UNCLASSIFIED
TM-ANT-045
MAGNETIC LOOP ANTENNA
High-Q Manually Tuned Magnetic Loop for HF, 40M–10M
Prepared by: Mervyn Martin, KO6NNH
Merced, California  •  26 May 2026
Amateur Radio / Antenna Engineering — Not for commercial use

Table of Contents

  1. Chapter 1 — General Information
  2. Chapter 2 — Theory of Operation
  3. Chapter 3 — Materials and Construction
  4. Chapter 4 — Assembly Procedures
  5. Chapter 5 — Calibration Procedure
  6. Chapter 6 — Tuning and Adjustment
  7. Chapter 7 — Verification
  8. Appendix A — Calculations and Formulas
  9. Appendix B — Example Results

CHAPTER 1 — GENERAL INFORMATION

1-1. SCOPE

This manual covers the design, construction, and safe operation of the Magnetic Loop Antenna for amateur radio HF use on 40M–10M (7–30 MHz). Magnetic loops are compact, low-noise antennas with narrow bandwidth, requiring careful tuning for each frequency. Due to high circulating currents and voltages at the tuning capacitor, strict safety procedures must be followed.

1-2. APPLICABLE REFERENCES

  • ARRL Antenna Book — Small Transmitting Loops Chapter
  • G4FGQ Technical Notes on Magnetic Loops
  • W0BTU / AA5TB Small Transmitting Loop Calculators
  • NEC2 model: magnetic_loop_antennas.nec (in antenna directory)

1-3. SAFETY PRECAUTIONS

WARNING HIGH VOLTAGE HAZARD: At 100 W output power the tuning capacitor of a magnetic loop antenna develops 4,000–12,000 V across its terminals, depending on loop Q and operating frequency. Use ONLY vacuum variable capacitors or high-voltage butterfly (split-stator) capacitors rated for at least 5 kV RMS. Do NOT use DIY stacked aluminum-plate capacitors, rolled-foil capacitors, or short lengths of coaxial cable as capacitors — these will arc and fail, causing equipment damage and potential personal injury. Allow the capacitor to fully discharge before touching any part of the loop structure after transmitting. Keep hands and metal objects away from the capacitor gap during tuning.
CAUTION — RF EXPOSURE Maintain minimum safe distance from all energized antenna elements during transmission. At QRP power levels (≤5 W) the MPE boundary is typically <1 m for HF antennas. At 100 W the controlled exposure limit for HF antennas requires maintaining ≥3–10 m distance depending on frequency (per FCC OET Bulletin 65). Never touch feed-point hardware or support structures while transmitting. Verify PTT key is open before antenna work.

CHAPTER 2 — THEORY OF OPERATION

2-1. RESONANT LOOP PHYSICS

A magnetic loop (small transmitting loop, STL) is an electrically small resonant circuit. The loop inductance L is tuned to resonance by a capacitor C. At resonance, circulating current Icirc = Vin × Q / Zloop where Q is the loop quality factor. Q values of 100–1000 are typical; this produces circulating currents far exceeding the feed current, and capacitor voltages VC = Icirc / (2πf·C) which can reach 4,000–12,000 V at 100 W.

2-2. RADIATION PATTERN AND GAIN

A small horizontal loop radiates as a magnetic dipole with a figure-eight pattern in the plane of the loop (null perpendicular to loop plane). Gain: 2–4 dBi (at height above 0.1λ). The null depth exceeds 20 dB and can be used for interference rejection by rotating the loop. Bandwidth is inversely proportional to Q: BW3dB = f0/Q; typical BW = 3–15 kHz at HF. This narrow bandwidth means the loop must be retuned for every frequency change.

2-3. EFFICIENCY AND CONDUCTOR SELECTION

Efficiency: 75–95% (depends on Q; copper best). Loop efficiency η = Rr/(Rr + Rloss). Radiation resistance Rr for a small loop scales as (A/λ²)², making large loop diameter critical for efficiency. Conductor resistance Rloss must be minimized: use copper or aluminum tubing (1–2 in OD for HF transmitting loops). Connections must be silver-soldered or bolted with contact resistance <0.001 Ω. Any high-resistance joint will dissipate power and possibly arc at high Q.

2-4. CAPACITOR REQUIREMENTS

Vacuum variable capacitor (recommended): rated for peak voltage ≥5 kV at operating power level. Split-stator butterfly capacitor: acceptable if plate gap is sufficient. Do NOT use: aluminum plate DIY capacitors, rolled-foil capacitors, silver-mica stacks, or coaxial stubs. Typical capacitance range: 10–500 pF for 40M–10M operation with 6–10 ft loop diameter.

CHAPTER 3 — MATERIALS AND CONSTRUCTION

3-1. BILL OF MATERIALS

Materials — Magnetic Loop Antenna
QtyItemSpecification
1 lengthMain loop conductorCopper or aluminum tubing 1–1.5 in OD; circumference per design frequency
1Vacuum variable capacitorRated ≥5 kV RMS, 10–500 pF range; Jennings, Cardwell, or equivalent
1Coupling loopDiameter 0.2× main loop; #12 AWG copper; used for 50 Ω feed
1Capacitor drive mechanismReduction drive ≥10:1 for fine tuning; motor drive for remote operation
1Support frameNon-conductive (PVC, wood, fiberglass); must resist loop torque
1SO-239 connectorFor coupling loop feed; coax shield bonds to coupling loop
WARNING HIGH VOLTAGE HAZARD: At 100 W output power the tuning capacitor of a magnetic loop antenna develops 4,000–12,000 V across its terminals, depending on loop Q and operating frequency. Use ONLY vacuum variable capacitors or high-voltage butterfly (split-stator) capacitors rated for at least 5 kV RMS. Do NOT use DIY stacked aluminum-plate capacitors, rolled-foil capacitors, or short lengths of coaxial cable as capacitors — these will arc and fail, causing equipment damage and potential personal injury. Allow the capacitor to fully discharge before touching any part of the loop structure after transmitting. Keep hands and metal objects away from the capacitor gap during tuning.

3-2. DIMENSION FORMULAS

Loop inductance (circular loop, μH)L = 0.002 × D × (ln(4D/d) − 2) where D = loop diameter (cm), d = conductor OD (cm)
Resonant capacitance (μF)C = 1 / (4π² × f² × L)
Capacitor voltage at resonance (V peak)VC = √(2 × P × Q² × Rr) where P = transmit power (W), Q = loop Q factor
Example: 100 W, Q=300, R_r=0.08 ΩVC ≈ 8,500 V peak — REQUIRES VACUUM CAPACITOR

CHAPTER 4 — ASSEMBLY PROCEDURES

WARNING HIGH VOLTAGE HAZARD: At 100 W output power the tuning capacitor of a magnetic loop antenna develops 4,000–12,000 V across its terminals, depending on loop Q and operating frequency. Use ONLY vacuum variable capacitors or high-voltage butterfly (split-stator) capacitors rated for at least 5 kV RMS. Do NOT use DIY stacked aluminum-plate capacitors, rolled-foil capacitors, or short lengths of coaxial cable as capacitors — these will arc and fail, causing equipment damage and potential personal injury. Allow the capacitor to fully discharge before touching any part of the loop structure after transmitting. Keep hands and metal objects away from the capacitor gap during tuning.
  1. Bend main loop tube to circular or square shape. Ensure all bends are smooth — kinks increase resistance.
  2. Mount vacuum capacitor at top of loop. Solder or silver-braze all connections to minimum contact resistance. Verify <0.001 Ω across each joint with milli-ohmmeter.
  3. Construct coupling loop as a circle of diameter 0.2× main loop. Mount coupling loop at the bottom of the main loop, centered and in the same plane.
  4. Connect coax braid and center conductor to coupling loop terminals. No balun required — the coupling loop is inherently isolated.
  5. Mount assembly on non-conductive support. Ensure no metal within 1 m of loop conductor.
  6. Install capacitor drive mechanism with ≥10:1 reduction. Label rotation direction for capacitance increase.
  7. Tag the capacitor: “HIGH VOLTAGE — UP TO 12 kV AT 100W. DO NOT TOUCH WHEN TRANSMITTING.”

CHAPTER 5 — CALIBRATION PROCEDURE

WARNING HIGH VOLTAGE HAZARD: At 100 W output power the tuning capacitor of a magnetic loop antenna develops 4,000–12,000 V across its terminals, depending on loop Q and operating frequency. Use ONLY vacuum variable capacitors or high-voltage butterfly (split-stator) capacitors rated for at least 5 kV RMS. Do NOT use DIY stacked aluminum-plate capacitors, rolled-foil capacitors, or short lengths of coaxial cable as capacitors — these will arc and fail, causing equipment damage and potential personal injury. Allow the capacitor to fully discharge before touching any part of the loop structure after transmitting. Keep hands and metal objects away from the capacitor gap during tuning.
  1. Begin with transmitter output ≤5 W for initial tuning. Do not apply full power until resonance is confirmed.
  2. SOLT calibrate NanoVNA. Connect to coupling loop feedpoint.
  3. Set NanoVNA sweep: target frequency ±10%.
  4. Slowly rotate capacitor while watching SWR display. Resonance appears as a sharp dip (narrow bandwidth — sweep may miss it at first).
  5. Narrow sweep span to ±2% once resonance found. Verify minimum SWR occurs at exact target frequency.
  6. Record: fres, SWRmin, 3 dB bandwidth (rotate cap slowly to find SWR = 1.4× minimum SWR points).
  7. Calculate Q: Q = fres / BW3dB. High Q (>200) indicates good construction quality.
  8. Verify capacitor voltage is within rating before applying full power: VC = √(2PQ²Rr).

CHAPTER 6 — TUNING AND ADJUSTMENT

Magnetic loops require tuning for every frequency change due to high Q and narrow bandwidth. Tune by rotating the variable capacitor until minimum SWR at the desired frequency. A good indicator of resonance is maximum receiver noise (on receive) or minimum reflected power (on transmit at low power). Fine-tune on the air by adjusting for maximum signal strength on a known station or beacon.

CAUTION Always tune at ≤5 W first. Verify SWR <2:1 at low power before increasing to full power. A mis-tuned loop at full power reflects most power back to the transmitter.

CHAPTER 7 — VERIFICATION

Acceptance Criteria — Magnetic Loop Antenna
ParameterRequirementPass/Fail
SWR at resonance (after coupling loop optimization)< 2.0:1____
Loop Q (minimum)≥100 (at center band)____
Bandwidth (3 dB)Consistent with Q____
Gain (NEC2)2–4 dBi (at height above 0.1λ)____
Efficiency75–95% (depends on Q; copper best)____
Capacitor voltage (calculated)<80% of cap rating____

APPENDIX A — CALCULATIONS AND FORMULAS

Loop inductance L (μH, circular loop)L = 0.002D[ln(4D/d) − 2] (D, d in cm)
Resonant capacitanceC = 1/(4π²f²L) (μF when L in μH, f in MHz)
BandwidthBW = f0/Q
Efficiencyη = Rr/(Rr + Rloss)
Capacitor peak voltageVpeak = √(2×P) × Q/√(Rr)

APPENDIX B — EXAMPLE RESULTS

Expected Measurements — Magnetic Loop Antenna
BandfresSWRQBW (kHz)Cap V (100W)
40M7.150 MHz<1.5:1200–40018–36 kHz4–8 kV peak
20M14.175 MHz<1.5:1300–60024–47 kHz6–12 kV peak
15M21.225 MHz<1.5:1400–70030–53 kHz8–12 kV peak
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