Calculator Input
Example Data Table
| Inductance | Capacitance | Stray Capacitance | Approximate Resonance | Typical Use |
|---|---|---|---|---|
| 10 µH | 100 pF | 5 pF | 4.91 MHz | RF tank testing |
| 47 µH | 220 pF | 8 pF | 1.54 MHz | AM band tuning |
| 1 mH | 10 nF | 0 pF | 50.33 kHz | Audio filter work |
| 220 nH | 33 pF | 2 pF | 57.45 MHz | VHF prototype checks |
Formula Used
The main resonant frequency is calculated with:
f = 1 / (2π√LC)
Here, f is frequency in hertz. L is inductance in henries. C is total capacitance in farads. Total capacitance includes the entered capacitor and optional stray capacitance.
Angular frequency uses:
ω = 2πf
Reactance uses:
XL = 2πfL
XC = 1 / (2πfC)
Series quality factor uses:
Q = √(L / C) / R
Bandwidth estimate uses:
BW = f / Q
Stored electric energy uses:
E = 0.5CV²
Air-core coil inductance can be estimated with Wheeler form:
LµH = d²N² / (18d + 40l)
The diameter and winding length are converted to inches before this estimate is used.
How To Use This Calculator
- Enter the coil inductance and select its unit.
- Enter the capacitor value used with the coil.
- Add stray capacitance when leads, winding, layout, or probe loading matter.
- Enter resistance to estimate Q and bandwidth.
- Enter a test frequency to compare XL and XC away from resonance.
- Enter voltage when stored energy and peak tank current are needed.
- Enable estimated inductance when coil dimensions should replace direct inductance.
- Press calculate. The result appears above the form.
- Use CSV or PDF export for reports and records.
Coil Resonance Guide
Why Coil Resonance Matters
A coil does more than oppose changing current. It also works with capacitance. That capacitance may be a selected capacitor. It may also come from winding spacing, leads, nearby metal, and measuring probes. When inductive and capacitive reactance balance, the circuit reaches resonance. At that point, energy moves between the magnetic field of the coil and the electric field of the capacitor.
Practical Circuit Insight
This calculator helps you inspect that point quickly. It accepts direct inductance values. It can also estimate an air-core coil with Wheeler’s equation. That is useful during early RF work, antenna matching, detector circuits, filters, oscillators, and tuned receivers. The estimate is best for ordinary single-layer coils. Real coils can shift because wire diameter, insulation, shield cans, nearby boards, and self-capacitance affect the final value.
Capacitance And Stray Effects
Small capacitance changes can move high frequency circuits a lot. A few picofarads may matter in radio designs. For this reason, the form separates external capacitance from stray capacitance. Add expected parasitic capacitance to get a more realistic result. When you measure a circuit, include probe capacitance if the probe is connected during operation.
Resistance, Q, And Bandwidth
Resistance controls loss. A lower series resistance normally gives a higher Q. Higher Q gives a narrower bandwidth and a sharper tuning point. A lower Q gives wider response and more damping. The calculator estimates series Q, bandwidth, and half-power points when resistance is supplied. It also shows a parallel Q estimate for comparison.
Reactance At A Test Frequency
The optional test frequency is helpful when checking whether a coil-capacitor pair is below, above, or near resonance. If inductive reactance is larger than capacitive reactance, the series network looks inductive. If capacitive reactance is larger, it looks capacitive. Near resonance, those values approach balance.
Energy And Lab Use
Voltage input adds stored energy and peak tank current estimates. These values help during bench checks, safe component selection, and oscillator planning. The exported CSV and PDF files make it easier to save each trial. You can compare coil turns, capacitor values, and layout assumptions before building the final circuit.
FAQs
What is coil resonant frequency?
It is the frequency where coil inductance and total capacitance balance. At this point, energy transfers between magnetic and electric fields, creating a tuned response.
Does stray capacitance affect the result?
Yes. Stray capacitance adds to the selected capacitor. It can lower the resonant frequency, especially in RF circuits using small picofarad values.
What unit should I use for inductance?
Use the unit that matches your measurement or datasheet. The calculator converts H, mH, µH, and nH into henries internally.
Why is resistance included?
Resistance estimates losses. It helps calculate Q and bandwidth. More resistance usually means lower Q, wider bandwidth, and less selective tuning.
What is Q factor?
Q factor describes how sharp or selective the resonance is. A higher Q gives a narrower response. A lower Q gives more damping.
Can this estimate an air-core coil?
Yes. Enable the air-core option and enter turns, diameter, and winding length. The result uses Wheeler’s single-layer coil estimate.
Why enter a test frequency?
A test frequency shows inductive reactance, capacitive reactance, and net reactance away from resonance. This helps diagnose circuit behavior.
Is the PDF generated on the server?
No. The PDF is created in the browser from the visible results table. The CSV is generated by the server for spreadsheet use.