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This paper deals with the radio-frequency disturbances caused by the normal operation of electrical equipment in aircraft. A brief account is given of the events leading to the adoption of the present system of suppression, reference is made to the special problems encountered, and attention is drawn to the more common sources and means of propagation of interference. Methods of reducing interference are described, and suppression at the source is indicated as being the most satisfactory method, emphasis being laid on the feasibility of designing radio-interference suppressors and suppression systems that will be effective over a wide frequency range with predictable behaviour. It is noted that some reduction of interference is possible by measures applied to receivers and aerials, and by careful design of equipment, and that these alone are rarely adequate. The design considerations for a radio-interference suppressor are stated and expanded for the particular application of a suppressor for aircraft use, and details are given of effective components and assemblies. Reference is made to test arrangements peculiar to the work. Experience has shown that, if adequate suppression is achieved continuously over the frequency range 150 kc/sÂ¿150 Mc/s, no trouble will be experienced at higher frequencies, and that the system adopted obviates the necessity for interference tests in production.