Coil Calculator & 3D Form Designer

Design air-core inductors and generate 3D-printable coil forms

Design Mode

Input Parameters

MHz
pF
Required inductance: -- μH
mm
mm

Wire & Constraints

mm
Typical range: 15-75mm for most projects

Coil Specifications

Inductance -- μH
Number of Turns --
Coil Diameter -- mm
Coil Length -- mm
Wire Length Needed -- m
Self-Resonant Freq (est.) -- MHz
Note: Use this coil well below the self-resonant frequency for predictable inductance.

3D Printable Coil Form

STL Export

Form Parameters

mm
mm
For mounting screw/rod. Set to 0 for solid core.
mm
0 = flush flanges (W9FFF style, no supports needed)
mm
Helps keep turns evenly spaced
mm
0.3-0.5mm works well (W9FFF style). Shallow grooves print reliably.
Holes in flanges for threading and securing wire ends

Form Dimensions

Outer Diameter --
Inner Diameter --
Total Length --
Winding Diameter --
License: Personal/non-commercial use only. Generated STL files from 73qrz.com may not be sold or used for commercial purposes.

Cross-Section View

WTF Does This All Mean?

Plain English version
A coil (or inductor) is just wire wrapped in a spiral. When electricity flows through it, it creates a magnetic field that resists changes in current. This property — inductance — is measured in microhenries (μH).
Why do I need a coil?
Common uses in ham radio:
  • Loading coils — Make a short antenna "look" electrically longer
  • Antenna tuners — Match your antenna to your radio
  • Filters — Block or pass certain frequencies
  • Baluns/chokes — Block common-mode current on feedlines
What affects inductance?
  • More turns = more inductance (it's proportional to turns²)
  • Larger diameter = more inductance
  • Longer coil = less inductance (counterintuitive!)
  • Closer turns = more inductance
What's "self-resonant frequency"?
Every coil has hidden capacitance between its turns. At some high frequency, this capacitance resonates with the inductance and the coil stops acting like an inductor. Use your coil well below this frequency (at least 3-5x lower).
What wire should I use?
  • 18 AWG — Good all-around choice, handles 100W+
  • 14-16 AWG — High power (500W+), lower loss
  • 22-24 AWG — QRP/low power, smaller coils
  • Use enameled magnet wire for close-wound coils
Using a coil to load an antenna?

A loading coil makes a short antenna "look" electrically longer. Where you place it matters a lot:

  • Base loading (at the feedpoint) — Easiest to build and adjust, but least efficient. More inductance needed.
  • Center loading (middle of the antenna) — Better efficiency than base loading. Classic mobile antenna design.
  • Top/tip loading (near the end) — Most efficient, but harder to support mechanically.

Higher placement = better efficiency but less inductance needed. Base loading might need 2-3x the inductance of center loading for the same result.

TL;DR: Enter your target inductance (or frequency + capacitor), pick your wire gauge, and this calculator tells you how many turns to wind. Then download the 3D-printable form with grooves that keep your turns perfectly spaced.

Design Tips

Choosing Diameter

Larger diameter = fewer turns needed, but bulkier coil. For QRP/portable, 20-30mm works well. For high-power, go larger (50mm+) for better heat dissipation.

Wire Gauge

Thicker wire (lower AWG) = lower resistance, higher Q, handles more power. 18 AWG is good for general use. Use 14-16 AWG for high power.

Turn Spacing

Close-wound gives maximum inductance per length. Spaced turns reduce inter-turn capacitance (higher self-resonant frequency) and can handle higher voltage.

For 3D printing: Spaced (1.5x) is the default because it produces grooves wide enough to actually print. Close-wound grooves often disappear in the slicer because the ridges are thinner than your nozzle. If you need close-wound, consider turning off grooves entirely.

3D Printing Tips

Use PETG or ABS. Avoid PLA, which softens at low temperatures and can warp from RF heating or summer sun. Print with 100% infill for strength. Orient vertically for best surface finish on winding area.

No supports needed: The default W9FFF-style design uses flush flanges and shallow grooves - just a simple tube that prints perfectly without any supports or overhangs.

How to Build Your Coil

Step by step, no frustration
Building a coil is easier than it looks. Take your time, and you'll have a great result.
1

Print Your Form

Download the STL and print it. PETG or ABS works great - avoid PLA which can soften from RF heating. Use 100% infill if you can. Orient the form standing upright for the smoothest winding surface.

2

Prepare Your Wire

Cut your wire about 10% longer than the calculator says. Straighten it by pulling it through a folded cloth a few times. Enameled magnet wire works best because it lets you wind turns close together without shorting.

3

Anchor the Starting End

If your form has anchor holes, thread the wire through one hole, pull about 3 inches through, then bend it back on itself. You can also use a small dab of hot glue or tape to hold it in place while you wind.

4

Wind the Turns

Hold tension on the wire and rotate the form away from you. If you have grooves, the wire will naturally settle in. Keep steady tension but don't pull too hard. Each turn should sit snugly against the previous one (for close-wound) or evenly spaced (for spaced turns).

5

Secure the Ending

Thread the wire through the anchor hole on the other flange. Pull it snug but not super tight. Leave about 3 inches of tail. You can wrap it back around itself once or twice to lock it in place.

6

Finishing Touches

If your wire has enamel coating, scrape or sand the last half inch of each end to expose bare copper. This is where you'll solder your connections. A bit of clear nail polish over the windings can help keep everything in place.

Quick Tips

  • Wire won't stay in groove? Increase groove depth in the settings, or add a tiny drop of super glue every few turns.
  • Running out of wire? Always cut extra. You can trim later but you can't add more.
  • Turns look uneven? Don't stress. You can gently push them into place with your fingernail or a plastic tool while keeping light tension.
  • Need to redo it? No shame in unwinding and trying again. The second attempt usually goes smoother.

Sources & References

Wheeler's Formula

This calculator uses Wheeler's 1928 formula for single-layer air-core inductors:

L (μH) = (r² × N²) / (9r + 10l)

Where r = radius in cm, l = length in cm, N = number of turns

H. A. Wheeler, "Simple Inductance Formulas for Radio Coils," Proceedings of the IRE, vol. 16, no. 10, pp. 1398-1400, Oct. 1928.

Accuracy

Wheeler's formula is accurate to within ±1% for coils where length ≥ 0.4 × diameter. For shorter coils, accuracy decreases but remains within ±5% for typical ham radio applications.

Further Reading