Calculator
Example Data Table
| Frequency | Diameter | Length | Turns | Wire | Example Use |
|---|---|---|---|---|---|
| 7.1 MHz | 75 mm | 140 mm | 18 | 2 mm | Large HF loading coil |
| 14.2 MHz | 50 mm | 90 mm | 12 | 1.6 mm | Portable whip coil |
| 28.5 MHz | 32 mm | 45 mm | 7 | 1.2 mm | Compact tuning coil |
Formula Used
Wavelength: λ = c / f
Corrected antenna length: Length = wavelength fraction × velocity factor × end effect factor
Wheeler air core coil: L(µH) = r²N² / (9r + 10l)
Here r and l are in inches. N is the number of turns.
Inductive reactance: XL = 2πfL
Capacitance for resonance: C = 1 / ((2πf)²L)
Skin depth: δ = √(2ρ / ωµ)
Approximate Q: Q = XL / RF resistance
How To Use This Calculator
Enter the operating frequency first. Select the antenna style. Add velocity factor and end effect when known.
Enter the physical coil diameter, winding length, turns, and wire diameter. Use the unit fields beside each value.
Add target inductance when you want a suggested turn count. Add resonating capacitance to estimate LC resonance.
Press Calculate. The result appears above the form and below the header. Use CSV or PDF for saving results.
What This Coil Calculator Does
A coil antenna often uses a compact helix or loading coil. It helps a short radiator act closer to its target band. This calculator combines coil geometry, radio frequency, conductor size, and tuning values. It is useful for hobby antennas, field repairs, mobile whips, and bench experiments.
Why Coil Geometry Matters
Turns, diameter, and winding length control inductance. A larger diameter usually raises inductance. More turns raise it quickly. A longer winding lowers it for the same turn count. Wire size also matters because it changes spacing, resistance, and possible heating. The calculator checks pitch and gap, so overlapping turns are easier to notice.
Tuning And Resonance
An antenna coil is not only a piece of wire. It stores magnetic energy. At radio frequency, that stored energy creates inductive reactance. A capacitor or antenna capacitance can tune against that reactance. The tool estimates needed capacitance and resonance with a chosen capacitor. These values help decide whether the coil is near the desired operating band.
Efficiency Checks
Coil loss can reduce radiated power. Thin wire, too many turns, and poor spacing can increase loss. The calculator estimates wire length, skin depth, AC resistance, and coil Q. These are practical indicators. They do not replace a network analyzer, but they guide better construction choices before cutting wire.
Using Results In Practice
Start with the frequency and the desired antenna style. Enter realistic diameter, length, turns, and wire size. Review inductance first. Then compare reactance, capacitor need, Q, and spacing. Change one input at a time. Larger coils may tune better, but they need more space. Small coils save room, but losses can rise. Build the coil with firm spacing. Keep leads short. Test the final antenna outdoors when possible.
Important Limits
The Wheeler formula works best for single layer air core coils. Nearby metal, insulation, weather, and mounting hardware can shift real results. A shortened antenna also depends on ground, height, and matching network. Use the output as a design starting point, then trim and measure for final tuning. For repeatable builds, record every trial. Note the measured SWR, final tap point, and nearby objects. Small notes make later antenna changes faster and safer during field work.
FAQs
What is a coil antenna?
A coil antenna uses a wound conductor as a loading or tuning part. It helps a shorter antenna work closer to the desired electrical length.
Which formula does this calculator use?
It uses Wheeler’s single layer air core coil equation. It also calculates wavelength, reactance, capacitance, pitch, skin depth, and approximate Q.
Can I use this for HF antennas?
Yes. It is useful for HF loading coils, mobile whips, portable antennas, and matching experiments. Always test the final antenna with proper RF equipment.
Why is my turn gap negative?
A negative gap means the wire is too thick for the selected turns and winding length. Increase length, reduce turns, or select thinner wire.
What does target inductance do?
Target inductance lets the calculator estimate the turns needed for your selected coil diameter and length. It helps compare design choices faster.
Is coil Q exact?
No. Q is an estimate. Real Q changes with wire surface, winding method, nearby metal, insulation, solder joints, and measurement conditions.
Why does real tuning shift?
Real antennas are affected by height, ground, feed line, supports, weather, nearby objects, and stray capacitance. Measure and trim after building.
Can I export the results?
Yes. Press Download CSV for spreadsheet records. Press Download PDF for a simple report that can be saved or printed.