GB890800A - Improvements in or relating to transmission media for electromagnetic wave energy inthe circular electric wave mode - Google Patents

Abstract

890,800. Waveguides. WESTERN ELECTRIC CO. Inc. Aug. 15, 1958 [Aug. 23, 1957], No. 26325/58. Class 40(8). A transmission medium for electromagnetic wave energy in the circular electric mode comprises a substantially cylindrical conductive helix 11 with the pitch distance less than a quarter wavelength of the design wavelength of the medium, a cylindrical sleeve 13 of lossy dielectric material surrounding the helix, and impedance transforming means 12, of a thickness greater than one half the distance between the helix turns, interposed between the helix 11 and the sleeve 13 for increasing the penetration into and absorption by the sleeve 13 of spurious mode energy. The impedance transforming means 12 can comprise a cylindrical dielectric sleeve having a radial thickness greater than the distance between the helix turns and less than one quarter wavelength of the highest operating frequency to be transmitted. A cylindrical insulating sleeve 14 surrounds the lossy sleeve 13 and the whole assembly is formed to have a bending stiffness comparable to that of a solid wall metallic waveguide to which it can be connected. Adjacent turns of the helix 11 are electrically insulated from each other by providing a small air gap or an insulating coating on the wire, e.g. the helix may be an enamelled or plasticcoated solid copper wire. The sleeve 14 has an internally threaded region 15 for connection to an adjoining section of waveguide, an internally smooth portion 16 of larger diameter for facilitating thread alignment and a larger diameter portion 18 forming a seat 17 for receiving a washer for sealing with the adjoining section. A conductive ring 19 between helix 11 and threaded portion 15 has an inside diameter equal to the inside' diameter of helix 11, terminates the end of the helix and forms a conductive abutting surface for the adjoining waveguide. The formation of the whole assembly upon a mandrel is described. (see Specification 890,799). The impedance transforming sleeve 12 can be formed by laminating thin layers of a dielectric material, e.g. glass fibres, reinforced plastic or glass fibres reinforced with plastic. as described in Specification 890,799. The material can be in the form of woven cloth or tape or in the form of roving. In the case of tape or roving the material can first be passed through a container of fluid plastic before winding or the plastic can be applied between the turns. The glass content should be between 50% and 75%. The plastic material can be an epoxide resin of the type that can be catalytically cured to form a thermosetting polymer. For the formation of the lossy sleeve 13 homogeneous materials having isotropic electrical properties have been used. The preferred material is as described in Specification 890,799 and comprises laminations of treated glass fibres. The material comprises plastics and glass cloth, tape or roving having an electrically conducting metallic oxide coating of the iodized type. Mixtures of the oxides of tin, titanium, cadmium, indium and antimony can be used. These materials can be applied to glass from a tetrachloride solution as described in U.S.A. Specification 2,564,707. The threaded portion 15, aligning portion 16 and seat portion 17-18 are filled in and built up to the outside diameter of the ring 19 by a process similar to that for the sleeve 12. For these portions glass fibre, roving and tape and an epoxide resin cured by a mixture of 10 parts per 100 metapheleylene diamine and 32 parts per 100 polyamide can be used. The remainder of sleeve 14 is then wound on using materials similar to those used for sleeve 12 but preferably using larger fibres of glass. The assembled waveguide is then cured. Specification 764,737 also is referred to.