Air Core Coil Design

Revised 7/21/97

Preface

This calculator helps you to design a single-layer air core coil of a given inductance. The formulae come from the <EM>ARRL Handbook for Radio Amateurs.</EM> These formulae provide close approximations of values for frequencies in the 1-30MHz range which are sufficiently accurate for most amateur radio purposes. The calculations are useful in the VHF and UHF range by only as a basis for further calculation and experimentation.

Turns per Inch

This program designs close-wound coils. For turn spacing (Turns per Inch) choose one of the following:

Bare Wire - Enter the guage, then double the diameter. During assembly, use a length of the same size wire as a spacer between turns.</LI>
Enamelled Wire - Manually add 0.005" to the diameter.</LI>
Insulated Wire - Either measure the OD including insulation and enter this directly, or wrap several turns around a ruler and enter the turns per inch.</LI>

In all cases, spread the winding evenly over the calculated winding length before securing in place.

Standard Forms

Standard plastic water and drain pipe is available in the following outside diameters: 0.66", 0.88", 1.66", 1.90", 2.38", 2.88", 3.50", 4.50"

Coil Design

To calculate diameter and number of turns:

Determine turns per inch.
- Enter AWG # for bare or enamelled wire. Note the resistance in Ohms per inch for the wire chosen.
- Adjust diameter: add 0.005 for enamelled, double for bare wire.
- For insulated wire, wrap a form and count the turns per inch. Enter turns per inch directly for insulated wire.
Enter the desired micro-Henries of inductance.

To calculate actual inductance for a standard sized wire form and whole number of turns:

Adjust coil diameter to a standard form. If you will make a form, you must still enter a number for diameter (possibly the number given as output).
Adjust the number of turns to the nearest integer

Note the total length of wire. Compute total length * ohms per inch to get the overall resistance of the coil.