A coil typically refers to a circular winding of conductive wire. Common applications for coils include motors, inductors, transformers, and loop antennas. In the context of electrical circuits, a "coil" specifically refers to an inductor. It is formed by winding a conductor-wire by wire-such that the individual strands remain electrically insulated from one another; the insulating core may be hollow, or it may contain an iron core or a powdered magnetic core. This component is commonly abbreviated as an "inductor." Inductors can be further categorized into fixed inductors and variable inductors; a fixed inductor coil is simply referred to as an "inductor" or "coil." Inductance is denoted by the symbol *L*, and its units include the Henry (H), millihenry (mH), and microhenry (µH), where 1 H = 10³ mH = 10⁶ µH.
Classification by Inductance Form: Fixed Inductors, Variable Inductors.
Classification by Magnetic Core Material: Air-core Coils, Ferrite-core Coils, Iron-core Coils, Copper-core Coils.
Classification by Functional Application: Antenna Coils, Oscillator Coils, Choke Coils, Notch Coils, Deflection Coils.
Classification by Winding Structure: Single-layer Coils, Multi-layer Coils, Honeycomb Coils.
Relevant Parameters
Inductance
Inductance (*L*) represents an intrinsic characteristic of the coil itself and is independent of the magnitude of the current flowing through it. With the exception of specialized inductor coils (such as color-coded inductors), the specific inductance value is generally not explicitly printed directly on the coil; instead, the component is identified by a specific model name or designation.
Inductive Reactance
Inductive reactance (*X*<sub>L</sub>) quantifies the extent to which an inductor coil impedes the flow of alternating current (AC). Its unit is the Ohm (Ω). The relationship between inductive reactance, inductance (*L*), and the AC frequency (*f*) is defined by the formula: *X*<sub>L</sub> = 2π*fL*.
Quality Factor (Q-factor)
The Quality Factor (*Q*) is a physical quantity used to characterize the quality of a coil. The *Q*-factor is defined as the ratio of the inductive reactance (*X*<sub>L</sub>) to the coil's equivalent series resistance (*R*); that is: *Q* = *X*<sub>L</sub> / *R*.
The higher the *Q*-value of a coil, the lower the energy loss within the associated circuit. A coil's *Q*-value is influenced by various factors, including the DC resistance of the wire, dielectric losses within the coil former (bobbin), losses induced by shielding enclosures or magnetic cores, and the effects of the high-frequency skin effect. Typically, the *Q*-value of a coil ranges from a few tens to several hundred. Distributed Capacitance
The capacitance existing between the turns of a coil, between the coil and its shielding enclosure, and between the coil and its baseplate is referred to as distributed capacitance. The presence of distributed capacitance reduces the coil's Q-factor and degrades its stability; consequently, the lower the distributed capacitance of a coil, the better.
