Advanced Coil Input Panel
Formula Used
The calculator uses a Wheeler style flat spiral estimate:
L µH = r² × N² / (8r + 11w).
Here, r is average coil radius in inches, N is turn count,
and w is winding build width in inches. The value is adjusted
by relative permeability and the layout correction factor.
Resonance is calculated with:
f = 1 / (2π√LC).
The tuning capacitor and stray capacitance are combined before
resonance is estimated. Wire resistance uses copper resistivity,
wire area, wire length, and temperature correction.
Quality factor is estimated with:
Q = XL / R.
Inductive reactance is calculated with:
XL = 2πfL.
Pulse time constant is calculated with:
τ = L / R.
How to Use This Calculator
- Select the coil shape that matches your planned winding.
- Enter the outside size and winding build width.
- Add turn count, wire diameter, and lead allowance.
- Enter the operating frequency and tuning capacitor value.
- Add stray capacitance if you know the cable or shield value.
- Use the correction factor for unusual spacing or former designs.
- Press the calculate button to view the result above the form.
- Download the CSV or PDF file for build records.
Example Data Table
| Shape |
Size mm |
Turns |
Wire mm |
Cap nF |
Use case |
| Circular |
280 diameter |
24 |
0.45 |
10 |
General coin search coil |
| Square |
300 side |
22 |
0.50 |
8.2 |
Wide sweep field test |
| Rectangular |
360 by 220 |
26 |
0.40 |
12 |
Narrow separation search |
| Circular |
180 diameter |
30 |
0.35 |
15 |
Small target recovery |
Metal Detector Coil Design Guide
Why Coil Planning Matters
A metal detector coil is more than a loop of copper. It controls the field shape, the tuning range, the sensitivity, and the stability of the search head. A larger coil can cover more ground and may notice deeper objects. A smaller coil can separate nearby targets with better control. This calculator helps builders compare those tradeoffs before winding wire or cutting a former.
Electrical Factors
Inductance depends mainly on loop size, turn count, and winding width. More turns raise inductance quickly. A wider winding spreads the magnetic field and changes the coil self behavior. Wire diameter affects resistance, which then affects quality factor. A low resistance coil usually gives a stronger response, but it may also need careful damping in the detector circuit.
Resonance and Tuning
Resonance is another important point. The coil and capacitor form a tuned circuit. When the chosen capacitor matches the coil inductance, the circuit responds strongly near its resonant frequency. This is useful for many VLF and tuned search designs. Pulse induction coils are different, but inductance and resistance still guide pulse speed and energy recovery.
Practical Accuracy
The calculator uses practical approximations. It treats square and rectangular layouts through an equivalent circular area. This keeps the tool simple while still giving useful planning values. Real coils can vary due to spacing, shielding, cable length, solder joints, ferrite, soil balance, and nearby metal parts. Final tuning should always be checked with a meter or oscilloscope.
Build Notes
Use the result as a design estimate, not a certification. Start with a safe current level. Keep lead wires twisted. Keep the shield open at one point if a shield is used. Avoid magnetic screws near the coil. After winding, measure the real inductance and resistance. Then adjust the capacitor or turn count. This process gives a coil that is easier to tune, quieter in use, and better matched to the detector circuit.
Testing Records
For best results, keep winding tension even and record every build detail. Note the wire type, turn count, former size, tape layers, measured values, and test frequency. These notes make later changes easier. They also help compare coils for parks, beaches, mineralized ground, and small target recovery without guessing. Simple testing turns rough estimates into dependable coil settings for regular field work today.
FAQs
1. What does this calculator estimate?
It estimates inductance, wire length, resistance, resonance, quality factor, current, heating, skin depth, and simple depth guidance for detector coil planning.
2. Is the inductance result exact?
No. It is a design estimate. Real inductance changes with spacing, winding neatness, shield design, cable length, nearby metal, and measuring method.
3. Which coil shape should I choose?
Use circular for common search coils, square for simple frames, and rectangular for narrow scanning or separation work.
4. What is winding build width?
It is the radial or edge width occupied by the wound turns. Wider builds usually reduce inductance compared with tighter windings.
5. Why add stray capacitance?
Cable, shielding, and layout can add capacitance. Including it gives a better resonance estimate for tuned detector circuits.
6. What does quality factor mean?
Quality factor compares inductive reactance with resistance. A higher value can mean a sharper tuned response, but damping may still be required.
7. Can this help with pulse induction coils?
Yes, for inductance, resistance, wire length, and time constant planning. Pulse coil switching behavior still needs circuit testing.
8. Should I measure the final coil?
Yes. Always measure real inductance and resistance after winding. Then adjust turns, capacitor value, or circuit damping as needed.