Calculator Inputs
Formula Used
This calculator uses Wheeler’s single layer air core solenoid estimate.
Inductance:
L(µH) = µr × r² × N² / (9r + 10l)
Here, r is coil radius in inches, l is winding length in inches, and N is turns.
Required turns:
N = √(Ltarget × (9r + 10l) / (µr × r²))
Reactance:
XL = 2πfL
DC resistance:
R = ρ × wire length / wire area
Estimated Q:
Q = XL / R
Self resonant frequency:
SRF = 1 / (2π√(LC))
How to Use This Calculator
- Select whether you want to calculate from entered turns or design from a target inductance.
- Enter the coil diameter and winding length in millimeters.
- Enter the number of turns or the desired inductance value.
- Add wire diameter, lead allowance, frequency, and expected current.
- Use copper resistivity as 17.24 nΩ·m unless you know another value.
- Press the calculate button.
- Review inductance, required turns, resistance, reactance, Q, and self resonance.
- Download the result as CSV or PDF for records.
Example Data Table
| Use Case | Diameter mm | Length mm | Turns | Frequency MHz | Typical Goal |
|---|---|---|---|---|---|
| HF antenna trap | 50 | 45 | 10 | 7 | Medium inductance with good Q |
| VHF tank coil | 18 | 20 | 5 | 100 | Low inductance and short leads |
| Filter inductor | 35 | 30 | 14 | 10 | Stable tuning and low loss |
| Oscillator coil | 25 | 25 | 8 | 14 | Repeatable construction |
About Air Core Inductor Design
An air core inductor stores energy in a magnetic field without a ferrite or iron core. This makes it stable at high frequency. It also avoids magnetic saturation. Radio circuits, filters, oscillators, antenna traps, and matching networks often use this coil style.
Why Geometry Matters
Inductance depends mainly on coil diameter, winding length, and turn count. A wider coil raises inductance because the magnetic field covers more area. More turns raise inductance even faster because turns are squared in Wheeler’s formula. A longer winding spreads the field and can lower inductance for the same diameter and turns.
Wire choice also matters. Thicker wire lowers resistance. Lower resistance can improve the estimated Q factor. At high frequency, skin effect and nearby conductors can change real results. For this reason, the output should be treated as a design estimate, not a final lab measurement.
Practical Design Notes
Keep turns evenly spaced when possible. Spacing lowers parasitic capacitance and helps the coil behave well at radio frequencies. Short leads are important because leads add extra inductance and resistance. Use a rigid form, ceramic support, or self-supporting wire when mechanical stability matters.
The calculator includes target design support. Enter a desired inductance to estimate the required turns. You can then round the answer to a practical whole number. The tool recalculates the expected inductance from that selected turn count. This helps you compare the ideal design with a buildable coil.
Understanding The Results
Reactance shows how strongly the inductor opposes alternating current at the chosen frequency. Higher frequency gives higher reactance. Resistance estimates copper loss from wire length and diameter. The Q estimate compares reactance with resistance. Higher Q usually means lower loss and narrower tuning.
Self-resonant frequency is estimated from your capacitance entry. Real self-capacitance depends on spacing, form material, winding style, and nearby parts. Measure the finished coil when accuracy is critical. Use the calculator first to choose a sensible starting design, then trim turns or spacing during testing.
Document your final coil dimensions after tuning. Small records make future repairs easier later. They also help you repeat successful filters, traps, and oscillators with less guessing again.
FAQs
What is an air core inductor?
An air core inductor is a coil without a magnetic core. It uses air as the core material. It is common in radio frequency circuits because it avoids core saturation and has stable behavior at higher frequencies.
Which formula does this calculator use?
It uses Wheeler’s single layer solenoid formula. The formula estimates inductance from coil radius, winding length, turn count, and relative permeability. It is practical for many hobby, radio, and electronics designs.
Why does turn count affect inductance strongly?
Turn count is squared in the formula. Doubling turns can raise inductance by about four times, if other dimensions stay similar. Real coils may vary because of spacing, leads, and nearby materials.
Should I enter outer or inner coil diameter?
Use the average winding diameter when possible. For thin wire, the form diameter is usually close enough. For thick wire, average the inner and outer winding diameter for a better estimate.
What does Q factor mean here?
Q compares inductive reactance with estimated copper resistance. A higher Q usually means lower loss and sharper tuning. This calculator gives a simple DC resistance based estimate, not a full RF loss model.
Why include estimated capacitance?
Every coil has stray capacitance between turns and nearby parts. That capacitance creates a self resonant frequency with inductance. Entering an estimate helps you avoid using the coil too near resonance.
Is this accurate for high power RF coils?
It is useful for starting values. High power RF work needs extra checks for heating, voltage spacing, skin effect, corona, and mechanical stability. Measure the finished coil before final operation.
Can I use aluminum wire?
Yes, but change the resistivity value. Aluminum has higher resistivity than copper, so resistance increases for the same size wire. This can lower the estimated Q factor and increase heating.