This invention relates to electromagnetic screening, including materials and structures suitable for .use in the screening and/or construction of electromagnetic transmission line components. The invention further relates to methods of electromagnetic screening and to novel constructions of electromagnetic transmission line components.

Electromagnetic transmission line components, such as, for example, wave guides, coaxial cables and microwave transmitting and receiving antenna components are customarily fabricated from or include sheet metal, braided metal wire or overlapping metal strips. Parts of these components which are electrically conductive are often designed to form barriers which screen regions lying on opposite sides thereof electrσmagnetically from one another.

Thus in a conventional wave guide the wave guide wall is effective in substantially preventing leakage of electromagnetic radiation from the interior of the wave guide to the surroundings and vice versa. Similarly in an electromagnetic transmission line component such as a microwave antenna, the conductive metal from which the component is formed, acts to screen the interior of the component from extraneous electromagnetic radiation. In the case of a coaxial cable, the conventional outer metallic sheath, which may, for example, be formed from braided or spirally wound wire, performs a similar function.

In certain forms of construction of electromagnetic transmission line, the barrier which screens regions lying on opposite sides thereof electromagnetically from one another may be discontinuous. One example of such a transmission line is a coaxial cable of the kind described above having a metallic sheath formed from braided or spirally wound wire. Another example is a waveguide formed from a convoluted metal strip. Wave guides having this form of construction can be flexible and may be bend about one or more axes and/or twisted.

The effectiveness of conductive metal barriers of the kinds described is such that an electromagnetic screening effectiveness of 80 dB can customarily be attained.

However in many applications, for example in situations where a high degree of security is required, or where highly sensitive or directional microwave receivers are required to be designed, an electromagnetic screening effectiveness of greater than 80 dB is desirable. There is accordingly a need for improved electromagnetic screening materials. This is particularly so in the case where the aforementioned barrier is discontinuous.

According to one aspect of the present invention there is provided a method of reducing or substantially eliminating leakage of electromagnetic radiation between first and.second regions lying on opposite sides of a conductive metal barrier, which comprises providing a layer of a conductive elastomeric material so as to overlie a substantial surface ¹ area portion of at least one surface of the barrier which faces one of the first and second regions.

The invention further provides a laminated structure for use in impeding the passage of electromagnetic radiation, said material comprising a conductive metal barrier for screening first and second regions lying on opposite sides of the barrier electromagnetically from one another, and a layer of a conductive elastomeric material overlying a substantial surface area portion of at least one surface of the barrier which faces one of the first and second regions.

The conductive elastomeric material preferably comprises a polymeric elastomer having dispersed therein a particulate electrically conductive filler. The electrically conductive filler may for example consist of metals in particulate form, for example nickel particles, or particles of non-metals rendered conductive by being provided, for example, with a metallic coating. Suitable fillers include silver, silver plated copper, silver plated nickel, silver plated brass, silver plated aluminium and silver plated glass particles.

The precise size and shape of the filler is not critical although these can be optimised according to performance requirements. For example the particles can be generally spherical or they may be of irregular shape or in the form of rods or fibres. For spherical or irregular particles, a maximum transverse dimension of less than 200 um is preferred.

A variety of polymeric elastomers may be used as binders for the dispersed electrically conducted filler. Preferably, however, polychloroprene-, silicone- or fluorosilicone- based elastomers are used. Phenyl-silicones are particularly useful. Preferably the electrically conductive filler and a curing agent are incorporated into uncured polymeric elastomer and the resulting mix cured by, for example, the application of heat.

Generally a high loading of electrically conductive filler is required in order for the resulting electrically conductive elastomeric material to have the required conductivity (for example a volume resistivity of less than 0.01 Q.cm).

The mode of application of the layer of conductive elastomeric material to the conductive metal barrier is not critical and generally depends upon the nature of the barrier and the desired shape of the eventual laminated structure. Thus, for example, the conductive elastomeric material may be applied by casting, by injection molding, by compression moulding, by using a spreading device or by applying a pre-formed layer of the conductive elastomeric material in sheet form.

The laminated structures of the invention are particularly suitable for use in the formation of electromagnetic transmission line components such as, for example, wave guides, coaxial lines and microwave antennae.

Thus according to a further aspect of the invention there is provided an electromagnetic transmission line component having interior and exterior regions separated by a conductive metal barrier arranged to screen the interior and exterior regions from one another, and a layer of a conductive elastomeric material overlying a substantial surface area portion of at least one surface of the barrier which faces one of the interior and exterior regions.

Where wave guide tubes are to be manufactured using laminated structures according to the invention, the physical form of the conductive metal barrier will depend, by and large, on the size and configuration of the desired wave guide tube. Thus, for example, where the wave guide tube is to be formed of metal strip,

the thickness of the metal strip will be determined by such factors as the required rigidity of the tube, the dimensions of the wave guide and the desired frequency characteristics of the wave guide. Similarly, the thickness of the applied layer of electrically conductive elastomer will depend primarily upon the degree of screening required and the necessity to provide a coherent layer of elastomer. Thus, for example, the applied layer may be from 1 to 10 mm in thickness.

The metal used to form the conductive metal barrier in the laminated structures according to the invention depends upon the intended use of the materials. Typical metals which may be used include beryllium-copper, brass, stainless steel, aluminium and titanium, any of which may be plated with another metal, for example a noble metal or silver.

The laminated structures of the invention may include one or more layers in addition to the conductive metal barrier and the layer of conductive elastomeric material. For example a layer of bonding agent may be incorporated between the conductive metal barrier and the layer of conductive elastomeric material. Alternatively or additionally at least part of the surface of the conductive elastomeric material on the opposite side from the side facing the conductive metal barrier may be provided with an applied layer of non-conductive elastomeric material.

The application of an electrically conductive elastomeric material in accordance with the invention has been found to be particularly effective in screening flexible wave guide tube having a corrugated wall formation, especially where the wall has discontinuities. A particular example of such a wave guide is the flexible corrugated wave guide tubing manufactured by Quasar Microwave Technology Ltd and sold under the trade name "Flex-Twist". This tubing is characterised by a corrugated wall formation formed by winding metal strip in a convoluted, overlapping formation onto a former. In the manufacture of such tubing, the corrugations can be formed by simultaneously wrapping the former with the metal strip which eventually forms the wave guide wall and a wire support which is located between the former and the applied metal strip and supports the convolutions.

Wave guide tubing formed in this way is flexible and twis able.

As indicated above the present invention, in one of its embodiments, increases the electromagnetic screening effectiveness of a conductive metal barrier by forming a laminated structure comprising the barrier and an electrically conductive elastomer. It has, however, been found that in the case of certain forms of electromagnetic transmission line components, especially coaxial electromagnetic transmission lines, the conductive metal barrier can be wholly or partially replaced by a. suitably configured structure formed of a conductive elastomeric material.

Thus according to a further aspect of the invention, there is provided a coaxial electromagnetic transmission line component having inner and outer electrically conductive elements electrically insulated from one another, characterised in that the outer electrically conductive element comprises a layer of electrically conductive elastomeric material.

The invention will now be described in more detail by way of example with particular reference to the accompanying drawings of which

Figure 1 represents a transverse cross-sectional view through a wave guide in accordance with the invention.

Figure 2 represents a longitudinal section through the wave guide of Figure 1, and

Figure 3 represents an enlarged view of a surface portion of the wall of the wave guide.

Figures 4 a-d represent schematic views of the wave guide of Figures 1-3 during stages of its manufacture.

Referring to the drawings, wave guide 10 consists of a rectangular spirally wound tube 12 formed of silver plated brass strip. The overall dimensions of the wave guide tube are width 22.3 mm, height 11.9 mm and length 80 mm. ^" The exterior surfaces of the wave guide tube are clad with a two-part elastomeric coating. The coating comprises an inner elastomeric layer 14 formed of an electrically conductive elastomer and an outer elastomeric layer 16 which is non-conducting. The ends of the tube (not shown) may be provided with flanges enabling the tube to be connected to, for example, another tube section.

Referring to Figure 3- the metal wall of the wave ^* guide is of convoluted configuration and is formed from silver plated brass strip 20. In producing the wave guide, the strip 20 is wound spirally around a rectangular former. Wires are .simultaneously wound onto the former so as to support the corrugations and an additional wire 22 is compressed between adjacent turns, of the tape in order to promote electrical continuity between adjacent turns.

The application of the layer of conductive elastomeric material in accordance with the invention will now be described in the following example.

Example

A wave guide tube 12,was degreased in a vapour degreasing bath using a chlorinated hydrocarbon solvent.

A bonding agent composed of synthetic organic silicones in a methanol solvent system was painted onto the exterior surface of the wave guide and allowed to dry in air for 10 to 20 minutes.

An electrically conductive elastomeric material was prepared by compounding a peroxide cured vinyl methyl silicone based polymer, a curing agent and an electrically conductive filler. The proportions were adjusted to provide a compression set resistance better than 35% after 25% compression for 22 hours at 125 ^° C and 30 minutes recovery, and a volume resistivity of less than O.OlΩ.cm.

The ingredients were mixed and then formed into sheets in a two-roll mill. Sheet sections were cut to size and laid onto the broad sides of the microwave as shown in Figure 4a and folded over the narrow sides leaving a 5 mm gap 24 between the edges of the upper and lower sheets. A non-conductive elastomeric mix was similarly formulated, rolled into-sheet form and wrapped around the resulting composite structure (Figure 4b) .

The double wrapped microwave tube was then compressed in a heated mould 26 (Figure 4c) . During curing, the gaps 24 referred to above closed as a result of plastic flow of the conductive elastomer (Figure 4d) .

When tested for leakage, it was found that the microwave tube formed in accordance with the invention had an electromagnetic screening effectiveness of greater than 110 dB. This represents a greater than 1000-fold improvement compared with the microwave tube without the applied conductive elastomeric material.