Air core

The air core to keep losses to a minimum it is necessary to keep the self-resistance of coils as low as possible. This means using the thickest possible wire within the space available. Another reason for using thick wire or even tubing is to reduce the skin effect losses at high frequencies. Direct current is spread uniformly over the cross-section of the conductor, but alternating current moves closer to the surface as the frequency increases. Thus it is necessary to provide a large surface to minimise radio frequency resistance which is known as skin effect. Inductors used can range from a 25 mm diameter tube for a slot antenna on VHF to 50 turns of 22 swg wire on a 7.5 cm diameter former for the tank circuit of a 1.8 MHz transmitter. The only adjustment available with air core inductors is by tapping all or part of a turn, or by varying the spacing between turns.