http://www.transformerinductor.com/air-core-coil/ Inductance is a coil of insulated enameled wire, which presents a certain amount of resistance to alternating current. The higher the frequency, the greater the resistance. If the current is direct current, the resistance is zero, and the inductance is only equivalent to a winding wire. There is a formula to calculate the impedance: Z=2蟺fL, The 鈥淶鈥?is the AC impedance of the inductor, 鈥渇鈥?is the frequency of the alternating current, and 鈥淟鈥?is the inductance of the inductor. The main parameter of inductance coil is its inductance, The amount of inductance depends on the structure of the coil, coil number, diameter and length, will affect the size of inductance.Air Core Coil suppliers website:http://www.transformerinductor.com/air-core-coil/