FR2829620A1 - Data transmission microwave guide filter is made from plastic foam with coating of conducting material - Google Patents

Abstract

A waveguide filter comprises a foam with a coating of conducting material (10). An Independent claim is included for a method of manufacturing the inventive waveguide filter comprising molding the foam, removing it from the mould after it has solidified and depositing the conductive material on the outer surface.

Description

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Waveguide filter
The invention relates to a waveguide filter and more particularly to its physical constitution.

 The proliferation of wireless data transmission applications leads to the study of systems using increasingly higher frequencies and offering wide transmission bands. Many filter technologies are known to perform the filtering functions. Among the many types of filters, the waveguide filters make it possible to produce filters having severe filtering constraints while having a small footprint.

 Waveguide filters can have very different shapes.

In general, the waveguide filters consist of one or more cavities of cylindrical or rectangular shape. The cavities may be coupled together by circular or rectangular irises which may be centered or eccentric. Walls, shutters, metal or dielectric partitions can also be placed in the cavities according to the plane of the electric field. The diversity of disposition of the different constituent elements can generate very complex forms. In addition, the dimensions of the constituent elements of the filters decrease when the frequencies are high. For more information on the design of waveguide filters, those skilled in the art will refer to the book titled Microwave Filters, Impedance Matching Networks and Coupling Structures by G. MATTHAEI, L. YOUNG and E. M. T. JONES.

 Achieving a waveguide filter for high frequencies requires complex and expensive machining that is not suitable for consumer products.

 Furthermore, it is known to use foams to fill the waveguide filters to modify the wave propagation medium, which allows the use of smaller waveguides.

 The invention proposes a new waveguide filter embodiment. The waveguide is made of foam by molding. The foam is then covered with a conductive layer. The invention is therefore a waveguide filter comprising a waveguide made of foam and a layer of conductive material deposited on the foam.

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 The molding of the waveguide eliminates complex machining of the waveguide. However, to have certain characteristics, the filter may comprise inserts passing through the foam, said inserts being at least partially covered by the conductive layer.

 The invention also relates to a method for manufacturing a waveguide filter which comprises the following steps: - molding of a waveguide-shaped foam, - extraction of the foam after solidification, - deposit of a layer of conductive material on the outer surfaces of the foam.

 For some filters, inserts are placed in the foam before the deposition of the layer of conductive material.

 The invention will be better understood, and other features and advantages will become apparent on reading the description which follows, the description referring to the appended drawings in which: FIGS. 1 to 4 represent the different steps of producing a first type of filter according to the invention, Figures 5 to 8 show the different steps of making a second type of filter according to the invention.

 In the present application, the drawings are not made to scale for reasons of comprehension. In particular, dimensions are stretched so that they can be viewed on the same figure.

 Figure 1 shows the walls of a coupled cavity waveguide filter. Four cylindrical cavities 1 to 4 are coupled by rectangular irises 5 placed in the middle of said cavities 1 to 4. Probes 6 and 7 provide the coupling of the filter to a high frequency circuit, for example of the microstrip type. Cavities 1 and 4 associated with probes 6 and 7 constitute two transition zones of a known type. In conventional technology, such a filter is made of at least two machined parts.

 According to the invention, a mold is made in two parts, part of which is shown in Figure 2. In addition to the waveguide shape, four orifices 8 are made to allow the filling of the mold. In the example described, the two parts of the mold are identical but it goes from

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 that the two parts may be different provided that they define the shape of the waveguide and one or more filling apertures.

 Once assembled, the mold is filled with a foam whose relative permittivity gr is close to 1. Preferably, the foam is a polymethacrylate imide sold under the Rohacelle brand 51. Once the mold is hot filled, it is expected the cooling of the mold and then demolding the waveguide thus produced.

 The waveguide 9 can then be deburred and covered with a conductive material. The conductive material is, for example, spray-applied conductive paint, as shown in FIG. 3. Any paint deposits on surfaces that are not to be covered can be removed by sanding or masked during conductive paint deposition.

 Figure 4 shows in section the completed filter. The waveguide 9 is covered with the conductive layer 10 except at least the transition planes. The waveguide is bonded to a substrate 11 which has, on the face in contact with the waveguide, a metal layer 12 which forms a ground plane pierced by two slots. On the other side of the substrate, micro-ribbon lines 13 and 14 are positioned in front of the cavities corresponding to the transitions. The person skilled in the art may notice that it is not necessary to cover the portion of the waveguide in direct contact with the ground plane 12 with the conductive layer.

 Other foams can be used. However, the choice of foam must take into account the relative permittivity of the foam.

When the permittivity moves away from the value 1, this leads to additional losses on the transmission chain. Depending on the application, these losses may be acceptable or not. By way of example, the following foams can be used:

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<tb> Foam <SEP> Er
<tb> Imide <SEP> of <SEP> polymethacrylate <SEP> 1
<tb> Polyurethane <SEP> 1, <SEP> 04
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Other molding possibilities are possible, in particular by injection or compression of the foam.

 Also, the metallization of the foam can be done by wet deposition, ie chemical and / or electrochemical soaking for example in a copper solution compatible with the foam. It is also possible to use a metal vacuum deposit of the CVD type (from

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 English Chemical Vapor Deposition) or PVD (Physical Vapor Deposition) type. For such deposits of metal layer, a surface treatment of the foam may be provided after demolding to facilitate the attachment of the metal layer.

 The filter thus produced does not require machining, at most one sanding. The use of foam molding mass production allows a very low manufacturing cost while having a high accuracy on the dimensions. In addition, the waveguides may have complex shapes, but it is much easier to mold the guide than to mold an element defining the guide.

 As indicated at the beginning of the application, the waveguide filters may include conductive or dielectric elements within the waveguide.

 The following example includes additional metal inserts. FIG. 5 shows a molded foam waveguide 20. The waveguide 20 thus produced defines three cylindrical cavities coupled by circular irises.

 In Figure 6, metal inserts 21 and 22 are placed in the central cavity of the waveguide 20. Two techniques are possible. A first technique is to push in the foam metal insert that makes its own hole. A second technique consists in making pre-holes slightly smaller than the insert and then pushing the insert into the hole. Depending on the size of the insert and its shape, one of the two techniques will be chosen. The inserts are held in position by the pressure exerted by the foam on them.

 Preferably, the metal inserts protrude from the foam in order to be covered at least in part by the deposition of the conductive layer as shown in FIG. 7. The filter shown in section in FIG. electrical contact with metal inserts. Compared to a conventional filter, inserting operations are facilitated by the fact that the guide is made of foam, which is easier to machine than metal.

 An advantage of producing a waveguide filter according to the invention is the stability of the temperature filter. Indeed, the waveguide filters are subjected to variations in external temperature, that is to say at temperatures ranging between -20 ° C. and + 80 ° C. The foam having a coefficient of thermal expansion generally lower than the metals used.

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 in conventional filters, the realized filter is ideal for applications using millimeter waves.

 Another advantage is the price of the material. Foam costs much less than metal alloys with a low coefficient of expansion.

Claims (10)

1. Waveguide filter comprising a waveguide made of foam (9) and a layer (10) of conductive material deposited on the foam (9).  2. Filter according to claim 1, characterized in that the foam (9) is polymethacrylate imide.  3. Filter according to claim 1, characterized in that the foam (9) is polyurethane foam.  4. Filter according to one of claims 1 to 3, characterized in that the conductive material (10) is a conductive paint.  5. Filter according to one of claims 1 to 4, characterized in that it comprises inserts (21,22) through the foam, said inserts being at least partially covered by the conductive layer.  6. A method of manufacturing a waveguide filter which comprises the following steps: molding a foam in the form of a waveguide, extracting the foam after solidification, depositing a layer of material conductor on the outer surfaces of the foam.  7. Method according to claim 6, characterized in that, before the deposition of the layer of conductive material, inserts are placed in the foam.  8. Method according to one of claims 6 or 7, characterized in that the foam is polymethacrylate imide.  9 Process according to one of claims 6 or 7, characterized in that the foam is a polyurethane foam. <Desc / Clms Page number 7>  10. Method according to one of claims 6 to 9, characterized in that the layer of conductive material is a conductive paint.