Mystery Coil Parameter Calculator

Use this calculator to find the inductances of the primary and secondary windings of a Mystery Crystal Set coil. Refer to the following diagram for the definition of the input variables.

In the diagram, primary turns are shown in blue. Secondary turns are shown in red.

For more information about the mystery coil please refer to the Mystery Crystal Radio page.

Input Parameters:
Number of Primary Turns - Np:
Number of Secondary Turns - Ns:
Secondary Start Position - a:
Coil Diameter - dc: mm
Mean Inter-turn Dielectric Constant - εR:
Winding Pitch - p: mm
Conductor Size - dw:
Conductor Diameter:   ***
Winding Pitch:   ***
Coil Length:   ***
Pri. + Sec. Wire Length:   ***
Primary Inductance LP:   *** µH
Secondary Inductance LS:   *** µH
Mutual Inductance M:   *** µH
Series (Pri. + Sec.) Inductance LT:   *** µH
Coupling Coefficient KC:   ***
Inter-winding Capacitance CPS:   *** pF


Conductor size is the size of the actual current carrying conductor, excluding insulation. For a close wound coil, set the pitch equal to the overall wire diameter including the insulation. For space wound coils, set the pitch to any larger value.

This calculator assumes that both primary and secondary conductors are the same size.

When calculating inductance, the variable pitch of the primary winding is taken into account.

To calculate inter-winding capacitance, an estimate of the effective dielectric constant εR is required. This will be a combination of the dielectric constants of air, the wire insulation and the coil form material. It's suggested that the calculation be done with the minimum possible value of 1 (air dielectric constant) and the maximum value (wire insulation, or coil form material, whichever is greater). The two resulting capacitance values will bracket the true value.

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This page last updated: January 29, 2023
Copyright 2010, 2023 Robert Weaver