A conducting loop with a perimeter small compared with wavelength forms a resonant circuit with a capacitor. Loop current can become very high even at moderate transmitter power.
Antennas · F1SYH
Magnetic loop
A magnetic loop is a compact antenna tuned by a variable capacitor. Its small size is achieved at the cost of narrow bandwidth and very high circulating current.
HF
Compacte
Q élevé
Operating principle
Main characteristics
Typical gain
Variable selon diamètre et pertes
Impedance
Adaptée par boucle ou gamma
Polarization
Principalement magnétique proche
Difficulty
Avancée
Dimensions and design
The perimeter is generally less than roughly one tenth wavelength. Large-diameter tubing reduces RF resistance and improves efficiency.
Construction and tuning
Step 1
Use large-diameter conductive tubing.
Step 2
Make all connections short and very low resistance.
Step 3
Install a capacitor rated for high RF voltage.
Step 4
Provide insulated or motorized tuning.
Step 5
Test resonance at low power before transmitting.
Advantages
- Small physical size.
- High selectivity.
- Useful side nulls.
- Can operate relatively close to ground.
Limitations
- Very narrow bandwidth.
- Efficiency highly sensitive to losses.
- Retuning required after frequency changes.
- Dangerous capacitor voltage.
History and origin
Small loops have long been used for reception, direction finding and transmission. Amateur use expanded where available space is limited.
Safety
The tuning capacitor may carry several kilovolts. The antenna must not be accessible during transmission.