Editing Inductor (section)

====''Q'' factor====
The winding resistance appears as a resistance in series with the inductor; it is referred to as DCR (DC resistance).  This resistance dissipates some of the reactive energy. The [[Q factor|quality factor]] (or ''Q'') of an inductor is the ratio of its inductive reactance to its resistance at a given frequency, and is a measure of its efficiency. The higher the Q factor of the inductor, the closer it approaches the behavior of an ideal inductor.  High Q inductors are used with capacitors to make resonant circuits in radio transmitters and receivers.  The higher the Q is, the narrower the [[bandwidth (signal processing)|bandwidth]] of the resonant circuit.

The Q factor of an inductor is defined as

:<math>Q = \frac{\omega L}{R}</math>

where <math>L</math> is the inductance, <math>R</math> is the DC resistance, and the product  <math>\omega L</math> is the inductive reactance

''Q'' increases linearly with frequency if ''L'' and ''R'' are constant. Although they are constant at low frequencies, the parameters vary with frequency. For example, skin effect, [[proximity effect (electromagnetism)|proximity effect]], and core losses increase ''R'' with frequency; winding capacitance and variations in [[Permeability (electromagnetism)|permeability]] with frequency affect ''L''.

At low frequencies and within limits, increasing the number of turns ''N'' improves ''Q'' because ''L'' varies as ''N''<sup>2</sup> while ''R'' varies linearly with ''N''. Similarly increasing the radius ''r'' of an inductor improves (or increases) ''Q'' because ''L'' varies with ''r''<sup>2</sup> while ''R'' varies linearly with ''r''.  So high ''Q'' air core inductors often have large diameters and many turns.  Both of those examples assume the diameter of the wire stays the same, so both examples use proportionally more wire. If the total mass of wire is held constant, then there would be no advantage to increasing the number of turns or the radius of the turns because the wire would have to be proportionally thinner.

Using a high permeability [[ferromagnetic]] core can greatly increase the inductance for the same amount of copper, so the core can also increase the Q. Cores however also introduce losses that increase with frequency. The core material is chosen for best results for the frequency band.  High Q inductors must avoid saturation; one way is by using a (physically larger) air core inductor. At [[VHF]] or higher frequencies an air core is likely to be used. A well designed air core inductor may have a Q of several hundred.