K7MEM Interactive Coil Inductance Calculator

Calculator Inputs

Calculation mode

Coil diameter

Diameter unit

Diameter basis

Winding length

Length unit

Turns

Target inductance

Target unit

Pitch mode

Fixed pitch

Pitch unit

Bare wire diameter

Wire unit

Extra insulation diameter

Insulation unit

Lead allowance each end

Lead unit

Operating frequency

Frequency unit

Estimated self capacitance

Relative permeability

Wire temperature

RMS current

Wire material

Example Data Table

Example	Diameter	Length	Turns	Wire	Frequency	Approximate Output
HF tank coil	50 mm	40 mm	18	1 mm copper	7.1 MHz	About 4.1 µH
Small VHF coil	12 mm	18 mm	6	0.8 mm copper	50 MHz	About 0.17 µH
Matching network coil	75 mm	90 mm	12	1.6 mm copper	3.5 MHz	About 3.4 µH

Formula Used

The main single layer solenoid estimate uses Wheeler style geometry.

L = r² × N² × µr / (9r + 10l)

Here, L is inductance in microhenries. r is coil radius in inches. l is winding length in inches. N is turns. µr is relative permeability.

Target turns use the same equation rearranged. With fixed pitch, the calculator solves the quadratic form.

Wire length = N × √((πD)² + pitch²) + lead allowance.

Reactance = 2πfL. Estimated Q = reactance / approximate AC resistance.

Skin depth = √(ρ / (πfµ0)). Self resonant frequency = 1 / (2π√(LC)).

How to Use This Calculator

Select whether you want inductance from turns or turns from target inductance.
Enter coil diameter, winding length, wire diameter, pitch, and unit choices.
Use form diameter when measuring the tube before winding.
Use centerline diameter when the winding path is already known.
Enter operating frequency to estimate reactance, skin depth, and Q.
Add estimated self capacitance for a rough self resonance check.
Press calculate to show results above the input form.
Use CSV or PDF buttons to save the current design report.

Advanced Coil Planning Guide

A radio coil is simple, but its behavior is detailed. Diameter, length, turns, wire size, pitch, and frequency all change the final result. This calculator helps you compare those choices before winding copper on a form. It follows the common K7MEM style of coil planning. It keeps the inputs practical. It also shows useful workshop values.

Why Geometry Matters

A wider coil usually gives more inductance. More turns also raise inductance quickly. A longer coil spreads the magnetic field. That can reduce inductance for the same turns. Pitch matters because it changes coil length and self capacitance. Close spacing may be compact. Wider spacing can improve high frequency behavior. These relationships are why one number is never enough.

Electrical Checks

The result includes reactance, estimated wire length, copper resistance, skin depth, and a rough Q estimate. These values help with matching networks, traps, filters, and tuned circuits. Q is only an estimate. Real Q depends on solder joints, nearby metal, coil form material, and measurement method. Still, the estimate is useful for early design choices.

Target Inductance Mode

You can enter a known number of turns. You can also choose target mode. In target mode, the tool estimates turns from the requested inductance. This is helpful when a design calls for a certain microhenry value. After winding, always measure the coil. Then stretch, compress, or remove turns as needed.

Practical Build Notes

Use consistent units. Measure the actual outside diameter of insulated wire. Include a small lead allowance. Keep the coil away from steel tools during testing. Use a low loss form for RF work. Air, ceramic, polystyrene, and dry plastic forms are common choices. For power circuits, check current heating as well.

Export and Record Keeping

CSV export saves the numbers for spreadsheet comparison. PDF export gives a quick report for bench notes. Save one report for each design change. That habit makes tuning easier. It also helps you repeat a good coil later. The best coil is measured, documented, and adjusted with care.

Safety and Limits

High voltage coils need spacing. RF coils may heat at high current. This tool is an estimate. Use safe clearances. Confirm performance with instruments before final service use.

FAQs

What does this coil calculator estimate?

It estimates inductance, turns, pitch, wire length, resistance, reactance, Q, skin depth, tuning capacitance, and self resonant frequency for single layer coils.

Is the result exact?

No. It is an engineering estimate. Real coils are affected by wire shape, nearby metal, coil form loss, lead length, and measurement method.

What is target inductance mode?

Target mode estimates the number of turns needed for a requested inductance. It is useful before winding a coil for a known circuit value.

Should I use form diameter or centerline diameter?

Use form diameter when you measured the tube before winding. Use centerline diameter when you already know the winding path diameter.

Why is wire spacing important?

Spacing changes coil length, capacitance, and high frequency behavior. Wider spacing may reduce stray capacitance, but it also changes inductance.

What does estimated Q mean?

Q compares inductive reactance with estimated AC resistance. Higher Q usually means lower loss, but real Q should be measured after construction.

Why enter self capacitance?

Self capacitance helps estimate self resonant frequency. Keep the operating frequency safely below that value for normal inductive behavior.

Can I export my result?

Yes. Use the CSV button for spreadsheet records. Use the PDF button for a simple report you can save with project notes.