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why we insert ferrite rod in coils

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mohammad mokhtar

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Hi all
why we insert ferrite rod inside wire wound coils especially the ferrite rod does
not (nearly)affect the value of the inductance ?
 

Because magnetization permeability of free air is equal to µ = μ0 = 4π×10−7 N A−2 but in case of ferromagnetic materias is µ » μ0. In other words to increase pemeability inside the coil and concentrate more magnetic flux lines inside of it.
 

You have higher inductance L, with smaller length of wire.
 
It also allows a convenient way to change the inductance.

Back around 1960 Collins Radio in the US pioneered variable inductance oscillator tuning. By changing the coil winding taper they could get a linear frequency change with linear position of the core in the coil.
 
As jetset pointed out, it increases magnetic permeability and thus magnetic filed is confined all around close it.
 

Inductance with ferrite core has smaller dimensions than the same inductance without the core.
 

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