magnetic loop antenna calculator

What this magnetic loop antenna calculator does

This magnetic loop antenna calculator is built for quick first-pass design work. Enter your target frequency, loop size, conductor diameter, and power level, and it calculates the most useful design values for a small transmitting loop:

• Loop inductance
• Required tuning capacitance for resonance
• Radiation resistance and estimated conductor loss resistance
• Efficiency estimate
• Q factor and approximate 3 dB bandwidth
• RMS and peak capacitor voltage at your chosen power

For amateur radio operators, this gives you a practical starting point before building, testing, and fine-tuning in the real world.

Input fields explained

Operating Frequency (MHz)

This is your target operating frequency (for example, 7.1 MHz for 40 m or 14.2 MHz for 20 m). The required capacitor value and loop current change strongly with frequency.

Loop Diameter (m)

The calculator assumes a circular loop. Larger loops generally improve radiation resistance and efficiency, but the antenna should still remain electrically small relative to wavelength for these formulas to stay accurate.

Conductor Diameter (mm)

Thicker conductors reduce RF loss resistance and can increase efficiency. Copper tubing is commonly used in practical high-Q magnetic loop builds.

Number of Turns

Most transmitting magnetic loops are one turn. Additional turns change inductance and resistance behavior significantly. Use this field for exploratory calculations or special designs.

Transmit Power (W)

Used to estimate loop current and capacitor voltage at resonance. Even with modest transmitter power, capacitor voltage can become very high due to high circulating current and high reactance.

Formulas used in this calculator

The script uses widely cited approximations for small loops:

• Wavelength: λ = c / f
• Loop area: A = πr²
• Inductance (circular loop): L ≈ μ₀N²r(ln(8r/a) − 2)
• Resonating capacitance: C = 1 / ((2πf)²L)
• Radiation resistance: R_rad ≈ 31200 × (NA / λ²)²
• Estimated RF conductor resistance: skin-effect approximation for round conductors
• Efficiency: η ≈ R_rad / (R_rad + R_loss)
• Q factor: Q ≈ ωL / (R_rad + R_loss)
• Approximate 3 dB bandwidth: BW ≈ f / Q

Practical design tips for magnetic loops

• Use low-loss, large-diameter conductor (copper tube is common).
• Keep joints mechanically solid and electrically clean.
• Choose a vacuum or high-quality RF capacitor with a large voltage margin.
• Expect narrow bandwidth, especially on lower HF bands.
• Add remote tuning if you plan to move frequency often.
• Keep safety distance: high RF fields can exist near the loop.

Interpreting results correctly

Treat results as design estimates, not final measured values. Real antennas are influenced by nearby objects, feed coupling method, capacitor ESR, solder quality, and installation height. Always validate with measurements (VNA, SWR bridge, field strength checks) before high-power operation.

When estimates can be off

• Loop circumference is not electrically small.
• Multi-turn geometry is not tightly coupled or evenly spaced.
• Conductor is not round (strip/tape/braided forms).
• Additional losses from capacitor, feed loop, enclosure, or nearby structures are significant.

Bottom line

A magnetic loop antenna calculator helps you move from guesswork to a realistic starting design in seconds. Use it to estimate key electrical values, then build safely with generous voltage margins and verify everything on-air with careful tuning.