Air-core coils

The simplest inductors (besides plain, straight lengths of wire) are coils. A coil can be wound on a plastic, wooden or other nonferromagnetic material, and it will work very well, although no air-core inductor can have very much inductance. In practice, the maximum attainable inductance for such coils is about 1 mH.
Air-core coils are used mostly at radio frequencies, in transmitters, receivers, and antenna networks. In general, the higher the frequency of an alternating current, the less inductance is needed to produce significant effects. Air-core coils can be made to have almost unlimited current-carrying capacity, just by using heavy-gauge wire and making the radius of the coil large. Air does not dissipate much energy in the form of heat; it is almost lossless. For these reasons, air-core coils can be made highly efficient.