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THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1953 



(b) This value will depend not only on the frequency and volume of 

 core, but on the inductance and current level as well. 



3. DC Resistance and Eddy Current Loss Predominate 



This condition is the most commonly occurring of all when metallic 

 magnetic materials are used but is of very little importance for ferrite 

 coil design. From (14) 



D = 



h 



V^i'nf 



+ fcsM/ 



Mopt. — 



/C4 



1/2 



i^opt. — 



yi/3 



(21) 

 (22) 

 (23) 



Here we note that, as in the case for residual loss, optimum permeability 

 is that which assures that the core loss is equal to the dc resistance. If 

 both eddy current and residual losses were significant the sum of these 

 should be equal to the DC resistance. It is also seen from (22) and (23) 

 that although the optimum permeability is a function of frequency the 

 resulting dissipation factor is independent of the frequency. 



We have developed expressions for optimum permeability for three 

 conditions of core loss. It is now of interest to know how critical the 

 adjustment of permeabiUty is. Fig. 1 shows the effect on the dissipation 

 factor of deviations from optimum in permeability. It is seen that for 



1.5 



D 



Dopt 



1.2 



1.0 



0.4 0.5 0,6 0,7 0.8 0.9 1.0 1.2 1.5 2.0 2.5 



M 

 ^opt 

 Fig. 1 — Effect of permeability on dissipation factor. 



