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HamCalc

Magnetic Loop Antenna Calculator

Design and optimize small transmitting loop antennas for amateur radio. Calculate efficiency, bandwidth, Q-factor, and capacitor stress for loops from 160m to 10m bands.

What is a Magnetic Loop Antenna?

A magnetic loop antenna (also called a small transmitting loop) is a compact, efficient antenna design popular for space-limited installations. These loops are typically less than 0.1 wavelength in circumference and use a high-voltage variable capacitor for tuning.

Key Characteristics:

  • Extremely compact size - ideal for apartments, balconies, and portable operation
  • Very narrow bandwidth - requires retuning when changing frequency
  • High Q-factor resulting in selectivity and noise reduction
  • Null in the plane of the loop - useful for direction finding and interference rejection
  • High voltages develop across the tuning capacitor - requires careful construction and safety precautions

Best Uses: Magnetic loops work well on HF bands (160m-10m) for operators with limited space. They are particularly effective for receive applications due to their low noise characteristics. For transmitting, larger loops with thick conductors provide better efficiency.

Magnetic Loop Antenna Design Formulas

Understanding the loop antenna formula helps optimize your magnetic loop design. Our magnetic loop calculator uses these key equations:

Loop Circumference

Circumference = π × Diameter (for circular loops)
Circumference = 4 × Side (for square loops)

Loop Inductance (Wheeler Formula)

L (μH) = 0.008 × N² × a² / (6a + 9h + 10w)

Where: N = turns (typically 1), a = loop radius (inches), h = conductor height, w = conductor width

Tuning Capacitance

C (pF) = 1 / (4π² × f² × L)

Where: f = frequency (Hz), L = inductance (H)

Radiation Resistance

Rr (Ω) = 31,171 × (C / λ)⁴

Where: C = circumference, λ = wavelength (same units)

Loop Efficiency

Efficiency (%) = Rr / (Rr + Rloss) × 100

Where: Rr = radiation resistance, Rloss = conductor loss resistance

Our loop antenna design calculator handles all these calculations automatically, including corrections for different conductor types, shapes, and coupling methods. Use the calculator below to design your optimal magnetic loop antenna.

Loop Design Parameters

14.2 MHz = 14200 kHz

Circular loops typically provide the best performance. Square and octagonal shapes may be easier to construct.

How the feedline couples to the main loop. Faraday loop (small coupling loop) is most common and provides good results.

Results

Enter design parameters to see results

Performance Charts

Enter design parameters and calculate to see performance charts

Important Notes:

  • These calculations are theoretical estimates based on small loop formulas from AA5TB and other sources.
  • Actual performance will vary based on construction quality, environment, ground effects, and nearby objects.
  • Always verify designs with an antenna analyzer and SWR meter before transmitting at full power.
  • High voltages can develop across the tuning capacitor. Use proper safety precautions and insulated adjustment tools.
  • Comply with local RF exposure regulations (FCC OET Bulletin 65 or equivalent in your jurisdiction).

Tuning Your Loop

Magnetic loops require precise tuning for optimal performance. Follow these steps:

  1. Start with low power (5-10W) to avoid damaging the capacitor or other components.
  2. Adjust the tuning capacitor slowly while monitoring SWR. The SWR will drop sharply when the loop resonates.
  3. Fine-tune for lowest SWR - magnetic loops can achieve SWR below 1.5:1 when properly tuned.
  4. Increase power gradually while monitoring SWR and temperature. Check for hot spots or arcing.
  5. Retune when changing frequency - even small frequency changes require retuning due to the narrow bandwidth.

Safety Warning: High voltages (several thousand volts) can develop across the capacitor during transmission. Never touch the loop or capacitor while transmitting. Use insulated adjustment tools and keep the antenna away from people and pets.

Related Resources

Learn more about magnetic loop antennas:

External References:

  • AA5TB Magnetic Loop Antenna Calculator (original source for many formulas)
  • ARRL Antenna Book - Chapter on Small Transmitting Loops
  • FCC OET Bulletin 65 - RF Exposure Guidelines for Amateur Radio