Folded Dipole Antenna Explained
A folded dipole antenna is a variation of the standard half-wave dipole that consists of two parallel conductors connected at both ends, forming a continuous loop. One of the conductors is split at the center to connect the feedline. Despite its simple appearance, it offers distinct electrical advantages over a standard dipole, particularly regarding impedance and bandwidth.
Technical Characteristics
1. High Input Impedance
The most significant feature of a folded dipole is its input impedance. While a standard half-wave dipole has a center-feed impedance of approximately 72Ω in free space, a folded dipole with two conductors of equal diameter has an impedance of approximately 300Ω.
- The 4:1 Ratio: The impedance is roughly four times that of a standard dipole (Zfolded = N2 x Zstandard, where N is the number of conductors).
- Matching: This makes it a perfect match for 300Ω twin-lead or ladder line, which was historically common for TV and FM broadcast reception.
2. Increased Bandwidth
Folded dipoles exhibit a flatter impedance curve across a range of frequencies compared to standard dipoles. This increased bandwidth makes them more “forgiving” and allows them to operate effectively over a wider portion of a band without a significant increase in SWR (Standing Wave Ratio).
3. Structural Rigidity
Because the antenna is essentially a closed loop, it is often more mechanically robust than a standard dipole, which has two “dead-end” wires. This loop structure also helps reduce static noise buildup on the antenna.
Comparison: Standard Dipole vs. Folded Dipole
| Feature | Standard Dipole | Folded Dipole |
| Impedance | ~ 72Ω | ~ 300Ω |
| Bandwidth | Narrower | Wider |
| Construction | Two separate wires | Continuous loop |
| Feedline Match | 50Ω or 75Ω Coax | 300Ω Twin-lead / Ladder line |
| DC Path | Open circuit | Short circuit (at DC) |
Common Applications
- FM Radio & Television: Many classic “T-shaped” indoor FM antennas are folded dipoles made from twin-lead wire.
- Amateur Radio: Used frequently on HF bands when using balanced feedlines, or as driven elements in Yagi-Uda arrays to raise the array’s overall low impedance to a more manageable level.
- VHF/UHF Operations: Due to the broad bandwidth, they are often used in commercial two-way radio base stations where multiple frequencies are used within the same band.