From frequency to antenna length
An antenna resonates when its length is a specific fraction of the wavelength. Start from the free-space wavelength:
λ = c / f (c = speed of light, 299,792,458 m/s)
then take the fraction you need — half for a dipole, a quarter for a ground-plane whip — and shorten it slightly for real-world effects. At 146 MHz, λ ≈ 2.05 m, so a half-wave dipole is about 0.97 m tip to tip and a quarter-wave whip about 0.49 m.
Why "shortened"
A real wire antenna resonates a few percent shorter than the ideal math, because the electric field fringes at the ends (the "end effect"). A factor around 0.95 for thin-wire dipoles captures it; fatter elements shorten more. Insulated wire and elements inside plastic shorten further still, toward the material's velocity factor. Adjust the factor field to match your build.
Common bands
| Band | Freq | ¼-wave (×0.95) |
|---|---|---|
| CB | 27 MHz | 2.64 m |
| 10 m ham | 28.5 MHz | 2.50 m |
| 2 m ham | 146 MHz | 0.49 m |
| 70 cm ham | 435 MHz | 0.16 m |
| LoRa | 915 MHz | 78 mm |
| GPS L1 | 1575 MHz | 45 mm |
| Wi-Fi | 2450 MHz | 29 mm |
Frequently asked questions
- Dipole or monopole — which length?
- A half-wave dipole (two quarter-wave legs) needs no ground plane. A quarter-wave monopole/whip needs a ground plane or radials to act as the other half. Both are in the table above.
- What velocity factor should I use?
- ~0.95 for a bare thin-wire dipole in air. For elements made of coax or inside dielectric, use the material's velocity factor (~0.66 for solid-PE coax, ~0.8 for foam). Thicker conductors resonate a bit shorter too.
- Does length need to be exact?
- Get close, then tune. A few percent off shifts resonance and raises SWR; antennas are usually cut slightly long and trimmed while measuring.
- Where does the wavelength come from?
- The same λ = c/f used by the frequency & period calculator, which also gives the period and angular frequency.