Antenna Length & Wavelength Calculator
Compute the wavelength and the physical length of a dipole / monopole / loop antenna from frequency, with velocity factor.
Physical length includes a velocity factor (~0.95 for thin wire) — real antennas are slightly shorter than the free-space wavelength. Build a bit long, then trim to tune SWR.
Disclaimer: This calculator is provided for general informational and educational purposes only, on an “as is” basis and without any warranty of accuracy or fitness for a particular purpose. Results may contain errors — always verify independently before relying on them in real designs. PartAndStock accepts no liability for any loss or damage arising from use of this tool, including when embedded on third-party sites.
How to use
- 1Enter the antenna's operating frequency in MHz.
- 2Pick the antenna type: half-wave dipole, quarter-wave monopole or full-wave loop.
- 3Enter the velocity factor (~0.95 for thin wire); wavelength and physical length are computed instantly.
How it works
Enter the operating frequency to instantly find the free-space wavelength and the practical physical length for a half-wave dipole, quarter-wave monopole or full-wave loop antenna.
How wavelength and antenna length are found
A radio wave's free-space wavelength is the speed of light divided by frequency: λ = c/f, in practice λ(m) ≈ 300/f(MHz). Antennas work best at fractions of that wavelength: a half-wave dipole is two legs totaling λ/2, a quarter-wave monopole is a single element of λ/4 (with a ground plane). For resonance the antenna length must match the wavelength; when it does, the antenna radiates energy efficiently.
Velocity factor and SWR tuning
A wave travels slightly slower inside a conductor than in free space, so a physical antenna is shorter than the theoretical wavelength. For thin bare wire the velocity factor k ≈ 0.95; it's lower for insulated or thick conductors. The right method: build the antenna a bit long, then measure with an SWR meter and trim the ends to minimum SWR. Nearby metal, ground and mounting height also shift resonance.
Worked examples
- f=100 MHz (FM) → λ = 3.0 m; half-wave dipole ≈ 1.43 m (each leg ~71 cm)
- f=433 MHz → quarter-wave monopole ≈ 16 cm
- f=2400 MHz (WiFi) → quarter-wave ≈ 3.0 cm
Antenna Lengths at Common Frequencies (k=0.95)
| Frequency | λ | ½-wave dipole | ¼-wave |
|---|---|---|---|
| 100 MHz (FM) | 3.00 m | 1.43 m | 71 cm |
| 145 MHz (2 m) | 2.07 m | 98 cm | 49 cm |
| 433 MHz | 69 cm | 33 cm | 16 cm |
| 915 MHz | 33 cm | 16 cm | 7.8 cm |
| 2.4 GHz (WiFi) | 12.5 cm | 5.9 cm | 3.0 cm |