Free Air Core Inductor Calculator
Calculate required coil turns, winding length, total copper wire length, and DC resistance for single-layer air-core RF and crossover inductors.
🌀 Coil Dimensions & Target Inductance
📊 Required Turns & Coil Specs
Recommended Tools & Equipment
Tested hardware and components for high reliability
Harold Wheeler’s Continuous Inductance Formula
In 1928, Harold A. Wheeler published his famous empirical approximation formula for single-layer solenoid inductors. It remains within 1% accuracy as long as the coil length is greater than 0.4 times the coil radius:
Why Air Core Inductors Never Saturate
Ferrite and iron core inductors suffer from magnetic flux saturation when high audio amplifier or RF transmitter currents pass through them, causing severe harmonic distortion. Air has constant magnetic permeability ($mu_0$), making air-core coils 100% linear at any power level.
The 1:1 Aspect Ratio Rule for Max Q
To maximize the inductor's Quality Factor ($Q$) while using the least length of copper wire (minimizing DC resistance), design your coil with a diameter-to-length ratio ($d / l$) close to 1.0 to 1.5.
Frequently Asked Questions
What type of wire should I use for winding inductors?
Use enameled magnet wire (copper wire coated with a microscopic insulation varnish). For audio crossovers, use 16 AWG or 18 AWG to keep resistance low; for RF transmitter tank circuits, use heavy silver-plated or bare copper wire.
How does spacing the turns affect inductance?
Stretching a coil outward increases the winding length ($l$), which decreases the mutual magnetic coupling between turns and lowers the total inductance ($L$). You can fine-tune RF coils by gently squeezing or stretching the turns.
Can I wind an inductor on a metal pipe?
NO! Winding around a conductive metal pipe (aluminum, copper, steel) creates shorted turns that act as an eddy current brake, destroying the inductance and overheating. Always wind on non-conductive formers like PVC pipe, acrylic, cardboard, or 3D-printed PLA.