Antenna Length Calculator
Antenna dimensions are set by the wavelength of the signal, which depends on the operating frequency and the speed of light. A half-wave dipole, a quarter-wave vertical, and a full-wave loop are all sized as fractions of one wavelength, with a small shortening factor applied to allow for the end effect on real conductors. Enter the operating frequency and a shortening factor; this calculator returns the free-space wavelength and the practical lengths for quarter-wave, half-wave, and full-wave antennas in metres.
Antenna length formula
c = 299,792,458 m/s
Wavelength = c / frequency (Hz)
Half-wave dipole = wavelength * 0.5 * factor
Quarter-wave = wavelength * 0.25 * factor
Full-wave = wavelength * factor
Frequency is entered in megahertz and converted to hertz inside the calculator. The shortening factor accounts for the end effect that makes resonant elements slightly shorter than their free-space length.
Antenna sizing context
- Wavelength equals the speed of light divided by frequency; higher frequencies have shorter antennas.
- The speed of light in vacuum is the exact SI value 299,792,458 metres per second.
- A typical shortening factor is around 0.95 to 0.98 for thin wire dipoles; thicker elements need a smaller value.
- A quarter-wave vertical relies on a ground plane to act as the missing half of the radiator.
- Verify and tune the final length with an analyser, since nearby objects and mounting affect resonance.
Antenna length: frequently asked questions
How is wavelength related to frequency?
Wavelength equals the speed of light divided by frequency. In free space the speed of light is exactly 299,792,458 metres per second, so a 100 MHz signal has a wavelength of about 3 metres. Antenna dimensions are set as fractions of this wavelength.
What is a half-wave dipole length?
A half-wave dipole is one half of a wavelength long in theory. In practice the physical length is slightly shorter because of the end effect, so a velocity or shortening factor near 0.95 is applied. This factor is a user-editable input so you can match your conductor and construction.
What is a quarter-wave antenna?
A quarter-wave antenna, such as a vertical monopole over a ground plane, is one quarter of a wavelength. Its length is half that of a half-wave dipole and it relies on the ground plane to act as the missing half of the radiator.
Why apply a shortening factor?
Real conductors have a finite diameter and an end effect that makes the resonant length slightly shorter than the ideal value in free space. A factor around 0.95 to 0.98 accounts for this. Thicker elements need a smaller factor, so the value is left editable.
Does this depend on the conductor material?
The dominant factor is the geometry and diameter rather than the metal itself. The speed of light and frequency set the wavelength, and the shortening factor captures the end effect. Enter a factor that matches your element thickness and mounting for best accuracy.
Official sources
- NIST Fundamental Physical Constants: speed of light in vacuum.
- NIST: SI units, the metre and hertz.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.