RADIOAMATEURS FRANCE REVIEW (RAF)
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DUAL MAGNETIC LOOP ANTENNA
by Antonio F4VVT

TECHNICAL DATA SHEET – DOUBLE MAGNETIC LOOP ANTENNA Ø110 cm

1. General Description
The antenna presented is a double magnetic loop made of 40×2 mm anodized flat aluminum.
The two main loops, 110 cm in diameter, are mounted in parallel and separated by an axial distance of 10 cm.
The variable capacitor (VC) of 13–140 pF is mounted in the lower 10 cm gap, while the coupling loop (Faraday loop)
with a diameter of 33 cm is positioned in the upper section, inside the main double loop.
The coupling loop is made of RG-58 coaxial cable and provides the connection to the transmitter via a UHF connector (female PL-259).

2. Observations and Adjustments
• The capacitor is installed inside a PVC (insulating) box to ensure high voltage withstand.
• The double loop is mounted perfectly coaxially to avoid the creation of parasitic coupled modes.
• Slight asymmetry or misalignment may generate two distinct resonance peaks; adjust the spacing to restore a single mode.

Recommended tuning procedure:
1. Mechanically assemble the two loops and the VC enclosure.
2. Connect the coupling loop and measure S11 using a NanoVNA over the 10–21 MHz range.
3. Adjust the position of the Faraday loop to obtain minimum SWR.
4. Check the absence of double resonance and Q stability as power increases.

3. Comparison with Other Antennas
Compared with a conventional half-wave dipole, the double magnetic loop offers a significantly reduced physical size
and better rejection of electric fields, at the cost of slightly reduced efficiency on the lower bands.
Compared with a single 110 cm copper loop made of 32 mm tubing, the aluminum double loop provides:
• slightly lower inductance but slightly wider bandwidth,
• lower weight and better mechanical strength,
• comparable efficiency, with the Q factor mainly dependent on conductivity and VC contact quality.

This configuration is therefore optimized for portable use or compact fixed stations from 10 to 21 MHz.

4. Dimensions and Resonant Frequency
A diameter of 115 cm gives a circumference of approximately 3.61 m.
At 10 MHz, the wavelength is about 30 m, so the loop is approximately 1/8 wavelength.
At 21 MHz, the wavelength is about 14.3 m, so the loop is approximately 1/4 wavelength.
The 13–140 pF variable capacitor allows tuning across the entire 10–21 MHz range.

5. Efficiency and Radiation
Magnetic loop antennas of this size have lower efficiency than traditional half-wave dipoles
due to their reduced size relative to wavelength, especially at lower frequencies (10 MHz).
Efficiency improves with frequency, approaching that of a dipole around 21 MHz.
Two parallel loops spaced 10 cm apart increase effective area, widen bandwidth,
and slightly improve efficiency.
Compared to a dipole, expected loss is typically 2–3 dB on lower bands,
decreasing toward 21 MHz.

6. Comparative Characteristics
Magnetic loop antennas have a horizontal figure-eight radiation pattern
with nulls at 90° relative to the loop plane.
Dipoles have a broader and simpler radiation pattern with generally higher efficiency.
Magnetic loops are advantageous in urban environments due to reduced electrical noise
and very compact size.

Approximate relative efficiency summary (dBi vs ideal dipole):
10 MHz: –2 to –3 dB
14 MHz: –1.5 to –2 dB
18 MHz: –1 to –1.5 dB
21 MHz: –0.5 to –1 dB

Conclusion
The double magnetic loop antenna is well suited for 10–21 MHz operation,
offering improved bandwidth thanks to its dual-loop configuration,
with inherently lower efficiency than a traditional dipole at low frequencies.
It remains an excellent compact solution, particularly in space-limited and electrically noisy environments.