Magnetic Loop Antenna Calculator

Enter Loop Details

Frequency

MHz

Main Loop Diameter

Loop Diameter Unit

Conductor Diameter

Conductor Unit

Number of Turns

Transmitter Power

Conductor Material

Custom Conductivity

MS/m

Capacitor ESR

Joint and Clamp Loss

Voltage Safety Margin

Formula Used

This tool uses common planning equations for a circular electrically small magnetic loop. Results are estimates and should be checked with real measurements.

Wavelength: λ = c / f
Loop area: A = πr²
Loop circumference: C = πD
Inductance: L = μ0 × N² × r × [ln(8r / a) − 2]
Tuning capacitance: Ccap = 1 / [(2πf)² × L]
Radiation resistance: Rr = 31,200 × (N × A / λ²)²
Skin depth: δ = √[2ρ / (2πfμ0)]
Conductor loss: Rac = conductor length / (conductivity × effective skin area)
Efficiency: η = Rr / (Rr + conductor loss + capacitor ESR + joint loss)
Q factor: Q = XL / total loop resistance
Bandwidth: BW = f / Q
Capacitor voltage: V = loop current × reactance

How to Use This Calculator

Enter the operating frequency in MHz.
Add the main loop diameter and choose its unit.
Enter the conductor diameter, such as copper tube size.
Select the material or enter a custom conductivity value.
Enter transmitter power for loop current and capacitor voltage estimates.
Add capacitor ESR and joint loss when known.
Press Calculate to view the result below the header.
Use CSV or PDF buttons to save the calculated report.

Example Data Table

Use Case	Frequency	Loop Diameter	Conductor	Turns	Power	Design Note
Portable HF loop	14.2 MHz	1.0 m	18 mm copper	1	20 W	Sharp tuning expected
Balcony low band loop	7.1 MHz	1.5 m	22 mm copper	1	50 W	High capacitor voltage possible
Receive focused loop	10.1 MHz	1.2 m	12 mm aluminum	1	5 W	Lower power reduces voltage stress

Magnetic Loop Antenna Design Guide

Compact Radio Performance

Magnetic loop antennas solve a useful radio problem. They place workable low band performance inside a small space. A loop can sit on a balcony, roof corner, garden stand, or portable mast. It is narrow, sharp, and sensitive to tuning. That behavior is why careful estimates matter before cutting tube or buying a capacitor.

Resonance and Tuning

The main loop acts as an inductor. A high voltage tuning capacitor resonates that inductance at the chosen frequency. When resonance is correct, the feed system sees a much easier match. A small coupling loop often transfers energy from coax to the main loop. Its diameter is commonly near one fifth of the main loop diameter, then adjusted during testing.

Loss and Efficiency

Efficiency depends on tiny resistances. Radiation resistance is often very small. Conductor loss, joints, clamps, capacitor loss, and skin effect can therefore consume real power. A wide copper tube usually performs better than thin wire. Clean joints also help. Higher efficiency comes from reducing loss, not from forcing more power into a poor loop.

Bandwidth and Q

Bandwidth is another important clue. A very high Q loop may tune sharply. That can improve selectivity, but it also means retuning is needed after small frequency changes. The calculator estimates Q and bandwidth from reactance and total resistance. These results help compare loop sizes, materials, and conductor diameters before building.

Voltage Safety

Capacitor voltage deserves special care. Even modest transmitter power can create several thousand volts across the tuning capacitor. The estimate shown here is a planning value. Real voltages can rise with mismatch, stray capacitance, weather, and construction details. Always choose parts with generous spacing and safe ratings.

Practical Testing

The small loop formulas work best when the loop circumference is much smaller than the wavelength. A common planning target is below one tenth of a wavelength. Larger loops may still work, but the simple equations become less reliable. Use the warnings as design guidance. Final adjustment should be made with an antenna analyzer, low initial power, and safe distance from people, pets, and metal objects. Good records help future changes. Save each trial with frequency, dimensions, material, and measured standing wave ratio. Compare those notes with calculator output. The pattern will show which upgrades give the best practical improvement for your station later.

FAQs

What is a magnetic loop antenna?

It is a compact resonant loop antenna. It uses loop inductance and a tuning capacitor to operate efficiently on a selected frequency. It is popular when full size antennas are not practical.

Why is capacitor voltage so high?

A magnetic loop has high circulating current and high reactance. Their product creates large voltage across the tuning capacitor. Even moderate transmitter power can require a high voltage capacitor.

What loop size should I use?

A common starting point is a circumference below one tenth wavelength. Larger loops may radiate better, but simple small-loop formulas become less accurate as size increases.

Why does conductor diameter matter?

Thicker conductor lowers RF loss. Since radiation resistance is often tiny, small loss changes can strongly affect efficiency. Copper tube usually performs better than thin wire.

What does Q mean?

Q describes how sharply the loop tunes. A high Q loop has narrow bandwidth. It may reject nearby signals, but it also needs frequent retuning.

Is the coupling loop diameter exact?

No. One fifth of the main loop diameter is only a practical starting point. Adjust the coupling loop position and size while checking match with an analyzer.

Can I use this for receiving only?

Yes. For receive-only loops, power and capacitor voltage are less critical. Tuning capacitance, inductance, Q, and bandwidth still help plan the loop.

Are these results final build values?

No. They are planning estimates. Real loops include stray capacitance, nearby metal, weather effects, imperfect joints, and construction differences. Always test at low power first.