws, may be employed to secure horn 1 to the waveguide interface.

Horn 1 is fabricated of a thermoplastic material, such as polyphenylene sulfide, polyarylsulfone, polyamideimide, polyetherketone or polyimide (including fluorinated polyimide) and mixtures thereof, or a cyanate ester polymer. Thermoplastics are often preferred by reason of their ease of fabrication, with polyetherimide and fluorinated polyimide being particularly preferred. If a thermoset unit is desired, however, cyanate ester polymers may be employed.

The structural strength of horn 1 is significantly improved by reinforcing the polymer material with lightweight reinforcement such as carbon or glass fibers. Carbon fibers woven as a web and impregnated with polymer are preferred.

Horn 1 may be fabricated by injection molding, especially when a thermoplastic resin is employed.

However, it is preferably fabricated by pressing multiple layers, e.g., a stack of carbon fiber webs impregnated with polyetherimide, against a contour shape, followed by heating and cooling. Details of the aforementioned process are disclosed in U.S. Patent 4,888,234, incorporated herein by reference. It is expected that a feed horn fabricated from carbon fiber-reinforced polyetherimide will be about 30-50% lighter than a metal feed horn of identical size and shape.

A plurality of feed horns 1 (usually a cluster) may be space-based, e.g., on a satellite in a geosynchronous orbit, or land-based, e.g., on a transmitterlreceiver. A space-based feed horn may be subjected to extremely rapid fluctuations in temperature (e.g., -60 C to +100 C), and a land-based feed horn to severe terrestrial conditions, such as hot and arid conditions of a desert or humid conditions of a tropical forest.

Feed horn 1 further comprises a transmissive layer 22 contiguously disposed on its surfaces. As shown in FIGURES 2 and 4, contiguously disposed transmissive layer 22 is preferably located on surfaces 19 on the inner side of horn body 1 and surfaces 21 of septum 20. FIGURE 5 is a cross-sectional view of a preferred embodiment of the invention in which transmissive layer 22 includes a primer layer 23 contiguously disposed on surfaces 19 and 21 of body 10 and septum 20 respectively.

The reliability of feed horn 1 is dramatically improved by increasing adhesion between primer layer 23 and surfaces 19 and 21. Therefore, surfaces 19 of body 10 and surfaces 21 of septum 20 are preferably subjected to a process of promoting adhesion of a metal layer thereon. For example, adhesion of a metal layer to polyetherimide surfaces may be improved by the processes disclosed in U.S.

Patents 4,842,946, 4,873,136, 4,959,121 and 4,999,251, all incorporated herein by reference. Adhesion of a metal layer to fluorinated polyimide surfaces may be improved by the process disclosed in U.S. Patent 5,124,192, also incorporated herein by reference. Said patent discloses a process for adhesion promoting the deposition of a metal layer on fluorinated polyimide surfaces by subjecting said surfaces to a series of treatments including initial contact with sulfuric acid, contact with aqueous base such as potassium hydroxide, oxidation with alkali metal permanganate such as potassium permanganate, and surface treatment with a reducing agent such as a hydroxylamine salt.

Primer layer 23 may be of nickel or copper.

Nickel, particularly an electroless nickel layer having a thickness in the range of about 0.1-2.0 microns, is preferred.

Transmissive layer 22 further includes an electrically conductive layer 24 disposed on top of primer layer 23. Said conductive layer 24, preferably a corrosion resistant layer, carries the signal received or transmitted by the telecommunication system. Surfaces 19 and 21, primer layer 23 and conductive layer 24 are sufficiently smooth to prevent distortion of the telecommunication signal during its transmission through hom 1. To improve the smoothness of surfaces 19 and 21, a polymer layer may be added to even out the irregularities of the carbon fiber web used for fabricating horn body 10.

Conductive layer 24 may be formed of non-lossy corrosion resistant metal, such as silver, gold or platinum.

Silver is preferred. Silver is generally applied on primer layer 23 by well known methods such as electroplating, electroless plating or chemical vapor deposition.

Electroplating is preferred. The conditions of electroplating may be chosen to achieve a uniformly smooth conductive layer 24.

The thickness of electrically conductive layer 24 is typically determined by the signal frequency and the conductivity of the metal thereof. The minimum thickness for transmission of a signal is determined by the following formula:

wherein: d is one "skin depth" thickness of outer layer 24 in meters required for reliable transmission of the signal; d is one "skin depth" thickness of outer layer 24 in meters required for reliable transmission of the signal; m is the permeability of outer layer 24 in micro Henrys per meter; s is the conductivity of the metal of outer layer 24 in Siemens per meter; and F is the frequency of transmission in Hertz (about 3.65 to about 14.5 GigaHertz).

To ensure reliability of transmission, the value of d is generally multiplied by a factor of safety, preferably 5. Thus, the thickness of conductive layer 24 is in the range of about 2.5-7.5 microns for the aforementioned frequency range.

In order to further improve the reliability of transmission, conductive layer 24 is preferably corrosion resistant and matched to a conductive layer disposed on the mating component, such as the waveguide interface, to reduce passive intermodulation (PIM), defined as electronic noise generated due to dissimilarities of metal layers between the feed horn and the mating component. A match is preferably achieved by using the same metal for conductive layer 24 as is used for the electrically conductive layer applied to the mating component to which feed horn 1 is secured.

In another embodiment of the invention, shown in FIGURE 6, transmissive layer 22 comprises primer layer 23 contiguously disposed on surfaces 19 and 21 of horn body 10 and septum 20, respectively; an electrically conductive middle layer 26 disposed on top of primer layer 23; a barrier layer 28 disposed on top of middle layer 26; and an external corrosion resistant layer 30 disposed on top of barrier layer 28.

Primer layer 23 in FIGURE 6 is preferably the same as the one disclosed in the aforementioned preferred embodiment. Electrically conductive middle layer 26 may be formed of non-lossy metal such as copper, silver, gold, or platinum; copper is preferred. Copper is generally applied to primer layer 23 by such well known methods as electroplating, electroless plating or chemical vapor deposition. Electroless plating is preferred. The thickness of electrically conductive middle layer 26 is typically determined as described hereinabove with reference to formula (1).

Barrier layer 28 may be formed of metal such as palladium or platinum; palladium is preferred. Palladium is generally applied on middle layer 26 by methods such as electroplating, electroless plating or chemical vapor depositing; electroless plating is preferred. The thickness of barrier layer 28 is sufficient to prevent migration of the metal of middle layer 26 into external layer 30. Thus, the thickness of barrier layer 28 in palladium is generally in the range of about 0.1-2.0 microns.

External layer 30 shown in FIGURE 6 may be formed of non-lossy corrosion resistant metal, such as silver, gold or platinum. Silver is preferred. Its conditions of deposition are the same as those previously described; typical thicknesses are about 0.1-1.0 micron.

FIGURES 7-10 show feed horn shapes constituting other embodiments of the invention. In FIGURE 7, the shape is of a hollow rectangular pyramid 32 with a single centrally disposed septum 34 that connects the two longer sides of the rectangle forming the base of the frustum of pyramid 32. It is contemplated that septum 34 may be also connect the two shorter sides of the rectangle. may be also connect the two shorter sides of the rectangle.

The embodiment of FIGURE 8 features the shape of a hollow rectangular pyramid 36 with no septum. In FIGURE 9, the feed horn has the shape of a hollow cone frustum 38; and in FIGURE 10 it has the shape of a hollow hexagonal frustum 40. A feed horn array comprising frustums 40 has the additional advantage of being adapted to close packing, i.e., like a beehive.

It will be readily apparent that the waveguides and multiplexers employed according to the invention will preferably have the same polymer material and transmissive layer described hereinabove with respect to feed horns. Maximum fuel capacity is achieved by having all signal-transmitting units constructed in accordance with the invention, which provides for minimum payload weight.

The invention will be further understood from the examples which follow. In said examples, plaques made of Cypacs 7005 carbon fiber reinforced polyetherimide, sold by American Cyanamid Company, having dimensions of 25.4 x 76.2 x 0.5 mm., were treated according to the described procedures. All temperatures, unless stated otherwise, are ambient.

Example 1 1. Clean for 5 minutes in a 2% (by volume) aqueous solution of Micron cleaner &commat; 50"C.

2. Rinse for 2 minutes in water.

3. Air dry with compressed air for 30 seconds.

4. Contact for 2 minutes with concentrated HNO3 (70%) mixed with 0.1% (by weight) of Fluoradd9 FC-95 fluorinated su rfactant.

5. Rinse for 5 minutes with aerated water.

6. Air dry with compressed air for 30 seconds.

7. Contact for 30 seconds with concentrated (reagent grade) H2SO4.

8. Rinse for 2 minutes with aerated water.

9. Contact for 5 minutes with 5M KOH.

10. Contact for 5 minutes &commat; 75"C with an aqueous KMnO4 (15 g./l.) solution in 1.2 ji KOH.

11. Rinse for 2 minutes with aerated water.

12. Contact for 5 minutes &commat; 52"C with Shipley Circuiposit < 9 MLB 216 neutralizer.

13. Rinse for 2 minutes with aerated water.

14. Contact for 5 minutes with Shipley 1175A Cleaner/Conditioner &commat; 750C.

15. Rinse for 2 minutes with non-aerated water.

16. Contact for 1 minute with Shipley Cataprepo 404 predip.

17. Contact for 3 minutes with Shipley Cataposits 44 catalyst &commat; 45 C.

18. Rinse for 2 minutes with non-aerated water.

19. Contact for 3 minutes with Shipley Accelerator 19.

20. Rinse for 2 minutes with non-aerated water.

21. Contact for 18 minutes with Enthone Enplate Ni-426 electroless nickel bath &commat; 52"C and a pH of 6.2.

22. Rinse for 2 minutes with non-aerated water 23. Air dry with compressed air for 30 seconds.

24. Heat-treat for 2 hours &commat; 75"C.

25. Electroplate &commat; 10 amperes per square foot (ASF) for 1 minute using Technic Silversene "C" strike silver bath.

26. Electroplate &commat; 5 ASF for 22.5 minutes using Technics Silversene "C" build silver bath.

27. Rinse for 2 minutes with aerated water.

28. Air dry with compressed air for 30 seconds.

29. Heat-treat for 2 hours &commat; 11000.

Example 2 The same process parameters as described in Example 1 were used, except that in step 21 the plaques were contacted for 12 minutes with Enplates Ni-422 electroless nickel bath &commat; 82"C and a pH of 4.8.

Example 3 Steps 1-24 were as in Example 1.

25. Contact for 30 seconds with 10% (by volume) aqueous H2S04.

26. Contact for 3.5 hours with Shipley CupositO 251 electroless copper bath &commat; 480 C.

27. Rinse for 2 minutes with aerated water Heat treat for 16 hours &commat; 110"C.

28. Contact for 1 minute with an aqueous solution of 1 g.Il.

of PdCI2 and 0.1% (by volume) HCI.

29. Rinse for 2 minutes with non-aerated water.

30. Contact for 25 minutes with a solution containing 2 g./l. of PdCI2, 4 ml./l. of HCI, 160 ml./l. of NH40H, 27 g./l.

of NH4CI and 10 g.!l. of NaH2PO2 &commat; 50"C.

31. Rinse for 2 minutes with non-aerated water.

32. Contact for 5 minutes with a solution containing 0.457 g./l. of NaAg(CN)2, 1.0 g./l. of NaCN, 0.75 g./l. of NaOH and 2.0 g./l. of dimethylamine borane &commat; 500C.

33. Contact for 30 minutes with a solution containing 1.83 g./l. of NaAg(CN)2, 1.0 g./l. of NaCN, 0.75 g./l. of NaOH, 2.0 g./l. of dimethylamine borane, and 1 mg./l. of thiourea &commat; 50"C.

34. Rinse for 2 minutes with aerated water.

35. Air dry with compressed air for 30 seconds.

36. Heat-treat for 16 hours &commat; 110 C.

In the aforementioned examples, the plated plaques were subjected to the following conditions: 1) 66 hours at 65"C and 100% relative humidity; 2) 5 cycles of a thermal test comprising exposure for 15 minutes &commat; 150"C followed by 15 minutes &commat; -196 C (maximum transfer time between the two extreme temperatures being 10 seconds); 3) 5 cycles of pressure test comprising exposure for 30 minutes &commat; 12 pounds per square inch of steam pressure followed by 30 minutes of cooling &commat; ambient temperature.

Adhesion of the metal layer to the substrate was then evaluated by ASTM D3359, Method B (Cross-Cut Tape Test), which comprises: cutting, with a sharp knife blade, a cross hatch pattern in single pass uniformly applied strokes through the metal layer disposed on the substrate area being evaluated by the test; removing the debris generated during the cutting procedure; evenly applying over the crosshatched area the recommended adhesive tape, having an adhesive strength of 40.2 + 2.8 g./mm.; pulling the tape away from the area at an angle of 90" to the surface in one quick stroke; and checking the tape for any metal flakes.

None of the plaques tested under the aforementioned tests showed any loss of metal layer adhesion. In addition, said plaques showed no trace of any metals other than silver on the exposed surface, as shown by Auger electron spectroscopy.

Claims

CLAIMS:

1. A lightweight, distortion resistant signalcarrying unit for a telecommunication system, said unit comprising a polymer selected from the group consisting of thermoplastic and cyanate ester polymers and an adherent transmissive layer disposed on surfaces thereof, said transmissive layer comprising a nickel layer contiguously disposed on said surfaces and a non-lossy, corrosion resistant, electrically conductive metal layer on said nickel layer.

2. A unit according to claim 1 which is a waveguide.

3. A unit according to claim 1 which is a multiplexer.

4. A unit according to claim 1 wherein the polymer material is selected from the group consisting of polyphenylene sulfide, polyarylsulfone, polyamideimide, polyetherketone, polyimide and mixtures thereof.

5. A unit according to claim 5 wherein the polymer material is polyetherimide.

6. A unit according to claim 5 wherein the polymer material . is fluorinated polyimide.

7. A unit according to claim 1 wherein the polymer material is reinforced with carbon fiber or glass fiber.

8. A unit according to claim 1 wherein said nickel layer has a thickness in the range of about 0.1-2.0 microns.

9. A unit according to claim 1 wherein said electrically conductive layer is silver, gold or platinum.

10. A unit according to claim 9 wherein said electrically conductive layer' is silver.

11. A unit according to claim 1 wherein said transmissive layer further comprises a barrier layer on said electrically conductive layer and an external corrosion resistant layer on said barrier layer.

12. A unit according to claim 11 wherein said electrically conductive layer is copper, said barrier layer is palladium and said external layer is silver.

13. A lightweight, distortion resistant feed horn for a telecommunication system, said horn comprising: a hollow shaped feed hom body fabricated from a thermoplastic polymer material selected from the group consisting of polyphenylene sulfide, polyarylsulfone, polyamideimide, polyetherketone and polyimide, having a first opening at one end and a second opening at the other end, said hollow shape being uniformly convergent from said first opening to said second opening; a flange surrounding said second opening of said body; securing means disposed on said flange for securing said feed horn to a waveguide interface; and an adherent transmissive layer disposed on surfaces of said body, said transmissive layer comprising a nickel primer layer contiguously disposed on said surfaces and a silver electrically conductive layer on top of said nickel layer.

14. A feed horn according to claim 13 wherein said surfaces on which said transmissive layer is disposed are located on an inner side of said body.

15. A feed horn according to claim 13 wherein said inner side further comprises at least one septum.

16. A feed horn according to claim 13 wherein said telecommunication system is land-based.

17. A feed horn according to claim 13 wherein said telecommunication system is space-based.

18. A lightweight, distortion resistant feed horn for a telecommunication system, said feed horn being fabricated from a carbon fiber-reinforced polyetherimide, comprising: a hollow pyramidally shaped feed horn body having a first opening at one end and a second opening at the other end, said hollow pyramid being uniformly convergent from said first opening to said second opening; at least one septum disposed on an inner side of said body; a flange surrounding said second opening of said body; securing means disposed on said flange for securing said feed horn to a waveguide interface; and an adherent transmissive layer disposed on the surfaces of said inner side and said septum of said body, said transmissive layer comprising a nickel primer layer contiguously disposed on said surfaces and a silver electrically conductive layer on top of said nickel layer.

19. A feed horn according to claim 18 further comprising at least one septum of carbon-fiber reinforced polyetherimide disposed on an inner side of said body.

20. A feed horn according to claim 18 wherein said transmissive layer further comprises a copper middle layer and a palladium barrier layer separating said nickel and silver layers.

21. A lightweight, distortion resistant feed horn for a telecommunication system, said feed horn being fabricated from a carbon fiber-reinforced fluorinated polyimide, comprising: a hollow pyramidally shaped feed horn body having a first opening at one end and a second opening at the other end, said hollow pyramid being uniformly convergent from said first opening to said second opening; at least one septum disposed on an inner side of said body; a flange surrounding said second opening of said body; securing means disposed on said flange for securing said feed horn to a waveguide interface; and an adherent transmissive layer disposed on the surfaces of said inner side and said septum of said body, said transmissive layer comprising a nickel primer layer contiguously disposed on said surfaces and a silver electrically conductive layer on top of said nickel layer.

22. A feed hom according to claim 21 further comprising at least one septum of carbon-fiber reinforced fluorinated polyimide disposed on an inner side of said body.

23. A feed hom according to claim 21 wherein said transmissive layer further comprises a copper middle layer and a palladium barrier layer separating said nickel and silver layers.

24. A telecommunication system of a telecommunication satellite, comprising: at least one incoming or outgoing reflector for reflecting the incoming telecommunication signals, said signals generally being in the microwave frequency range; at least one lightweight, distortion resistant incoming or outgoing feed horn located in the focal region of said reflector and having its axis pointed at said reflector; at least one waveguide comprising transmissive metal at least in part, any such waveguide which is secured to said feed horn having said transmissive metal matched to the metal on said feed horn to reduce passive intermodulation, said waveguide being effective to transmit signals received by said telecommunication system; receiving means for the incoming signal; transmitting means for the outgoing signal; and signal processing means including at least one multiplexer comprising metal at least in part; wherein at least one feed horn, waveguide or multiplexer comprises a polymer selected from the group consisting of thermoplastic and cyanate ester polymers and an adherent transmissive layer disposed on surfaces thereof, said transmissive layer comprising a nickel layer contiguously disposed on said surfaces and a non-lossy corrosion resistant metal layer on top of said nickel layer.

25. A telecommunication system according to claim 24 wherein said transmissive layer further comprises a copper middle layer and a palladium barrier layer separating said nickel and silver layers.

26. Telecommunications apparatus substantially as hereinbefore described with reference to the accompanying drawings.