Using of ferrite rods in vlf antennas

hi,

what is the purpose of using ferrite rods in vlf antennas? Is there any calculation required for designing such an antenna?

Ferrite rods offer the small size and a good directivity. In basic textbooks you can learn the details

Thanks for reply.
Can you suggest any text book for this topic?

Because of the effect of the powder iron/ferrite core, the inductance is increased by a large factor , meaning that less turns can be used to achieve the same inductance. though for maximum Q only air cored coils can be used.
Frank