Folded Dipole Length Calculator
Use this tool to estimate folded dipole dimensions and feedpoint impedance. Enter your design frequency, velocity factor, and number of parallel conductors.
A folded dipole is one of the most practical wire antennas in RF work. It is mechanically simple, broadens bandwidth compared to a thin single-wire dipole, and is often easier to match in real systems. If you want to build one for ham radio, FM reception, VHF experiments, or educational projects, this page gives you both a working calculator and practical design guidance.
What is a folded dipole?
A folded dipole is usually made from two parallel conductors joined at both ends, forming a loop-like shape. The feedpoint is placed in the center of one conductor. The total electrical length is approximately a half-wavelength at the target frequency, similar to a regular half-wave dipole, but the current/voltage distribution changes due to the folded geometry.
The classic two-wire folded dipole has an input impedance near 300 ohms at resonance (in free space, idealized). This is why older TV antennas often used 300-ohm twin-lead with a folded dipole element.
How this calculator works
Core equations
| Parameter | Formula |
|---|---|
| Wavelength | λ = 300 / fMHz |
| Folded dipole total length | L = (150 / fMHz) × VF |
| Each long side | L / 2 |
| Each quarter section | L / 4 |
| Approx. feedpoint impedance | Z ≈ 73 × n2 |
Where:
- fMHz is center frequency in MHz
- VF is velocity factor (accounts for conductor/environment effects)
- n is number of identical parallel conductors in the folded structure
How to use the results
- Enter your target frequency in MHz.
- Choose a velocity factor. For many practical wire builds, 0.93 to 0.98 is a common starting range.
- Set conductor count (2 is most common).
- Cut slightly long (the calculator includes a 2% long suggestion) and trim while monitoring SWR or return loss.
- Add a balun/choke near the feedpoint to control common-mode current.
Practical construction tips
1) Conductor spacing matters
The spacing between parallel conductors affects impedance and bandwidth. Keep spacing uniform to maintain predictable behavior. Even simple spacers every few inches can improve repeatability.
2) Keep it symmetric
Folded dipoles perform best when both sides are physically symmetric. Avoid sharp bends near the feedpoint and keep lead routing balanced to reduce pattern distortion.
3) Install at useful height
At VHF and above, even modest height changes can noticeably alter pattern and feedpoint impedance due to ground interaction. Final tune at or near operating height when possible.
4) Tune in tiny steps
Do not remove too much conductor at once. Small trims (a few mm to a few cm, depending on frequency) are safer and easier to control.
Matching and feedline guidance
- A 2-wire folded dipole is often close to 300 Ω at resonance.
- With 75 Ω coax, a 4:1 balun is a common match strategy.
- With 50 Ω coax, direct feed usually shows mismatch unless a matching network is used.
- A current balun (or common-mode choke) helps keep feedline radiation under control.
Example use case
If you design for 146.52 MHz with VF = 0.95 and n = 2:
- Total length is close to 0.97 m (about 3.19 ft)
- Each long side is about half that
- Estimated feedpoint impedance is near 292 Ω
That gives you a strong starting geometry for a VHF folded dipole, with final trim based on your analyzer readings.
Common mistakes to avoid
- Cutting exactly to theoretical length without leaving trim margin
- Ignoring feedline/choke effects during measurement
- Tuning too close to metal gutters, masts, or walls and expecting free-space behavior
- Changing conductor spacing during installation
Final thoughts
A folded dipole is a great blend of simplicity and performance. Use the calculator to get your first-cut dimensions, then validate with an antenna analyzer and adjust carefully. With good symmetry and a proper feed arrangement, you can get reliable performance across many applications.