Folded Dipole Length Calculator
Enter your target frequency and construction details to estimate folded dipole dimensions, total wire needed, and feedpoint impedance.
What this folded dipole calculator does
This tool gives you a fast starting point for building a two-wire folded dipole for HF, VHF, or UHF work. A folded dipole is popular because it is mechanically sturdy, naturally balanced, and often provides a wider usable bandwidth than a thin straight dipole at the same center frequency.
- Calculates wavelength and practical half-wave length from frequency.
- Applies a shortening factor (velocity factor) for real-world materials.
- Estimates long-side length, leg length, and total conductor length.
- Provides a simple feedpoint impedance estimate based on conductor ratio.
Folded dipole formulas used in this page
1) Wavelength
The free-space wavelength is calculated from frequency in MHz:
λ (meters) = 299.792458 / f(MHz)
2) Practical resonant length
A classic dipole starts near half wavelength:
Lhalf-wave = λ / 2
Real antennas are usually shortened slightly due to end effects and conductor geometry, so we apply a user-defined factor:
Lpractical = Lhalf-wave × velocity factor
3) Folded dipole geometry
In a standard folded dipole, each long parallel conductor is approximately the full practical half-wave length. The total wire estimate includes both long sides and the two short end connections:
Total wire ≈ 2 × Lpractical + 2 × spacing
4) Feedpoint impedance estimate
For equal diameter conductors, the feedpoint impedance is typically near 300 Ω. This calculator uses a simplified engineering estimate:
Zin ≈ 73 × (1 + diameter ratio)2
This is useful for quick planning, but final impedance depends on spacing, element diameter, nearby objects, and installation height.
Practical build guidance
Material and diameter
Thicker conductors generally improve bandwidth and make tuning less sharp. Copper tubing, aluminum rod, or heavy copper wire are all common. Keep conductor diameters consistent unless you intentionally design for a specific impedance transformation.
Spacing between conductors
Maintain uniform spacing along the full length. Uneven spacing can shift resonance and pattern symmetry. Insulating spacers every 15 to 40 cm are typical for wire builds, depending on wind loading.
Balun and feedline
A folded dipole is a balanced antenna. If you feed with coax (unbalanced), use a balun or current choke at the feedpoint to reduce common-mode current on the coax shield. For a classic ~300 Ω folded dipole into 75 Ω coax, a 4:1 balun is often used. For 50 Ω systems, matching approach depends on your exact measured impedance and bandwidth target.
Tuning workflow that works
- Cut slightly long (1-3% extra length).
- Install at intended height above ground before final trim.
- Measure with an antenna analyzer, not just an SWR meter if possible.
- Trim both sides equally to move resonance upward in frequency.
- Re-check feedpoint choke/balun performance after tuning.
Example (2-meter band)
Suppose you target 146.0 MHz with a velocity factor of 0.95. The calculator returns a practical folded-dipole length a little under 1 meter for each long side, plus a total wire estimate that includes short end sections. This usually puts you close enough that only small trimming is needed.
Important note on accuracy
Any online antenna calculator should be treated as a starting model, not the final answer. Nearby metal, mast coupling, roofline effects, and feedline routing can change resonant frequency and impedance. Always verify with measurements in the real installation environment.