Calculator Input
Formula Used
The calculator uses Wheeler’s single-layer air-core coil equation:
L(µH) = r²N² / (9r + 10l).
Here, r is coil radius in inches, l is coil length in inches,
and N is the number of turns.
Inductive reactance is calculated as XL = 2πfL.
DC resistance is calculated from wire length, wire area, and material resistivity.
AC resistance is increased by skin effect and the selected proximity factor.
The quality factor is calculated as Q = XL / Rac.
Estimated self-resonant frequency uses SRF = 1 / (2π√LCp).
The capacitance value is an estimate. Real coils can vary because nearby objects,
insulation, shields, and winding style affect capacitance.
How to Use This Calculator
- Enter the number of turns on the coil.
- Enter coil diameter, coil length, and wire diameter in millimeters.
- Select the wire material used in the winding.
- Enter the operating frequency in megahertz.
- Add estimated stray capacitance for resonance checking.
- Enter RMS current if you want power loss and voltage estimates.
- Use the proximity factor to model extra AC loss from close winding.
- Press the calculate button and review the result above the form.
Example Data Table
| Turns | Diameter | Length | Wire | Frequency | Inductance | Q Factor | SRF |
|---|---|---|---|---|---|---|---|
| 12 | 25 mm | 30 mm | 1 mm | 10 MHz | 2.147 µH | 486.5 | 62.70 MHz |
| 24 | 50 mm | 65 mm | 1.6 mm | 3.5 MHz | 16.198 µH | 824.5 | 17.68 MHz |
| 8 | 20 mm | 18 mm | 0.8 mm | 21 MHz | 0.933 µH | 446.5 | 111.08 MHz |
Understanding Coil Inductance and Q
Why Coil Geometry Matters
A coil stores energy in its magnetic field. Its shape controls how much energy it can store. More turns usually increase inductance. A larger diameter also raises inductance. A longer coil spreads the field and can reduce the final value. These effects must be balanced for a stable design.
Why Q Factor Matters
Q factor shows how efficiently a coil stores energy compared with its losses. A higher Q value is useful in tuned RF circuits, filters, matching networks, and oscillators. Low Q means more energy becomes heat. That can reduce selectivity and shift circuit performance.
Resistance and Skin Effect
At radio frequencies, current does not use the full wire cross section. It crowds near the surface. This is called skin effect. The effective conducting area becomes smaller. AC resistance rises. The calculator estimates this effect from frequency, resistivity, and wire size.
Proximity Loss and Spacing
Close turns can also increase loss. Magnetic fields from nearby turns push current into uneven paths. This is called proximity effect. The proximity factor lets you add a practical correction. Use a lower value for well-spaced turns. Use a higher value for compact windings.
Resonance Check
Every coil has unwanted capacitance. This capacitance forms a resonant circuit with the inductance. Above self resonance, the coil may stop acting like a normal inductor. Keep the operating frequency safely below the estimated self-resonant frequency. This gives better stability and predictable tuning.
FAQs
1. What does coil inductance mean?
Coil inductance measures magnetic energy storage. It depends on turns, diameter, length, and core material. This calculator estimates air-core coil inductance.
2. What is Q factor?
Q factor compares inductive reactance with AC resistance. A higher Q means lower loss and sharper tuning. It is important in RF circuits.
3. Why is AC resistance higher than DC resistance?
AC resistance rises because current crowds near the wire surface. This skin effect becomes stronger as frequency increases.
4. What is the proximity factor?
The proximity factor models extra loss from nearby turns. Close winding usually needs a larger value than widely spaced winding.
5. Can this calculator handle ferrite cores?
This version is intended for air-core coils. Ferrite and powdered iron cores need permeability, gap, saturation, and core loss data.
6. What is self-resonant frequency?
Self-resonant frequency is where coil inductance resonates with stray capacitance. Above it, coil behavior becomes less inductive.
7. How can I increase Q?
Use thicker wire, smoother conductors, better spacing, shorter leads, and lower-loss layouts. Keep the coil away from metal objects.
8. Are results exact?
No. They are engineering estimates. Real measurements can differ because winding style, insulation, layout, and nearby parts affect performance.