A closed loop of wire one wavelength around, fed at a single point. The shape can be a square, diamond, triangle (a delta loop), or any outline you can support; the circumference is what counts. It has a little more gain than a dipole and a different pattern.
Fed at the bottom of a vertical loop, polarization is horizontal; fed in a side, it is vertical.
How it works
Current on the wire is a standing wave with its peak at the feed point and another peak directly opposite, and it is near zero at the middle of the sides in between. The currents in the two peak sides (bottom and top of the diagram) flow the same way at the same moment, so their fields add at right angles to the plane of the loop and are weaker in the plane. Two in-phase currents a quarter-wave apart make the extra gain.
The feed point impedance is typically around 100 Ω, so 75 Ω coax is a natural match, or 50 Ω coax at about 2:1 SWR. A balun or common-mode choke at the feed is wise.
Sizes
Band
Frequency
Circumference
Square side
80 m
3.6 MHz
279 ft (85 m)
70 ft
40 m
7.15 MHz
141 ft (43 m)
35 ft
20 m
14.2 MHz
71 ft (22 m)
17.7 ft
Cut a bit long and trim for resonance.
In practice
Polarization follows the feed. Fed at the bottom or top of a vertical loop, it is horizontal; fed at a side, it is vertical.
Orientation. A vertical loop puts its lobes broadside to its plane; a loop lying horizontal sends most energy upward.
Shapes. More enclosed area generally means a little more gain. A delta needs one high support, a diamond or square needs two or more.
Noise. Many operators find a loop quieter to receive on than a dipole; being closed, it has no ends to build up static.
Other bands. A loop cut for the lowest band can also be used on higher ones, with a tuner and a changed pattern.