tools / RF & SDR
Antenna Length Calculator
Punch in a frequency and get the physical length for a quarter-wave whip, a half-wave dipole, or a 5/8-wave vertical, in cm, inches and metres. Set the velocity factor to match your wire so the numbers land close to resonance before you even reach for an analyser.
How it works
The maths
Wavelength is the speed of light divided by frequency (λ = c / f). A quarter-wave element is 0.25 λ, a half-wave dipole is 0.5 λ tip to tip (two 0.25 λ legs), and a 5/8-wave vertical is 0.625 λ.
Velocity factor
A real conductor slows the wave slightly, so elements come out about 5% short of the free-space number. 0.95 is a safe start for thin wire. Thicker rod, insulation or nearby metal pull it lower, so cut long and trim to resonance.
FAQ
How long is a quarter-wave antenna for 433 MHz?
Roughly 16.4 cm at a 0.95 velocity factor. Type 433 into the calculator to see it.
What velocity factor should I use?
0.95 for thin wire. Drop toward 0.90 to 0.93 for thick, taped or insulated elements, then trim to resonance.
