Figure 1. Quadruplexer, designed by Matra Marconi Space and manufactured in silver-plated carbon-fibre composite material by Polymer Kompositer AB
Contractor
Polymer Kompositer AB Publi (S)
Funding
Advanced Systems and Technology Programme Phase 4
These problems have now been overcome by new technology which has made possible the manufacture of microwave components out of metallised carbon-fibre composite material. This technique yields microwave devices with reduced weight, improved stability and good electrical performance.
Under a programme initiated by ESA, process technology has been developed and an L-band feed operating at 1.5-1.7 GHz and a Ku-band quadruplexer have been manufactured from silver-plated carbon-fibre-reinforced plastic.
A new technique for manufacturing and plating high quality composite material has been developed by Polymer Kompositer AB for component manufacture. This process uses prepreg materials which are subjected to high pressures of between 6 to 25 bar during the temperature-controlled curing cycle. To do this, special equipment was designed for controlling the pressure and temperature variations needed.
The properties of the autoclave can be experimentally determined and optimised for every prepreg material. By controlling the onset and duration of the time periods of heating and pressurisation.
Increased heat conduction combined with shorter process times improves the production capacity over that of a conventional gas autoclave. The option for high pressure levels gives freedom in the selection of materials with high fibre content and reduces the occurrence of pinholes and gas and air voids. The process can accurately produce components of complex shapes.
A new process for electroless copper plating, called microwave coppermix, has also been developed. In contrast to common electroless copper plating, this technique permits the build-up a copper layer of unlimited thickness without any sacrifice in quality.
The adhesion of copper to carbon-fibre-reinforced plastic of cyanate ester or epoxy matrices was equal to that of high performance adhesives-over 30 MPa-which exceeds the requirements for antennas and microwave components by a wide margin.
In contrast to galvanic plating, electroless plating yields a very even surface, and as such, it is suitable for plating the insides of parts with complex shapes such as feeds, waveguide assemblies, beamformers or the iris of a filter cavity. The process is fully controllable and it can achieve sub-micron tolerances. The metallisation build-up is uniform even when compared to pulsed electrolytic plating, a property which is extremely important for the manufacture of an iris for a filter cavity.
Under a separate ESA contract with Matra Marconi Space, a quadruplexer, which is a filter for separating closely spaced transmit and receive bands, was designed, manufactured and tested in both aluminium and invar. It had been designed to operate at the frequencies used by the Astra 1C and 1D satellites. Matra Marconi Space (UK) acted as consultants for an activity to realise and test the same design in carbon-fibre-reinforced plastic. For this component, pulsed silver plating, deposited on a layer of electroless copper, provided the conductive coating.
The quadruplexer had no trimming screws and had to be manufactured to very tight tolerances to meet its specification. The resulting design had low-mass and high-strength and could be manufactured without the need for subsequent rework or trimming (Figure 1). It was tested by Matra Marconi Space to determine its stability during thermal cycling and to see if there would be long-term changes in response. The test conditions are given in Table 1. Results show no degradation in the electrical (microwave) properties of the quadruplexer over the test period. The temperature coefficients of frequency drift of the invar, aluminium and carbon-fibre versions of the quadruplexer, listed in Table 2, show the superior stability of the carbon-fibre construction.
TABLE 1 Test conditions used to evaluate the metallised carbon-fibre composite material. ------- Test Sequence Conditions ------------- ---------------------- Thermal Cycling* 9 cycles, -40° to 140° Storage 1 month at room temperature Thermal cycling 5 cycles, -40° to 140° Storage 1 month at room temperature Thermal cycling 5 cycles, -40° to 140° (*)ElectricaL Measurements were taken over the first five cycles with a 2-hour dwell period.
TABLE 2 Measured frequency stability of quadruplexers, comparing the use of carbon-fibre material with conventional metallic construction. ------- Material Frequency Drift ------------------------ ---------------- Invar, silver-plated 2 x 10(exp -6) /°C Aluminium, silver-plated 23 x 10(exp -6) /°C Carbon fibre reinforced plastic, copper- and silver-plated 1 x 10(exp -6) /°C
Figure 1. Quadruplexer, designed by Matra Marconi Space and
manufactured in silver-plated carbon-fibre composite material
by Polymer Kompositer AB
A silver-plated, circularly polarised, L-band feed was designed and manufactured in carbon fibre material having a wall thickness of 0.5 mm (Figure 2 and Figure 3). The co- and cross-polar radiation patterns were measured and the performance of the feed over the band and was found to be within specification. The cross-polar (i. e. axial) ratio was 0.3, better than the prototype aluminium feed, indicating that the stepped septum polariser was accurately positioned.
Figure 2. Circularly polarised L-band feed system made of
metallised carbon-fibre composite material
Figure 3. Details of the inside of the L-band feed
The total mass of a flight model feed is expected to be 200 g. This compares favourably with the 300 g mass of a thin-wall aluminium part, which could only be realised by expensive spark erosion techniques. Attempts to manufacture the component from aluminum, using laser welding and electron beam welding, produced a component with inferior mechanical strength and poorer electrical performance.
New technology for metal plating carbon-fibre composite material has made possible the manufacture of high-performance passive microwave components whose dimensional stability and whose mechanical and electrical properties are better than those of aluminium and invar devices. The use of composite material also brings a substantial saving in mass. A new electroless silver-plating process is currently under development and will be completed soon.
Preparing for the Future Vol. 5 No. 4.