A single turn of thick tube or coax, much smaller than a wavelength (typically a circumference of about 1/10 to 1/4 wavelength), tuned to resonance by a variable capacitor. A small second loop couples it to the radio. It fits where no dipole will, indoors, on a balcony or in a backpack, but it pays for the size with efficiency and bandwidth.
How it works
The loop and its capacitor form a parallel resonant circuit, like a coil and capacitor in a filter. Current circulates round the loop, and it is that current that radiates. Seen from far away the loop makes a figure-eight, with its strongest signal in the plane of the loop and nulls broadside to it.
Small loop: signal arrives best in the plane of the loop. The nulls are broadside to it.
A small loop has a tiny radiation resistance, because it goes up with the fourth power of its size: halve the diameter and it falls by 16 times. The wire's own resistance is not tiny, so efficiency is a competition between two very small numbers.
Size, band and efficiency
Illustrative model: one turn of 22 mm copper tube, 100 W. Real loops differ, mostly through capacitor and joint losses.
Try the model. Make the loop bigger or the frequency higher and radiation resistance climbs quickly; shrink it or drop to a lower band and efficiency collapses. Even a hundredth of an ohm of extra loss, from thin tubing, a poor capacitor or an untidy joint, can cost a decibel or more on the low bands. So loops are built from fat copper or aluminum tube with short, solid, well-made connections.
The other side of the same coin is Q. Radiation and loss resistance are both small, so the circuit resonates sharply. That gives:
Narrow bandwidth. A loop is tuned for the signal you are on; if you move a few kHz you retune. Motorized tuning capacitors are common.
Selectivity. The loop rejects out-of-band energy, and a small loop is often quieter on receive in a noisy house, because it is less sensitive to nearby electrical noise.
Safety: high voltage
In practice
Tuning. Tune for the lowest SWR with the radio on low power; a loop that is off resonance by a few kHz can look badly mismatched.
Location. Keep it clear of large metal, wiring and people, which detune it and add loss.
Polarization. A vertical loop is vertically polarized; one laid flat is horizontally polarized.