Folded Dipole Antenna Explained: How It Works & Why We Use It
A folded dipole is a half-wave dipole antenna that includes an additional parallel wire connecting its two ends, forming a continuous, flattened loop. While its physical footprint and radiation pattern are almost identical to a standard half-wave dipole, the addition of the second wire fundamentally alters its electrical characteristics, making it highly advantageous for wideband and specific impedance-matching applications.
Feedpoint Impedance & Current Division
The most defining characteristic of a folded dipole is its feedpoint impedance, which is exactly four times that of a standard dipole.
- Standard Dipole: ~73 ohms (in free space).
- Folded Dipole: ~292 ohms (often rounded to 300 ohms).
The Physics Behind the 4:1 Step-Up: When you apply RF power to the feedpoint, the current flows into the driven wire and splits equally between the two parallel conductors (assuming they are of the same diameter). Because the current is split in half, the current at the feedpoint (I) is exactly half of what it would be in a standard single-wire dipole radiating the same total power.
According to the power formula P = I2R, if the current (I) is halved, the resistance (R) must be multiplied by four to radiate the same amount of power. This native ~300-ohm impedance makes the folded dipole a perfect match for 300-ohm twin-lead transmission lines, or it can be perfectly matched to standard 50-ohm coaxial cable using a 4:1 balun.
Bandwidth and Q-Factor
A folded dipole offers significantly wider bandwidth than a standard single-wire dipole. Because the two parallel wires are separated by a small distance, they act electrically like a single, much thicker conductor. In antenna theory, increasing the effective diameter of the radiating element lowers the antenna’s Q-factor (quality factor). A lower Q-factor results in a broader resonance curve, allowing the antenna to maintain a low SWR across a much wider frequency range without requiring an antenna tuner.
Asymmetrical Conductors (Impedance Tuning)
The 4:1 impedance transformation assumes both parallel wires have the same diameter. However, by using conductors of different thicknesses, you can change the impedance step-up ratio. If the unbroken, folded wire is thicker than the fed wire, it carries a larger share of the current. This further reduces the current at the feedpoint, resulting in an impedance step-up ratio much greater than 4:1 (sometimes 6:1 or 12:1). This technique is frequently used to match specific feedline requirements.
Common Applications
- Yagi-Uda Driven Elements: The parasitic elements (reflectors and directors) in a Yagi antenna drastically lower the feedpoint impedance of the driven element, sometimes dropping it to 15 or 20 ohms. By using a folded dipole as the driven element, the 4:1 multiplier steps that 15-ohm impedance back up to 60 ohms, allowing for a near-perfect direct match to 50-ohm coax.
- FM and TV Broadcast Reception: Older VHF/UHF television and FM receiver antennas commonly used folded dipoles matched directly to 300-ohm twin-lead cable.
- Wideband Wire Arrays: Folded dipoles are foundational to wideband wire designs like the T2FD (Terminated Folded Dipole), which uses a non-inductive terminating resistor in the center of the folded wire to provide continuous coverage across the entire HF spectrum.
Physical Construction
For wire antennas on the HF bands, a folded dipole is easily constructed using standard 300-ohm ladder line or 450-ohm window line. The line is cut to a standard half-wavelength (468/Frequency in MHz for feet, or 143/Frequency in MHz for meters). The two wires at both far ends are stripped and soldered together to create the loop. In the exact center of the antenna, only one of the wires is cut to serve as the feedpoint, while the other remains unbroken.
When built with rigid aluminum tubing for VHF/UHF, the unbroken center of the folded wire is at a point of zero RF voltage (a voltage node). This allows the exact center of the folded element to be bolted directly to a grounded metal boom without requiring heavy insulators, providing excellent physical stability and direct DC grounding for lightning protection and static bleed-off.