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For a simple air-core inductor, wind enamel-coated magnet wire around a removable form, use a spacer line to keep turns even, and add epoxy support ribs before removing the form. Treat the turn-count calculation as a starting estimate: measure the finished coil and adjust turn spacing uniformly to reach the target inductance. This method is for air-core coils, not a complete design recipe for ferrite-core or high-current power inductors.
What this method can—and cannot—do
Robert H. Johns’s August 1997 QST article describes making self-supporting, single-layer air-core coils. The method is useful when a coil’s dimensions and application are known but a ready-made part is unavailable. It does not establish safe current ratings or provide enough information to design a ferrite-core or switching-power-supply inductor.
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Coil dimensions, conductor choice, current, operating frequency, losses, heat, insulation, and mechanical support all depend on the intended circuit. Do not infer a current or thermal rating from the example wire sizes or construction details below. Hackaday’s overview of homebrewing inductors also notes that wire and core losses can vary with frequency, but it does not supply a complete design method.
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ARRL gives this approximate relationship for a single-layer air-core coil, with inductance L in microhenrys, coil diameter d and length l in inches, and turn count n:
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L = d²n² / (18d + 40l)
Use this as an estimate when the coil length is at least 0.4 times its diameter. The equation above uses the plus sign under the denominator’s terms, as corrected in the article’s feedback. Calculated turns do not guarantee the finished coil’s inductance; measure the coil and adjust its spacing uniformly for the application.
ARRL’s example targets 10.0 µH with a 1-inch diameter and 1.25-inch length, producing approximately 26.1 turns by the formula. The article treats 26 turns as a practical starting point followed by spacing adjustment; this is a calculation example, not a reported measurement of a newly built coil.
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Choose the construction scale and materials
The ARRL method uses magnet wire on a PVC form. Its example mentions wire from #20 to #12 AWG or larger, but explicitly leaves wire and coil size to the application. That range is not a general recommendation: choose conductor and dimensions for the actual circuit rather than copying the example.
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- Smaller coils: Magnet wire and epoxy ribs can make a self-supporting air-core coil.
- Larger coils: Johns describes thicker conductor such as thin-wall copper tubing and reinforced ribs using PCB or perfboard strips.
- Workspace: The article warns that fiber-filled fiberglass resin is too noxious for indoor use.
The source does not provide current limits, thermal calculations, or a method for selecting wire based on those limits. Larger conductor or stronger supports alone do not establish that a coil is electrically safe for a particular service.
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Build a removable coil form
- Prepare the form. PVC pipe can serve as the winding form. Cut slots at least 3/8 inch wide so the form walls can collapse after the support ribs cure. Roughen the outer surface to reduce wire slipping. Put a block or tube inside the form to support it during winding.
- Set the spacing. Wind nylon monofilament alongside the magnet wire as a temporary spacer, then remove the line after winding. The article also mentions trimmer line; zip cord or speaker wire can create wider spacing.
- Wind under controlled tension. Keep the wire taut enough to stay in place while applying epoxy, but do not overtighten it. Johns describes holding the form in a vise with a protective rag.
- Make the support ribs. Use electrical tape or tape-covered craft sticks as an inner mold to stop epoxy dripping between turns. Mix regular-setting epoxy in an amount sufficient for one rib at a time.
- Let the ribs cure, then release the coil. Allow the epoxy to cure thoroughly before collapsing and removing the form. Johns suggests waiting overnight as an assurance of a thorough cure.
Balance turn spacing, inductance, and efficiency
Spacing is both a mechanical and electrical choice. Johns writes, “Wider turn spacing improves coil efficiency; closer turn spacing increases the coil inductance.” In the context of his air-core construction method, he suggests approximately one wire diameter between turns as a compromise. That is practical historical guidance, not a universal spacing rule for every geometry or frequency.
If the initial measured inductance is not the target, change the spacing evenly along the coil rather than concentrating a correction in one section. Recheck the inductance after each adjustment. A separate historical personal project account describes measuring hand-wound coils with an LCR meter; it is not a current instrument recommendation. When selecting a meter, check its specified inductance range, test frequency, accuracy at the expected value, and how it handles lead effects.
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- All coils are plated to eliminate oxidation and guarantee contact
- The coils are all flexible and can be bent or separated for a variety of applications
Add connections, taps, or later changes
To make a connection or tap, remove the enamel insulation only where needed, taking care not to damage the conductor. Johns describes scraping the coating, then wrapping and soldering a tap wire to the exposed turn.
The article also describes cutting support ribs to divide a long coil and freeing or cutting turns at the ribs to remove them. After any mechanical or winding change, check the coil again in the intended circuit rather than assuming its previous inductance remains unchanged.
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- 【Insulation Protection】These coils are insulated for your protection and also coated to resist abrasion, to ensure your safety, if the insulating layer is damaged, a new coil needs to be used.
- 【Reusable】 Each coil can resist overheating and premature wear，they are all durable enough to be used at least several hundred times.
- 【Great Flexibility】 These induction coils are all flexible and can be bent for various purposes，you just need to bend them till they meet your expectations.
- 【Prevent Oxidation】Each induction coil kit is plated and coated, to avoid oxidation after prolonged use, so that it can provide maximum working efficiency.
- 【Application】The induction heater coil used to remove rusty bolts, nuts, bearings and gears ect, easily and efficiently.
Know when this is not enough
The 1997 ARRL guide is an air-core construction article, not a general inductor design standard. It does not prescribe ferrite material, core gap, saturation margin, winding loss, thermal limits, insulation distances, or switching-supply design. For ferrite-core or high-current use, obtain design-specific data and engineering guidance; the air-core formula and construction examples do not establish a safe rating.
Sources: Robert H. Johns, “Homebrew Your Own Inductors!”, QST, August 1997; Al Williams, “Tips For Homebrewing Inductors,” Hackaday, byline dated September 4, 2025; and N5ESE, “N5ESE Builds the M-Cubed LCR Meter,” dated January 28, 2007.
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