Antenna Length Calculator
Enter a frequency and a velocity factor to get the half-wave dipole length in metres — plus the quarter-wave length — straight from the free-space wavelength.
Dipole and quarter-wave at once
Enter the frequency and velocity factor and the calculator returns the half-wave dipole length and the quarter-wave length in metres together.
Cut a little long
The result is a precise starting point — leave a few extra centimetres, then trim to resonance with an SWR meter once the antenna is in place.
What is an antenna length calculator?
From frequency to wire length
An antenna length calculator turns a radio frequency into the physical length of wire you need to cut. It starts from the free-space wavelength — the distance one full wave travels — and then works out the half-wave dipole, the most common wire antenna, and the quarter-wave length used for verticals and ground-plane antennas. You give it the operating frequency in megahertz and a velocity factor (around 0.95 for bare wire), and it returns both lengths in metres. It is the number behind every home-brewed dipole, the starting point for a 20-metre band antenna, and the quickest way to size an element before you reach for the wire cutters.
Enter a frequency in MHz and a velocity factor to get the half-wave dipole length and the quarter-wave length in metres instantly.
The free-space wavelength is the speed of light divided by the frequency; the half-wave dipole is that wavelength times the velocity factor, halved.
L = (c / f) × VF / 2The speed of light is about 299.792458 metres per microsecond, so dividing it by a frequency in MHz gives the wavelength directly in metres. Multiplying by the velocity factor accounts for the wave travelling a little slower in real wire, and halving gives the dipole. The quarter-wave length is simply half of that again.
Suppose you want a dipole for the 20-metre band at 14.2 MHz with a velocity factor of 0.95.
Find the wavelength
299.792458 ÷ 14.2 = 21.1121 m — the free-space wavelength.
Apply the velocity factor and halve
21.1121 × 0.95 ÷ 2 = 10.0283 m — the half-wave dipole length.
Halve again for a quarter wave
10.0283 ÷ 2 = 5.0141 m — each leg, or a quarter-wave vertical.
The two outputs answer two practical questions. The half-wave dipole length (10.03 m for the example above) is the total wire you cut for a centre-fed dipole — the whole antenna, both legs together. The quarter-wave length (5.01 m) is half of that: it is the length of each individual leg of the dipole, and it is also the length of a quarter-wave vertical or ground-plane element, which uses the earth or a set of radials to stand in for the missing half. Higher frequencies give shorter antennas, because length is inversely proportional to frequency — double the frequency and the antenna halves. The velocity factor nudges everything a few percent shorter than the ideal free-space figure, which is why a real dipole never measures exactly half a wavelength. Treat the numbers as a tuned starting point: they get you within a few centimetres, and a final trim to resonance does the rest.
The formula is exact, but a real installation always needs a little adjustment.
An ideal starting point, not the final cut
This calculator gives the ideal length for a thin wire in free space, scaled by the velocity factor. Real antennas are affected by height above ground, nearby metal and buildings, wire diameter, and feed-line effects, all of which shift the resonant length slightly. Cut the wire a few centimetres long, then measure the SWR and trim to resonance. The model also assumes a simple half-wave dipole and does not cover loaded, folded, or multi-band designs.