Magnetic loop antenna
| Magnetic loop antenna | |
|---|---|
| Also called | Small transmitting loop (STL), mag loop |
| Size | Circumference about a tenth of a wavelength or less; transmitting loops sometimes approach a quarter wavelength at the top of their range |
| Tuned by | High-voltage variable capacitor, usually a vacuum or butterfly type |
| Fed by | Small coupling loop or gamma match |
| Characteristics | |
| Bandwidth | Very narrow; small frequency changes need retuning |
| Q | High, typically in the hundreds |
| Efficiency | Falls steeply as the loop gets smaller compared with the wavelength |
| Main hazards | Thousands of volts at the capacitor; strong RF fields close to the loop |
A magnetic loop antenna is a small loop of heavy conductor, often a ring of copper tubing about a meter across, tuned to resonance by a variable capacitor. Among hams the term usually means the small transmitting loop (STL), a compact HF antenna that fits on a balcony, in an attic or on a picnic table where a full-size dipole will not. It is popular with operators facing antenna restrictions and for portable operating. The name comes from the field close to the antenna, which is mostly magnetic. It should not be confused with a full-wave loop, which is about one wavelength around.
How it works
A loop counts as "small" when its circumference is about a tenth of a wavelength or less. Transmitting loops are often somewhat larger, and one that covers several bands may approach a quarter wavelength around at its highest frequency. A loop 3.3 feet (1 m) across is about 10 feet (3.1 m) around: under a tenth of a wavelength on the 40-meter band but over a fifth of a wavelength at 21 MHz.
The loop itself acts as an inductor. A capacitor across a gap at the top resonates with it, and at resonance a large current circulates around the ring and does the radiating. Power from the coax is usually coupled in by a smaller loop, typically about one fifth the diameter of the main loop, mounted opposite the capacitor. It works as a transformer, raising the main loop's very low impedance to the 50 ohms the radio expects. A gamma match is a common alternative.
Mounted upright, a small loop radiates best off its edges and least through its face, a figure-eight pattern with two deep, narrow nulls.
Efficiency and size
The loop's radiation resistance, the part of its impedance that represents power actually radiated, is tiny: a small fraction of an ohm, often only thousandths of an ohm on the lower bands. Any resistance in the conductor, joints and capacitor competes with it, turning power into heat instead of signal, so good loops use thick copper tubing, soldered or welded joints, and capacitors without sliding contacts.
For a loop of a given size, radiation resistance falls with the fourth power of frequency: halving the frequency cuts it to one sixteenth. A loop that performs well on the 20-meter band is therefore much less efficient on 40 meters and usually poor on the 80-meter band. A well-built loop can perform respectably on its higher bands, but it is rarely the equal of a full-size antenna in the clear, and the gap grows on its lowest band.
Tuning and bandwidth
A magnetic loop is a high-Q circuit. Q is a measure of how sharply a tuned circuit responds, and a high Q means a very narrow bandwidth. On the lowest band a loop covers, the range with an acceptable SWR may be only a few kilohertz, so moving even a short way along the band means retuning. Many have a motor-driven capacitor with a remote control.
The capacitor must withstand very high RF voltage. A vacuum variable capacitor has its plates sealed in a glass or ceramic envelope and handles high voltage. A butterfly capacitor has two stator sections bridged by the rotor, so the loop current never passes through a lossy sliding contact.
The narrow bandwidth has an upside: the loop acts as a filter against strong signals outside the frequency it is tuned to, and the FT8 segment of a band usually fits within its bandwidth.
Safety
The voltage across a loop's capacitor can reach several thousand volts at 100 watts. Touching the capacitor, or the loop near it, while transmitting can cause a deep and painful RF burn. Keep the loop out of reach of people and pets, especially on a balcony or at a portable site.
Because a loop is often used indoors or within a few feet of the operator, RF exposure deserves particular attention. The fields close to a loop are intense, and a station comfortably within the limits with a dipole at 40 feet may not be with a loop beside the operating position. Evaluate the station as the FCC rules require, keep as much distance as practical, and use no more power than needed; many loop users run QRP or modest power.
Receiving loops
Small loops are also widely used for receiving only. A receiving loop needs no high-voltage capacitor or heavy tubing, because on the lower HF and medium-wave bands natural and man-made noise usually outweighs the signal lost to inefficiency. Many are untuned wideband loops with a built-in preamplifier, called active loops, popular with listeners using software-defined radios. The nulls are a great asset: turning the loop so that a null points at a noisy device nearby can reduce interference dramatically.