A half-wave dipole with a second wire joining its two ends, so it looks like a long thin loop. The feed point is at the center of one wire. It radiates like a plain dipole, but the feed-point impedance is about four times higher.
Same radiation, but each wire carries only half the feed current, so the feed point impedance is about four times higher.
Why four times the impedance
Both wires carry current in the same direction, so both radiate, but the source feeds only one of them. Call the total radiating current I. It splits between the two wires, so each carries I ÷ 2, and the source supplies only I ÷ 2. The antenna radiates power = I² × 73 Ω, exactly as a plain dipole would. The source delivers that power from half the current, so impedance = power ÷ (I ÷ 2)² = 4 × 73 ≈ 292 Ω.
With three wires each carries I ÷ 3, and the impedance is 9 times higher. For equal-diameter wires the rule is 73 Ω × n².
Illustrative: equal-diameter wires, a free-space half-wave. The same radiated power comes from n times less current at the feed.
Feed point impedance292 Ω
Why use it
Matching. About 300 Ω matches 300 Ω twin-lead directly, or a 4:1 balun brings it to 75 Ω: 300 ÷ 4 = 75. Onto 50 Ω coax that leaves an SWR of 75 ÷ 50 = 1.5.
Bandwidth. The folded form is usually a little more tolerant of frequency than a thin single-wire dipole, which is often the reason for choosing it.
Impedance step-up. Parasitic elements pull a Yagi driven element's impedance down; a folded driven element lifts it back up. See The Yagi-Uda beam.
In practice
Twin-lead or ladder line is a convenient way to build one: short the two ends together, and cut one conductor at the center for the feed. The insulation slows the wave, so a twin-lead version comes out shorter than a bare wire. Cut long and trim.
Feed through a balun and a choke, because the feed is balanced and coax is not.
Height and pattern behave as they do for the The half-wave dipole. Like it, it is a single-band antenna.