Free Small Magnetic Loop Antenna Calculator
Size compact HF transmitting loops, calculate radiation resistance, antenna Q, 2:1 SWR bandwidth, and determine peak capacitor voltage to prevent high-voltage arcing.
🧲 Frequency, Loop & Power Parameters
✓ 0.141 λ Perimeter (Within true small-loop domain)
Lethal RF Voltage Warning: Small magnetic loop antennas operate with exceptionally high $Q$. Even at just 50W to 100W transmitter power, RF voltages across the tuning capacitor exceed 3,000 to 7,000 Volts! Standard receive-only air capacitors will arc instantly. Use wide-spaced transmitting capacitors or vacuum variables.
Recommended Tools & Equipment
Tested hardware and components for high reliability
Physics of the Small Transmitting Loop (STL)
A magnetic loop antenna is electrically small when its circumference is less than a quarter wavelength ($C le lambda / 4$). Unlike standard wire antennas that radiate primarily from electric fields, the small loop couples directly into the surrounding space as a magnetic dipole:
Conductor Loss: R_loss ≈ (Perimeter / Conductor_Dia) × √(f_MHz) × 10^-3
Efficiency: η = R_rad / (R_rad + R_loss) × 100%
Quality Factor: Q = 2 × π × f × L / (R_rad + R_loss)
Peak Capacitor Volts: V_peak ≈ √(P_watts × Q × 2 × π × f × L)
Why Heavy Copper Tubing is Required
Because the radiation resistance ($R_{rad}$) of a small loop is minuscule—often between 0.02 and 0.20 Ohms (20 to 200 milliohms)—any ohmic loss in the conductor or solder joints directly destroys efficiency. If your copper conductor has 0.100 $Omega$ of RF skin-effect loss, and $R_{rad}$ is 0.050 $Omega$, two-thirds of your transmitter power is dissipated as pure heat in the copper pipe! This is why magnetic loops must be constructed from thick 3/4" to 1" copper plumbing tube or Heliax cable with welded/brazed joints.
50-Ohm Coupling Loop
To feed the loop with standard 50-ohm coaxial cable without baluns, construct a small Faraday coupling loop from RG-58 or copper wire with a diameter equal to approximately 1/5th the diameter of the main loop, positioned at the bottom opposite the tuning capacitor.
Frequently Asked Questions
Why does the antenna need retuning when changing frequency by only 20 kHz?
Because magnetic loops have an extraordinarily high Q-factor (often between 300 and 1,000), their 2:1 SWR bandwidth is typically only 10 kHz to 30 kHz on HF bands. A reduction-drive motor or stepper motor is usually installed to adjust the variable capacitor as you tune across the band.
Why are magnetic loops so quiet on receive compared to long wires?
Most urban electromagnetic interference (man-made noise from LED light switchers, EV chargers, and solar inverters) resides in the electric near-field. Because magnetic loops couple to the magnetic field and reject the electric near-field by 20 to 30 dB, they drastically drop urban background noise.
Can I touch the antenna while transmitting?
NEVER touch any part of a magnetic loop while transmitting! In addition to lethal RF voltages across the capacitor, extremely intense localized magnetic fields surround the copper conductor that can cause severe deep-tissue RF burns.