Antenna Length Calculator

Cut an antenna too long or too short and it won't resonate on your target frequency — SWR will spike, transmitter output will drop, and your signal will suffer. The calculator for antenna length finds the exact physical length for a resonant dipole, quarter-wave vertical, or full-wave loop at any operating frequency, factoring in the velocity factor that accounts for the wave traveling slightly slower in real conductors than in free space.

The Fundamental Antenna Length Formulas

All resonant antenna lengths derive from the relationship between wavelength λ and frequency f:

λ = c / f — where c = 299,792,458 m/s (speed of light in free space)

For wire antennas, the physical length is shortened by the velocity factor (VF) — typically 0.95–0.98 for bare copper wire and 0.66–0.80 for insulated wire:

• Half-wave dipole: L = 0.5 × λ × VF ≈ 142.5/f(MHz) meters (for VF = 0.95)
• Quarter-wave vertical: L = 0.25 × λ × VF ≈ 71.25/f(MHz) meters
• Full-wave loop: L = 1.0 × λ × VF ≈ 285/f(MHz) meters

The classic shortcut formula — 468/f(MHz) feet for a half-wave dipole in feet — embeds VF ≈ 0.95 and unit conversion. Use this online calculator for any antenna type, frequency, and velocity factor. The frequency-wavelength converter provides the λ calculation as a standalone tool.

Velocity Factor: Why Antennas Are Shorter Than a Free-Space Half-Wavelength

Electromagnetic waves travel at exactly c in vacuum, but in real conductors and dielectric materials they travel slower — a fraction of c known as the velocity factor. This slowing shortens the physical antenna length required to achieve electrical resonance. Typical velocity factors:

• Bare copper wire in free space: 0.95–0.97 (end effects reduce electrical length)
• PVC-insulated wire: 0.66–0.80 depending on insulation thickness and dielectric constant
• Coaxial cable (RG-58): 0.66 — relevant for coaxially fed antennas
• Printed circuit board (FR4): 0.55–0.65 — critical for PCB antenna design in IoT devices

The quarter-wave stub, commonly used for impedance matching and RF grounding, must account for VF to achieve the correct electrical length at the target frequency. The dipole antenna calculator provides the complete dipole design including feedpoint impedance.

Amateur Radio Bands: Quick Reference Lengths

For the most common amateur radio HF bands, approximate half-wave dipole lengths (bare wire, VF = 0.95):

• 80 m band (3.5–4.0 MHz): ≈ 40–41 m (131–135 ft)
• 40 m band (7.0–7.3 MHz): ≈ 20–21 m (66–68 ft)
• 20 m band (14.0–14.35 MHz): ≈ 10.2–10.5 m (33–34 ft)
• 10 m band (28.0–29.7 MHz): ≈ 5.0–5.3 m (16–17 ft)
• 2 m band (144–148 MHz): ≈ 0.97–1.0 m (38–39 inches)

The quarter-wave antenna calculator and antenna and RF calculators category cover the full range of antenna design and RF engineering tools.

End Effect and Practical Length Adjustment

Real-world antennas must be trimmed to final resonance after construction because the theoretical length rarely produces exactly 1:1 SWR at the target frequency. End effects from wire termination, surrounding objects, antenna height, and ground conductivity all shift the resonant frequency slightly. The standard practice is to cut the antenna 2–5% longer than the calculated length, then trim 1–2 cm at a time while monitoring SWR with an analyzer until minimum SWR is achieved at the target frequency. This iterative physical trimming is the final step in all wire antenna construction.