GB403426A - Improvements in or relating to magnetic cores for high frequency inductance coils and transformers - Google Patents

Abstract

403,426. Main inductances. JOHNSON LABORATORIES, Inc., 609, West Lake Street, Chicago, U.S.A.-(Assignees of Polydoroff, W. J. ; 420, Fullerton Parkway, Chicago, U.S.A.) March 16, 1932, No. 26229/33. Convention date, March 16, 1931. Divided on 403,368. [Class 37.] A compressed magnetic core for an inductance or transformer for radio frequency circuits is mounted for movement relatively to the coil to vary the inductance, as described in Specification 366,475, and is constructed so that the core losses per unit volume of core material are different for different parts of the core in order to maintain a uniform ratio of inductance to core losses throughout the tuning range. The core is constructed of finely divided magnetic material, the particles of which are insulated from one another so that the core losses at frequencies of 150 to 1500 kilo-cycles are comparable with the copper losses of an air core coil, as described in Specification 403,368. Those parts of the core which first enter the region of maximum magnetic field intensity are so arranged as to have a lower permeability than the portions that enter this region subsequently, in order to compensate for the relatively high losses at the higher frequencies. As shown in Fig. 3, the core may be made of varying density by subjecting separate sections 3, 4, 5 to different pressures. Alternatively the core may be shaped as shown in Fig. 4 so that the section 6 is of smaller diameter than the sections 7, 8. In a modified method of producing a core of variable density, the core is pressed in a mould of such a shape that the pressure is not evenly distributed throughout the core. Such a core is shown in Fig. 5, in which the portions 9, 10 of the centre core, due to frictional losses on the wall of the mould, have a lower density than the portions 11. Alternatively the mould may be charged with two or three different mixtures in which the insulating coatings are of different thicknesses or the particles of magnetic material are of different sizes. In order to facilitate the matching of ganged variable inductances, the core is formed with a cavity 13 to receive a pellet 12, or a quantity of loose insulated powdered material which is sealed in after adjustment. Alternatively a hole may be bored into the core until the core matches the standard.