Ferrite Cores

In the high radio-frequency bands, ferrite cores have the advantage of high resistivity and practically no eddy-current component of core loss. Several grades are manufactured commercially, usually mixtures of manganese, nickel, and zinc ferrites.

Fig. 163. Magnetic ferrite normal permeability.

Figure 163 is a set of normal permeability curves for different grades of ferrites, and Fig. 164 gives initial permeability. Usually the lower-permeability materials have lower loss at higher frequencies, so that permeability is an inverse indication of the relative frequencies at which ferrites are useful.

Fig. 164. Magnetic ferrite initial permeability.

Losses in ferrites are often related to the product μ₀Q. (See Fig. 165.)

Fig. 165. Loss factor of ferrite cores.

This relation is approximately as follows:

[95]

Instead of μ₀Q, the quantity R_ser/μfL is sometimes plotted, where R_ser is the equivalent series resistance corresponding to core loss. Equation 95 then becomes

[96]

At the lower radio frequencies, finely divided powdered iron has loss lower than some ferrites. Owens⁽¹⁾ gives 1.0 mc as the highest frequency for which this holds. Both ferrites and powdered iron have temperature limits far below that of strip-wound cores: ferrites because of low Curie temperature, and powdered iron because of possible damage to the bonding material. Powdered iron with certain bonds has the better temperature coefficient of permeability. Both materials are available in the forms shown in Fig. 144.

(1)	See "Analysis of Measurements on Magnetic Ferrites," by C. D. Owens, Proc. I.R.E., 41, 360 (March, 1953).