Editing Inductor (section)

===Air-core inductor===
{{multiple image
| align = right
| direction = horizontal
| header   =
| image1   = Radio transmitter tank coil.png
| caption1 = High Q tank coil in [[tuned circuit]] of radio transmitter
| width1   = 220
| image2   = Antenna tuning coil - station WOR.jpg
| caption2 = An [[antenna tuner|antenna tuning]] coil at an AM radio station. 
| width2   = 140
| footer   = These coils illustrate high power [[Q factor|high Q]] construction: single layer winding with turns spaced apart to reduce [[Proximity effect (electromagnetism)|proximity effect]] losses, made of silver-plated wire or tubing to reduce [[skin effect]] losses, supported by narrow insulating strips to reduce [[dielectric losses]]
}}

The term ''air core coil'' describes an inductor that does not use a [[magnetic core]] made of a ferromagnetic material. The term refers to coils wound on plastic, ceramic, or other nonmagnetic forms, as well as those that have only air inside the windings. Air core coils have lower inductance than ferromagnetic core coils, but are often used at high frequencies because they are free from energy losses called [[core loss]]es that occur in ferromagnetic cores, which increase with frequency. A side effect that can occur in air core coils in which the winding is not rigidly supported on a form is 'microphony': mechanical vibration of the windings can cause variations in the inductance.

====Radio-frequency inductor====
[[Image:Hf spoler og transformatorer.jpg|thumb|upright=1.5|Collection of RF inductors, showing techniques to reduce losses. The three top left and the [[loop antenna|ferrite loopstick]] or rod antenna,<ref>{{cite web
  | title = An Unassuming Antenna – The Ferrite Loopstick
  | publisher = Radio Time Traveller
  | date = January 23, 2011
  | url = http://radio-timetraveller.blogspot.com/2011/01/unassuming-antenna-ferrite-loopstick.html
  | access-date = March 5, 2014 
}}</ref><ref name="Frost">{{cite web
  | last = Frost
  | first = Phil
  | title = What's an appropriate core material for a loopstick antenna?
  | work = Amateur Radio beta
  | publisher = Stack Exchange, Inc.
  | date = December 23, 2013
  | url = http://ham.stackexchange.com/questions/1156/whats-an-appropriate-core-material-for-a-loopstick-antenna
  | access-date = March 5, 2014
}}</ref><ref name="Poisel">{{cite book
  | last = Poisel
  | first = Richard 
  | title = Antenna Systems and Electronic Warfare Applications
  | publisher = Artech House
  | date = 2011
  | pages = 280
  | url = https://books.google.com/books?id=1YA1NZuo6u0C&q=%22ferrite+rod+loop+antenna&pg=PA280
  | isbn = 978-1608074846
}}</ref><ref name="Yadava">{{cite book
  | last = Yadava
  | first = R. L. 
  | title = Antenna and Wave Propagation
  | publisher = PHI Learning Pvt. Ltd
  | date = 2011
  | pages = 261
  | url = https://books.google.com/books?id=MMtjYYrE2r8C&q=%22ferrite+loop+antenna&pg=PA261
  | isbn = 978-8120342910
}}</ref> bottom, have basket windings.]]

At [[high frequency|high frequencies]], particularly [[radio frequency|radio frequencies]] (RF), inductors have higher resistance and other losses. In addition to causing power loss, in [[resonant circuit]]s this can reduce the [[Q factor]] of the circuit, broadening the [[Bandwidth (signal processing)|bandwidth]]. In RF inductors specialized construction techniques are used to minimize these losses. The losses are due to these effects:
*'''Skin effect''': The resistance of a wire to [[high frequency]] current is higher than its resistance to [[direct current]] because of [[skin effect]].<ref name="Zurek1">{{cite web
  | last = Zurek
  | first = Stan
  | title = Skin effect
  | website = Encyclopedia Magnetica website
  | publisher = 
  | date = 2023
  | url = http://www.e-magnetica.pl/doku.php/proximity_effect
  | format = 
  | doi = 
  | accessdate = 21 May 2024}}</ref><ref name="Kazimierczuk">{{cite book
  | last = Kazimierczuk
  | first = Marian K.
  | title = High-Frequency Magnetic Components
  | publisher = John Wiley and Sons
  | date = 2011
  | location = 
  | pages = 
  | language = 
  | url = https://books.google.com/books?id=t2TgU-uuNQ0C&pg=PA141
  | archive-url= 
  | archive-date=
  | doi = 
  | id = 
  | isbn = 978-1-119-96491-9
  | mr = 
  | zbl = 
  | jfm =}}</ref>{{rp|p.141}}  Due to induced [[eddy current]]s, radio frequency alternating current does not penetrate far into the body of a conductor but travels along its surface.  For example, at 6&nbsp;MHz the skin depth of copper wire is about 0.001 inches (25&nbsp;μm); most of the current is within this depth of the surface.  Therefore, in a solid wire, the interior portion of the wire may carry little current, effectively increasing its resistance.
*'''Proximity effect''': Another similar effect that also increases the resistance of the wire at high frequencies is [[proximity effect (electromagnetism)|proximity effect]], which occurs in parallel wires that lie close to each other.<ref name="Zurek2">{{cite web
  | last = Zurek
  | first = Stan
  | title = Proximity effect
  | website = Encyclopedia Magnetica website
  | publisher = 
  | date = 2023
  | url = http://www.e-magnetica.pl/doku.php/proximity_effect
  | format = 
  | doi = 
  | accessdate = 21 May 2024}}</ref><ref name="Kazimierczuk" />{{rp|p.98}} The individual magnetic field of adjacent turns induces [[eddy current]]s in the wire of the coil, which causes the current density in the conductor to be displaced away from the adjacent surfaces.  Like skin effect, this reduces the effective cross-sectional area of the wire conducting current, increasing its resistance.
*'''Dielectric losses''': The high frequency electric field near the conductors in a [[LC circuit|tank coil]] can cause the motion of polar molecules in nearby insulating materials, dissipating energy as heat.  For this reason, coils used for tuned circuits may be suspended in air, supported by narrow plastic or ceramic strips rather than being wound on coil forms.
*'''Parasitic capacitance''': The capacitance between individual wire turns of the coil, called [[parasitic capacitance]], does not cause energy losses but can change the behavior of the coil. Each turn of the coil is at a slightly different potential, so the [[electric field]] between neighboring turns stores charge on the wire, so the coil acts as if it has a capacitor in parallel with it. At a high enough frequency this capacitance can resonate with the inductance of the coil forming a [[tuned circuit]], causing the coil to become [[self-resonant frequency|self-resonant]].

{{multiple image
| align = right
| direction = horizontal
| header   =
| total_width = 300
| image1   = Spider coil.jpg
| image2   = Kreuzwickelspule.png
| footer   = ''(left)'' Spiderweb coil ''(right)'' Adjustable ferrite slug-tuned RF coil with basketweave winding and litz wire
}}
To reduce parasitic capacitance and proximity effect, [[Q factor|high Q]] RF coils are constructed to avoid having many turns lying close together, parallel to one another. The windings of RF coils are often limited to a single layer, and the turns are spaced apart. To reduce resistance due to skin effect, in high-power inductors such as those used in transmitters the windings are sometimes made of a metal strip or tubing which has a larger surface area, and the surface is silver-plated.
; Basket-weave coils: To reduce proximity effect and parasitic capacitance, multilayer RF coils are wound in patterns in which successive turns are not parallel but crisscrossed at an angle; these are often called ''honeycomb'' or ''[[Basket winding|basket-weave]]'' coils.  These are occasionally wound on a vertical insulating supports with dowels or slots, with the wire weaving in and out through the slots.
; Spiderweb coils: Another construction technique with similar advantages is flat spiral coils. These are often wound on a flat insulating support with radial spokes or slots, with the wire weaving in and out through the slots; these are called ''spiderweb'' coils. The form has an odd number of slots, so successive turns of the spiral lie on opposite sides of the form, increasing separation. 
; Litz wire: To reduce skin effect losses, some coils are wound with a special type of radio frequency wire called [[litz wire]]. Instead of a single solid conductor, litz wire consists of a number of smaller wire strands that carry the current. Unlike ordinary [[stranded wire]], the strands are insulated from each other, to prevent skin effect from forcing the current to the surface, and are twisted or braided together. The twist pattern ensures that each wire strand spends the same amount of its length on the outside of the wire bundle, so skin effect distributes the current equally between the strands, resulting in a larger cross-sectional conduction area than an equivalent single wire.

; Axial Inductor
Small inductors for low current and low power are made in molded cases resembling resistors.  These may be either plain (phenolic) core or ferrite core.  An ohmmeter readily distinguishes them from similar-sized resistors by showing the low resistance of the inductor.