How a folded dipole works
A folded dipole is a half-wave dipole with a second conductor joined to it at both ends. The current divides between the two conductors, so for the same power the feed point sees half the current, and four times the impedance: about 290 Ω instead of 73 Ω. That's why folded dipoles are fed with 300 Ω ribbon or through a 4:1 balun. With different conductor diameters, the step-up changes, which is used in some Yagi driven elements to get a different feed impedance.
Uses
- As the driven element of a Yagi, where the low impedance of a close-spaced beam is stepped up towards 50 Ω.
- For FM broadcast and TV reception, made from 300 Ω ribbon.
- Because the whole antenna is DC-connected, it drains static, which reduces noise.
For a plain dipole, use the dipole calculator, and for the balun, the balun calculator.
Questions
What is the impedance of a folded dipole?
About 290 Ω in free space with equal conductors, four times a plain dipole's 73 Ω.
How do I feed a folded dipole?
With 300 Ω ribbon or ladder line, or with a 4:1 balun to 50 or 75 Ω coax.
Is a folded dipole longer than a normal dipole?
No, about the same length, roughly 0.95 of a half wavelength. Made from 300 Ω ribbon, it's a little shorter because of the ribbon's velocity factor.
Why use different conductor sizes?
It changes the impedance step-up, which can be used to match a Yagi's driven element.