Enter Spiral Loop Antenna Values
Example Data Table
| Frequency | Turns | Inner Diameter | Outer Diameter | Use Case |
|---|---|---|---|---|
| 7.1 MHz | 7 | 40 mm | 260 mm | Compact HF receive loop test |
| 14.2 MHz | 5 | 30 mm | 180 mm | Bench prototype spiral loop |
| 27.12 MHz | 4 | 25 mm | 130 mm | ISM band experiment |
| 144 MHz | 2 | 15 mm | 65 mm | Small VHF geometry check |
Formula Used
The calculator models the spiral as an Archimedean spiral. The path is described by:
r = a + bθ
The conductor length is calculated with the spiral arc length integral:
L = ∫ √(r² + b²) dθ
Free-space wavelength is:
λ = c / f
The full-wave resonance estimate is:
fres = c × velocity factor / L
Inductance is estimated with a Wheeler style planar spiral approximation:
LµH = r²N² / (8r + 11w)
Radiation resistance for a small loop is estimated by:
Rr = 31200 × (A / λ²)²
These formulas are planning estimates. Final antenna behavior depends on nearby objects, feed style, ground, enclosure, and matching parts.
How to Use This Calculator
- Enter the operating frequency in MHz.
- Add the number of spiral turns.
- Enter inner and outer diameter values in millimeters.
- Set conductor width and thickness.
- Choose the conductor material or enter custom resistivity.
- Use velocity factor to adjust resonance for real conductors.
- Press the calculate button.
- Review length, resonance, inductance, tuning capacitance, and loss values.
- Download the CSV or PDF file for project records.
Spiral Loop Antenna Design Guide
A spiral loop antenna packs a long conductor into a compact round space. It is useful when the available panel, enclosure, or experiment area is smaller than a straight wire. This calculator treats the spiral as an Archimedean path. That means the radius grows evenly as each turn moves outward.
What The Results Mean
The conductor length is the most important value. It helps compare the spiral with the operating wavelength. A full wave loop often starts near one wavelength of conductor length. A smaller receiving loop may work far below resonance, but it needs tuning and a low loss connection. The electrical length value shows whether the design is small, moderate, or large at the selected frequency.
The inductance estimate uses a Wheeler style planar spiral method. It is an approximation, not a final lab value. Real results change with substrate, nearby metal, wire shape, feed layout, and solder joints. The calculator also estimates reactance, radiation resistance, skin depth, and conductor loss. These values help you judge efficiency before cutting material.
Geometry Choices
Outer diameter increases length, area, and radiation ability. More turns increase length and inductance. A smaller inner diameter can add conductor length, but tight spacing may raise capacitance. Wider conductor lowers resistance. Larger spacing can reduce stray capacitance and make construction easier. Copper usually gives lower loss than aluminum, but mechanical strength and cost also matter.
Practical Build Notes
Use the calculator as a planning tool first. Leave trimming room on the outer tail. Measure the finished path after bending or etching. Keep the feed wires short and balanced when possible. For transmitting, check current, voltage, heating, and matching network ratings. For receiving, focus on low loss connections and stable tuning. Compare the estimated resonance with a real analyzer reading. Then adjust length, spacing, or tuning capacitance.
Good records improve every antenna build. Export the values, save the example, and note the final measured frequency. This makes later changes easier and avoids repeated trial work. When space is limited, test several layouts on paper before building. Small changes in diameter or turn count can move the design noticeably. A simple prototype often prevents wasted board, tubing, or wire during early setup.
FAQs
What is a spiral loop antenna?
It is a loop antenna shaped as a spiral. The layout places more conductor length inside a compact area, which helps when space is limited.
Is this calculator for transmitting or receiving?
It can guide both cases. For transmitting, confirm voltage, current, heat, matching parts, and legal limits before applying power.
Why does conductor length matter?
Conductor length controls electrical size. It helps estimate resonance, wavelength ratio, radiation behavior, and matching difficulty.
What does velocity factor change?
Velocity factor adjusts resonance for real conductors and layouts. A lower value makes the estimated resonant frequency lower.
Is the inductance exact?
No. It is a planning estimate. Nearby metal, substrate material, lead length, spacing, and construction style can change measured inductance.
What happens if spacing is negative?
The selected conductor width is too large for the spiral pitch. Increase diameter, reduce turns, or use a narrower conductor.
Why is radiation resistance very small?
Small loops often have tiny radiation resistance. Loss resistance can dominate, so conductor quality and matching design become important.
Should I still test the antenna?
Yes. Use an analyzer or measurement setup. Trim, tune, and record real results after building the physical antenna.