Magnetic Loop Antenna Calculator
Estimate inductance, tuning capacitance, radiation resistance, efficiency, Q, bandwidth, and capacitor voltage for a circular small transmitting loop.
What this mag loop calculator does
A magnetic loop antenna is popular with amateur radio operators who need a compact antenna for limited space. The challenge is that small loops are high-Q devices with narrow bandwidth and potentially very high tuning capacitor voltage. This calculator helps you quickly evaluate whether a design is practical before you start cutting copper tube.
Enter your operating frequency, loop diameter, conductor diameter, number of turns, and intended transmit power. The tool then computes core electrical quantities that determine performance and safety.
Outputs you can use immediately
- Required capacitance: tuning capacitor value to resonate the loop at your frequency.
- Loop inductance: estimated inductance from geometry.
- Radiation resistance: how much resistance represents actual radiated power.
- Conductor loss resistance: RF loss estimate from skin effect in copper.
- Efficiency: ratio of radiation resistance to total series resistance.
- Q and 3 dB bandwidth: indicates how narrow your tuning will be.
- Capacitor RMS/peak voltage: critical for component voltage rating and spacing.
Formulas used
The calculator uses common first-order loop equations suitable for design screening:
Where r is loop radius, a conductor radius, A loop area, and λ wavelength. These are simplified models, but they are very useful to compare design options before detailed EM simulation or measurement.
Practical design guidance
1) Keep the loop electrically small
As a rule of thumb, loop circumference should stay well below one-tenth of a wavelength to stay in the classic small-loop regime. If the ratio gets too high, simple equations become less accurate and current distribution assumptions begin to break down.
2) Use thick, low-loss conductor
Larger conductor diameter reduces RF resistance and usually improves efficiency. Copper tubing is a common choice because it offers low resistance, mechanical strength, and easy bending.
3) Respect capacitor voltage
Even at modest transmit power, circulating current can create very high RF voltage across the tuning capacitor. Choose a capacitor with generous voltage margin, smooth plate edges, and safe spacing to avoid arcing.
4) Expect narrow bandwidth
High Q means retuning may be required every few kHz (especially on higher HF bands). A smooth reduction drive or motorized tuning solution can make operation much easier.
Example interpretation
Suppose you enter 14.2 MHz, 1.0 m loop diameter, 22 mm conductor, one turn, and 10 W. Typical results show:
- Capacitance in the tens to low hundreds of pF,
- Very small radiation resistance,
- A narrow 3 dB bandwidth,
- Capacitor voltage that can already be significant.
If efficiency is poor, increase loop diameter, increase conductor diameter, reduce losses in connections, and verify all joints are low resistance.
Important notes
- This is an educational calculator, not a substitute for field strength measurement or vector network analysis.
- Real-world performance depends on coupling loop design, nearby objects, soil, enclosure effects, and construction quality.
- At higher powers, treat the capacitor section as a high-voltage RF hazard.