The ratio of inductance after and before a core with the diameter of the coil has been inserted inside it is called relative permeability (denoted μ r). Winding a coil, over a ferromagnetic coil focuses the magnetic field, increasing the inductance. Air cores are the most broadband but getting high inductances means using a lot of wire, they are also not the most efficient do to the magnetic field escaping the coil – this escaping magnetic can cause interference by inducting in nearby wires and other coils.
There are different methods of making coils, depending on the area of use and inductance needed. Winding coils is not hard but quite time-consuming. Every hobbyist wanting to dabble in radio has to – at some point – wind a coil or two, be it the antenna coil of an AM radio, a coil on a toroidal core for a bandpass filter in a communications transceiver or a centrally tapped coil for use in a hartley oscillator.
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