Quarter-Wave Antenna Length Calculator
A quarter-wave monopole is one of the most common antenna elements. Its length is a quarter of the wavelength at the operating frequency, scaled by a velocity factor that accounts for end effects on a real conductor. Enter the frequency in megahertz and the velocity factor to find the wavelength, the ideal free-space quarter wave, and the practical physical element length in metres, centimetres, and inches.
Quarter-wave formula
Wavelength lambda = c / f
Physical quarter-wave = (lambda / 4) * velocity factor
c = 299,792,458 m/s (exact), f in Hz
The velocity factor shortens the ideal free-space quarter wave to the practical resonant length on a real conductor.
Worked example
At 146 MHz with a velocity factor of 0.95: lambda = 299,792,458 / 146,000,000 = 2.05 m. Free-space quarter wave = 0.5133 m. Physical length = 0.5133 * 0.95 = 0.49 m, about 48.77 cm or 19.20 inches, typical for a 2-metre band whip.
Quarter-wave antenna: frequently asked questions
How is a quarter-wave antenna length found?
First find the free-space wavelength, lambda = c / f, where c is the speed of light. A quarter wave is lambda / 4. The physical element is shorter than the free-space quarter wave by a velocity factor, so length = (c / f / 4) times velocity factor.
What velocity factor should I use?
A common figure for a thin wire monopole is about 0.95, which is the editable default. The exact value depends on conductor diameter and surroundings. Antenna handbooks often quote 0.95 to 0.98 for thin elements.
What speed of light does this use?
It uses the exact defined speed of light in vacuum, 299,792,458 metres per second, as fixed by the SI definition of the metre.
Why is the real antenna shorter than a quarter wavelength?
End effects and the velocity of the wave along a real conductor make the resonant physical length slightly shorter than the ideal free-space quarter wave. The velocity factor captures this shortening.
Sources
- NIST: speed of light in vacuum (299,792,458 m/s, exact).
- The wavelength and quarter-wave relations are standard results of electromagnetics. The velocity factor is a user-editable estimate.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.