GB647960A - Improvements in permeability tuning arrangements - Google Patents

Abstract

647,960. Inductances. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Aug. 14, 1947, No. 22574. Convention date, Aug. 14, 1946. [Class 38 (ii)] A permeability tuning arrangement comprises a pair of coils connected in either series or parallel aiding relation, each coil being provided with a ferromagnetic core whereby the self and mutual inductance may be varied by relative axial movement of the cores and coils in unison, wherein each coil has a part of its surface flatter than another part thereof, the coils being disposed with the flatter parts of their surfaces facing one another, thereby improving the tuning range over that available in an arrangement employing coils of circular cross-section. A tuning device comprises two coils 20, 21 and a movable core, preferably of the powdered iron type, having four members 24-27 which may or may not be joined by a common yoke 28 but which are always arranged to move in unison. The effect of the external members 26, 27 is to provide, for a given core movement, greater changes in the self and mutual inductances of the coils and in the self capacitance of the assembly than would be obtained if the arrangement 'comprised a single coil and core, the changes being greater, but in a less and less proportion as the number of coil and core groups is increased above two to increase the tuning range. The coils may be formed to any shape (e.g. square, circular, rectangular, triangular) the tuning range increasing as the proportion of the coil peripheries in intimate contact with each other increases. The electrical connections between the coils may be series or parallel, aiding or opposing, the greatest tuning range being obtained with the parallel aiding connection. Yet further increases in the tuning range may be obtained by (i) increasing the ratio of length to diameter of the coils ; (ii) making the cores longer than the coils ; (iii) earthing the cores to increase the change in self capacitance ; (iv) using core materials of higher permeability ; (v) moulding the coils into a former of low-loss, high-dielectric-constant material ; (vi) the provision of additional cores on each side of the coils.