Antenna Length Calculator
MHz
0.95
Quarter-Wave (1/4 λ)0.7125 m
Half-Wave Dipole1.4300 m
Full Wavelength (λ)2.8500 m
The Antenna Length Calculator turns an operating frequency into the physical wire lengths you need to build a resonant antenna. It reports the quarter-wave (ground-plane/vertical) length, the practical half-wave dipole length, and the full wavelength. A velocity-factor slider accounts for the wire slowing the wave slightly compared with free space.
Formula
λ = (300 / f)·vf; quarterWave = λ/4; halfWaveDipole = 143 / f
- f
- Operating frequency in megahertz
- vf
- Velocity factor of the conductor (0–1)
- λ
- Wavelength in metres
How it works
- Enter the operating frequency in megahertz and set the velocity factor (typically about 0.95 for bare wire).
- The free-space wavelength is approximated as 300 / f(MHz) metres, then multiplied by the velocity factor; one quarter of that gives the quarter-wave element.
- The half-wave dipole uses the standard practical formula 143 / f(MHz) metres, which already bakes in the end-effect shortening real conductors experience.
Worked example
An FM transmitter at 100 MHz built with bare wire (velocity factor 0.95).
- Wavelength = (300 / 100) × 0.95 = 3 × 0.95 = 2.85 m.
- Quarter-wave = 2.85 / 4 = 0.7125 m.
- Half-wave dipole = 143 / 100 = 1.43 m.
Quarter-wave 0.7125 m, half-wave dipole 1.43 m, full wavelength 2.85 m.
Frequently asked questions
- What is the velocity factor and why does it matter?
- The velocity factor is the ratio of wave speed in the conductor to the speed of light. Real wire is slightly slower than free space, so a value near 0.95 to 0.98 shortens the resonant length a little.
- Why is the dipole formula 143 instead of 150?
- A theoretical half-wave is 150 / f, but the end effect from the physical wire ends makes a real dipole resonate slightly short, so the widely used practical constant is about 143 for bare wire.
- Should I use the quarter-wave or half-wave figure?
- Use the quarter-wave length for a vertical or ground-plane antenna with a counterpoise, and the half-wave dipole length for a center-fed horizontal dipole. They are different antenna types for the same frequency.