Antenna Length Calculator (Dipole & Monopole)

Half-wave dipole and quarter-wave monopole length

A resonant antenna's length is set by the free-space wavelength at the operating frequency: λ = c / f, with c = 299,792,458 m/s. The classic half-wave dipole — two straight legs fed at the center — is resonant when its total length is one half wavelength, split evenly between the two legs. A quarter-wave monopole (a ground-plane or vertical antenna working against a ground plane or radials) is resonant at one quarter wavelength, because the ground plane acts as a mirror and forms the "missing" second half of the equivalent dipole.

Real wire isn't infinitely thin, so current doesn't reach exactly zero at the physical ends — extra capacitance there makes the antenna electrically longer than its physical length for a given resonance, meaning the actual metal must be cut a bit shorter than the ideal free-space figure. This is captured by the end-effect/velocity factor k: L_dipole = k × λ/2 and L_mono = k × λ/4. For thin wire k ≈ 0.95–0.98 (the familiar ham-radio rule "468/f(MHz)" for a dipole in feet is exactly this formula with k ≈ 0.952); thicker elements like aluminum tubing shorten further, down toward k ≈ 0.90–0.95.

Treat the result as a solid cut-to-length starting point, then trim: real-world resonance also shifts with nearby objects, antenna height above ground, and feedline effects, so builders typically cut a few percent long and trim for lowest SWR at the target frequency rather than relying on the formula alone.