An antenna radiates best when its physical size matches the wavelength of the signal it carries. The starting point is always the wavelength λ = c / f, the distance the wave travels in one cycle. A quarter-wave (λ/4) element over a ground plane and a half-wave (λ/2) dipole are the two most common resonant antennas. Real elements are cut about 5% shorter than the free-space figure because of end effects, captured by a velocity (shortening) factor of roughly 0.95 for thin wire.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: Wikipedia: Dipole antenna and ARRL antenna references.
The antenna-length formulas
A handy shortcut in metres is λ ≈ 300 / f(MHz), so a quarter-wave is roughly 71.25 / f(MHz) metres after the 0.95 factor. The dipole is fed in the centre as two quarter-wave legs; the monopole is a single quarter-wave leg that uses a ground plane as its mirror image.
Worked example — a 433 MHz remote antenna
Scenario: A 433 MHz ISM-band module needs a simple quarter-wave whip.
So a 16.5 cm wire makes a resonant quarter-wave antenna at 433 MHz — the classic length you see on key-fob and sensor modules. A half-wave dipole would be 33 cm total (two 16.5 cm legs). Always trim slightly and check the SWR for the best match in the real installation.
Frequently Asked Questions
λ = c ÷ f = 300 ÷ f(MHz) metres. Quarter-wave = λ/4 and half-wave dipole = λ/2, each × ~0.95. At 100 MHz: λ = 3 m, quarter-wave ≈ 0.71 m, dipole ≈ 1.42 m.
Real antennas are slightly shorter than free space due to end effects (wire diameter and capacitance). A ~0.95 factor (the 95% rule) gives the physical length; insulated or thick elements use a bit less.
A quarter-wave monopole is one λ/4 element over a ground plane (which mirrors it). A half-wave dipole is two λ/4 legs centre-fed, λ/2 total, with no ground plane. The monopole is half the height.
At 2.4 GHz, λ ≈ 12.5 cm, so a quarter-wave ≈ 3.0 cm and a half-wave dipole ≈ 5.9 cm — which is why Wi-Fi whips are only a few cm long.
Yes — length is inversely proportional to frequency. Doubling the frequency halves the antenna. A 1 MHz quarter-wave is ~71 m; a 5 GHz one is ~1.4 cm.