An antenna length calculator that gives you the exact physical wire or element length for any common antenna type — half-wave dipole, quarter-wave vertical, full-wave loop, 5/8-wave whip, Yagi driven element, or a fully custom fraction — and works in reverse too, finding the resonant frequency for a length of wire you already have. Everything runs locally in your browser; no data is sent anywhere.
How it works
Every antenna is a resonator. A radiating element resonates when its physical length corresponds to a specific fraction of the radio wavelength at the operating frequency. The free-space wavelength for any frequency is:
λ = c ÷ f
where c = 299,792,458 m/s (speed of light) and f is the frequency in Hz. The physical length of the antenna element is then:
L = k × VF × λ
or equivalently:
L = (k × VF × c) ÷ f
Here k is the wavelength fraction (0.5 for a half-wave dipole, 0.25 for a quarter-wave vertical, 1.0 for a full-wave loop, 0.625 for a 5/8-wave element), and VF is the velocity factor — the ratio of propagation speed along the physical conductor to the speed of light in free space. For bare copper or aluminium wire the VF is typically 0.95–0.97; for coaxial stubs or heavily loaded elements it can be 0.66 or lower.
To solve for frequency instead: f = (k × VF × c) ÷ L.
The calculator applies a type-appropriate default VF for each antenna family and lets you override both VF and k freely in the “Custom” mode, covering anything from a loading-coil-shortened mobile whip to a multi-element Yagi reflector.
Worked example — 2 m half-wave dipole at 145 MHz
A 145 MHz half-wave dipole with VF = 0.95:
- Free-space wavelength: λ = 299,792,458 ÷ (145 × 10⁶) = 2.067 m
- Physical total length: L = 0.5 × 0.95 × 2.067 = 0.982 m (98.2 cm)
- Each arm: 0.982 ÷ 2 = 49.1 cm
The classic shorthand — L (feet) = 468 ÷ f(MHz) = 468 ÷ 145 = 3.228 ft = 0.983 m — agrees to within 0.1%, confirming the 468 constant already encodes k = 0.5 and VF ≈ 0.95 in imperial units.
| Band | Frequency | Antenna type | Physical length |
|---|---|---|---|
| AM broadcast | 1 MHz | Half-wave dipole | 142.3 m |
| 40 m amateur | 7.1 MHz | Half-wave dipole | 20.1 m |
| 2 m amateur | 145 MHz | Half-wave dipole | 0.98 m |
| 70 cm amateur | 435 MHz | Quarter-wave vertical | 16.3 cm |
| Wi-Fi 2.4 GHz | 2,437 MHz | Quarter-wave monopole | 2.9 cm |
| 5G mid-band | 3,500 MHz | Quarter-wave monopole | 2.0 cm |
Formula note
The formula L = k × VF × c ÷ f is exact from Maxwell’s equations for a lossless conductor with a fixed velocity factor. Real antennas deviate slightly due to element diameter (thick elements electrically shorten the resonant length), nearby conductors, height above ground, and end effects. In practice, builders add 2–3% extra length and trim down to the SWR minimum — a process called pruning. The calculator gives the theoretical starting length; expect to trim 1–5 cm on VHF antennas and 0.5–1 m on HF wire dipoles.