What is an advantage of using a ferrite core toroidal inductor?
Large values of inductance may be obtained
The magnetic properties of the core may be optimized for a specific range of frequencies
Most of the magnetic field is contained in the core
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Explication
A ferrite toroid gives all three benefits at once: the high permeability of ferrite yields large inductance from few turns, the mix can be chosen for a given frequency range, and the closed ring keeps nearly all the flux inside the core so the coil neither radiates nor picks up stray coupling.