Antenna Length Calculator
Calculate antenna lengths for dipole, quarter-wave, and other types based on frequency.
Covers HF, VHF, UHF, and WiFi.
Antenna Length is determined by the operating frequency. Antennas are typically sized as fractions of the wavelength of the signal.
Wavelength Formula:
λ = c / f
Where:
- λ (lambda) = Wavelength
- c = Speed of light = 299,792,458 m/s (≈ 300,000 km/s)
- f = Frequency in Hz
Common Antenna Types and Lengths:
| Type | Length | Formula (meters) | Use |
|---|---|---|---|
| Full wave | 1λ | 300 / f(MHz) | Loop antennas |
| Half-wave dipole | ½λ | 150 / f(MHz) | Most common HF antenna |
| Quarter-wave vertical | ¼λ | 75 / f(MHz) | Ground-plane, mobile |
| 5/8 wave | ⅝λ | 187.5 / f(MHz) | VHF/UHF mobile, higher gain |
Velocity Factor: Real antennas are slightly shorter than the theoretical calculation due to end effects and wire thickness. A typical correction factor is 0.95 (multiply theoretical length by 0.95).
Practical length = Theoretical length × 0.95
Common Frequencies and Antenna Lengths:
| Application | Frequency | Half-wave Dipole |
|---|---|---|
| AM Radio (center) | 1 MHz | 150 m (492 ft) |
| 40m Ham Band | 7.15 MHz | 19.9 m (65.4 ft) |
| 20m Ham Band | 14.175 MHz | 10.1 m (33.0 ft) |
| FM Radio (center) | 100 MHz | 1.43 m (4.7 ft) |
| 2m Ham Band | 146 MHz | 0.97 m (3.2 ft) |
| WiFi 2.4 GHz | 2,450 MHz | 5.8 cm (2.3 in) |
| WiFi 5 GHz | 5,800 MHz | 2.5 cm (1.0 in) |
Practical Example: For a 2m ham radio half-wave dipole at 146 MHz: Theoretical: 150 / 146 = 1.027 m. With velocity factor: 1.027 × 0.95 = 0.976 m (38.4 inches) total, or 0.488 m (19.2 in) per side.
Tips:
- A dipole antenna has two equal halves. Each side is a quarter wavelength.
- Mount dipoles at least a quarter wavelength above ground for best performance.
- Thicker wire has a broader bandwidth but slightly different velocity factor.
- For indoor antennas, nearby metal objects affect the resonant length. Trim for lowest SWR.
- A simple antenna analyzer or SWR meter helps fine-tune length after installation.