Ferrite core

It is used for its properties of high magnetic permeability coupled with low electrical conductivity (which helps prevent eddy currents).

Ferrites that are used in transformer or electromagnetic cores contain iron oxides combined with nickel, zinc, and/or manganese compounds.

The low coercivity means the material's magnetization can easily reverse direction while dissipating very little energy (hysteresis losses); at the same time, the material's high resistivity prevents eddy currents in the core, another source of energy loss.

The ferrite cores used for power transformers work in the low-frequency range (1 to 200 kHz usually[2]) and are relatively large in size, can be toroidal, shell, or shaped like the letters 'C', 'D', or 'E'.

This core effectively concentrates the magnetic field of the radio waves[6] to give a stronger signal than could be obtained by an air core loop antenna of comparable size, although still not as strong as the signal that could be obtained with a good outdoor wire aerial.

Several ferrite cores
Loopstick antenna from an AM radio having two windings, one for long wave and one for medium wave (AM broadcast) reception. Typically 10 cm long, these loop antennas are usually hidden inside the radio receiver.