📡️ Whip Antenna Length Calculator

A whip antenna is the familiar flexible vertical radiator used on walkie-talkies, car radios, routers and base stations. Its length is set by the operating frequency: for a quarter-wave monopole the element is one quarter of the free-space wavelength, L = 0.25 × λ, which for most mobile bands lands in a convenient physical size. This calculator returns the theoretical length for the quarter-wave, half-wave and 5/8-wave configurations, and applies a velocity factor so the tuned length accounts for the real electrical length of the conductor.

The classic shop shortcut is the meters formula L = 142.65 / f(MHz) for a half-wave dipole, or L = 71.3 / f(MHz) for a quarter-wave monopole, where the extra ~2% accounts for end effects. The 5/8-wave vertical is a favorite on 2 m and 70 cm because it concentrates radiation toward the horizon and adds 1–2 dB over a quarter-wave on a good ground.

The calculator first converts frequency to free-space wavelength λ = c/f in meters. Each radiator fraction (0.25, 0.5, 0.625) is multiplied by λ and by the velocity factor, then converted to centimetres, inches and feet. The velocity factor for a typical wire or stainless whip is around 0.95; a coil-loaded or dielectric-jacketed element may need 0.85–0.92.

The tuned length with velocity factor is the practical cutting length — start slightly long and trim, because any connector, coil or nearby metal shifts resonance. For a full link budget, combine the result with the Friis path loss calculator or check the frequency-wavelength converter.

1. Enter the operating frequency in MHz.
2. Choose the radiator fraction: quarter-wave, half-wave or 5/8-wave.
3. Set the velocity factor (0.95 for bare wire, lower for jacketed elements).
4. Click Calculate to see the length in cm, inches and feet.
5. Cut the element slightly long, then trim to the tuned SWR.

Why is a quarter-wave whip 71.3/f rather than 75/f meters?

The 71.3 constant is a practical tuning figure that folds in the end effect — the electric field fringes off the tip, so a physically shorter element resonates at the intended frequency. Using the plain 0.25λ formula leaves the antenna slightly too long.

What velocity factor should I use?

For a bare wire or metal whip, 0.95 is the standard starting point. A fiberglass or shrink-jacketed element behaves more like 0.90–0.92. When in doubt, use 0.95 and trim.

Why does my handheld still need the whip replaced by a coil-loaded one?

A coil-loaded whip keeps the same electrical quarter-wave length in a shorter physical space by adding inductance. The calculator gives the electrical length; the physical length depends on the coil, so use the coil manufacturer's data for the metal length.

What is the advantage of 5/8-wave over quarter-wave?

A 5/8-wave vertical lifts the radiation pattern toward the horizon and, on a good ground system, gains 1–2 dB compared with a quarter-wave. The trade-off is a slightly narrower SWR bandwidth and a need for proper counterpoise.

Does the antenna length change if I mount it on a car roof?

The vehicle body acts as the ground plane and becomes part of the radiating system. In practice the roof lowers the resonant frequency slightly, so base-loading or a slightly longer whip is often needed. Tune on the actual vehicle, not on the bench.

Can I use the same whip for both 2 m and 70 cm?

Dual-band whips use a trap or a collinear design so one physical element resonates on two bands. The single-length calculator targets one band; for dual-band operation use the 1/2-wave-on-2m / 5/8-wave-on-70cm compromise lengths published by dual-band antenna makers.

Always cut the whip a little long — about 3–5% — and trim in small steps while watching an SWR meter; you can only remove metal, not add it. The velocity factor of a bare wire is close to 0.95, but shrink tube, paint or a fiberglass jacket lowers it. A quarter-wave monopole needs a ground plane (vehicle roof, radial kit, or counterpoise wires) to work well; without one the antenna becomes part of the feed line and the SWR wanders. For repeaters and base stations the 5/8-wave vertical gives a useful horizon gain when the ground system is good. Do not confuse electrical length with physical length on coil-loaded whips — the loading coil replaces most of the missing conductor.