Antenna Length Calculator — quarter wave, half wave and dipole
Quarter-wave, half-wave and dipole lengths for any frequency, with presets for 433, 868, 915 and 2400 MHz and the ham bands.
Example: At 433 MHz the wavelength is 692.4 mm, so a quarter-wave monopole is 164.4 mm (164.4 mm).
Formula
λ = c / f length = λ × fraction × velocity factorWorked example
At 433 MHz the wavelength is 692.4 mm, so a quarter-wave monopole is 164.4 mm (164.4 mm).
λ = c / f
λ = 299.8 Mm/s / 433 MHz
λ = 692.4 mm
length = λ × 0.25 × velocity factor
length = 692.4 mm × 0.25 × 0.95
length = 164.4 mm
Frequently asked questions
How long should a 433 MHz antenna be?
A quarter-wave element is about 165 mm allowing for velocity factor — free-space quarter wavelength is 173 mm, and a real wire is a few percent shorter. It also needs a ground plane; a quarter-wave whip on a small board without one will underperform badly.
What is velocity factor and why does it shorten the antenna?
Radio waves travel slightly slower along a conductor than through free space, so a physically shorter element is resonant at the same frequency. Thin wire is around 0.95; thicker elements need more shortening. Coax is much lower — RG-58 is about 0.66.
Does a quarter-wave antenna need a ground plane?
Yes. A quarter-wave monopole works against its ground plane, which mirrors the element to make the electrical equivalent of a half-wave dipole. Without an adequate plane the pattern and impedance are both wrong. If you cannot provide one, use a dipole instead.
Should I cut the antenna to exactly this length?
Cut slightly long and trim. Nearby metal, the enclosure, the ground plane and the feed all shift resonance, so the calculated figure is a starting point. Trim a few millimetres at a time while watching SWR or return loss.
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