GB952327A - Filter and method for making filters - Google Patents

Abstract

952,327. High frequency attenuators. E. B. McMILLAN. Jan. 4, 1960 [Jan. 20, 1959], No. 245/60. Heading H1W. A low-pass aperiodic filter comprises a central conductor 11, e.g. of copper, in contact with a surrounding energy absorptive medium 14 which includes a powdered magnetic material, such as powdered iron, dispersed in a solidified binder of thermosetting resin composition and an electrically conductive shield 12, e.g. of copper, steel or brass, separated by insulation 13 from the absorptive medium 14, the weight ratio of the powdered magnetic material to non-magnetic material in the absorptive medium being selected to provide a ratio of substantially 100: 1 between the electrical conductivities of the conductor 11 and the absorptive medium 14. The weight ratio between the magnetic and non-magnetic materials of the absorptive medium prior to hardening of the binder is substantially 84: 16. The insulation 13 can be sufficiently thin to prevent the shield 12 from operating as a waveguide. The insulation 13 can be of mica-coated Fiberglas or can be a film of lacquer or plastic material sprayed or brushed on to the shield 12, or can be of barium titanate. In an example a coating of polyvinyl butyral is applied to the shield 12 followed by coatings of phenolic resin, and polyvinyl butyral. In a modification a coating loaded with aluminium powder or a coating containing barium titanate can be provided between the first coating and the other two coatings. Alternatively an insulating coating can be applied to the outer surface of the medium 14 and the shield 12 formed by spraying or painting. The conductor 11 can be of helical configuration. The quantity of binder, which is preferably an epoxy resin composition, when hardened is substantially 8.9% by weight of the total weight of the absorptive medium. Methods of forming the device are described. In the construction shown in Fig. 3, the outer surface of the absorptive medium 14 and the inner surface of the shield 12 are curved about the individual turns of the helical conductor 11 and feed-through insulators 23, 24 are provided between the shield 12 and the conductor 11 at the ends of the filter. The medium 14 is retained in place by rubber discs 21, 22. The filter can be flexible and formed into a coil. In the construction shown in Fig. 4 capacitors 31 and 32 are connected to the ends of the filter to improve the low-frequency performance and retaining rubber discs 34, 34<SP>1</SP> are provided. Filters constructed as above can be connected in parallel.