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With the advent of r.f. transistor circuitry, the small toroid coil and transformer have come into great favor due to the toroid's high coefficient of coupling, low losses and high permeability. However, miniature toroid cores are sometimes difficult to find.
A good source of suitable cores, both ferrite and powdered iron, is the threaded slug with a hexagonal hole through it, intended for use in an inductively-tuned coil. As shown in Fig. 1, these % inch long by % inch diameter cores make excellent coil forms. The coils can be wound bifilar, and since only a few turns are needed, fine wire can be used without significantly reducing the Q.
Fig. 1. (A) Threaded core as removed from slug-tuned coil. (B) Miniature core with bifilar-wound coils.
Although permeabilities vary greatly with different materials, the threaded cores usually fall into two general categories: the ferrites, which are dark gray in color and are quite shiny, and the powdered irons, which are lighter in color and dull in texture. The ferrites are good to about 2 Mc., while some of the powdered irons are usable to several hundred Mc. Formulas for approximate inductance using the 3¼-inch diameter by 3/8 inch long forms are:
L = 0.65N2 (for ferrite)
L = 0.02N2 (for powdered iron)
where L = Inductance in microhenrys.
N = Number of turns.
Dan Tomcik, K8ZQE.