Air Core Coil

The Air core coil is a coil which does not use a ferromagnetic material as the core.  The core is empty and is therefore filled with air.  The inductance of the air core coil is not affected by the current unlike coils with Ferromagnetic cores in which coils have a tendency to saturate.  Hence, the inductance value changes as the current is increased. 

Because, the coil does not have a metallic core, the hysteresis losses are negligible.  This results in more efficiency, less distortion and increased power capacity. 

Air cores coils can be used at very high frequencies up to 1 GHz in contrast to ferromagnetic core coils which can be used up to a maximum of 100 MHz.

However, Air core coils also have some disadvantages. For the same value of inductance, the air core coil needs to have more number of turns.  This results in higher copper loss.  The air core inductance is also susceptible to greater interference from stray fields.