RF choke design inputs
Use the geometry method when you know the core dimensions. Use the AL method when the datasheet already provides inductance per turn squared.
Plotly reactance graph
The graph plots choke reactance across a one-decade sweep below and above the operating frequency.
Example data table
| Item | Example value |
|---|---|
| Operating frequency | 10 MHz |
| System impedance | 50 Ω |
| Target reactance multiplier | 10× |
| Target reactance | 500 Ω |
| Required inductance | 7.9577 µH |
| Relative permeability | 125 |
| Magnetic path length | 50 mm |
| Core area | 25 mm² |
| Selected turns | 11 |
| Achieved inductance | 9.5033 µH |
| Achieved reactance | 597.1111 Ω |
| Wire diameter | 0.50 mm |
| DC bias current | 0.12 A |
| Estimated DC resistance | 0.057950 Ω |
| Flux density | 0.004147 T |
| Winding fill | 39.9971% |
Formula used
Target reactance:
XL,target = Multiplier × Zsystem
Required inductance:
L = XL,target / (2πf)
Turns from geometry:
N = √(L le / (μ0 μr Ae))
More design equations
Turns from AL:
N = √(LnH / AL)
DC resistance:
R = ρl / A
Reactance:
XL = 2πfL
Bias and winding checks
Flux density:
B = μ0 μr N I / le
Saturation current:
Isat = Bmax le / (μ0 μr N)
Fill percentage:
Fill % = Used copper area / Usable window area × 100
How to use this calculator
- Enter the operating frequency and system impedance of the circuit you want to isolate.
- Choose a reactance multiplier. Many RF designs use several times the circuit impedance.
- Select the turns method. Use geometry when you know the core dimensions. Use AL when a datasheet provides AL directly.
- Add the DC bias current, wire diameter, mean turn length, and window data for thermal and winding checks.
- Press the calculate button. The result block appears above the form with electrical and practical checks.
- Review the graph, notes, fill percentage, and saturation current before building the choke.
Frequently asked questions
1. What reactance multiplier should I choose?
A common starting point is 5× to 10× the system impedance at the operating frequency. Higher values usually improve RF isolation, but they can increase turns, resistance, size, and parasitic capacitance.
2. Why does the selected turns value differ from the ideal turns?
Real coils must use whole turns. The calculator rounds upward so the finished choke meets or exceeds the target inductance more reliably during construction.
3. Does the calculator include skin effect or core loss?
No. The resistance and Q outputs are first-pass estimates based mainly on copper DC resistance. At higher frequencies, skin effect, proximity effect, and core loss can significantly change the actual result.
4. Can I use this tool for air-core chokes?
Yes, but use a permeability near one and enter realistic geometry. Magnetic flux and saturation checks become less critical, while stray capacitance and self-resonance become more important.
5. What does fill factor mean?
Fill factor is the fraction of the winding window that you expect usable for copper. It accounts for insulation, bobbin shape, imperfect packing, and build tolerance.
6. Why is the estimated Q sometimes very high?
The calculator uses reactance divided by DC resistance as a fast approximation. That method ignores many RF losses, so measured Q is usually lower than the quick estimate.
7. Is saturation current important for an RF choke?
Yes, especially when the choke carries DC bias. Even if RF current is small, too much DC can push the core toward saturation and reduce effective inductance.
8. What should I check after building the choke?
Measure inductance, impedance at frequency, DC resistance, and self-resonant behavior. Then confirm temperature rise and performance inside the actual RF circuit.