Dipole Antenna Length Calculator (Half-Wave, Quarter-Wave, Loop)

How it works

λ (m) = 299.79 ÷ f (MHz) · half-wave dipole = λ ÷ 2 × k (k ≈ 0.95 → 142.4 ÷ f metres, 468 ÷ f feet) · each leg = half · quarter-wave = λ ÷ 4 × k · full-wave loop ≈ 1.02 λ · inverted V ≈ 4% shorter

A dipole resonates when each leg is about a quarter of a wavelength, so the whole antenna is half a wavelength long, and the wavelength is the speed of light divided by the frequency. A real wire is slightly shorter than the free-space figure because capacitance at its ends and the finite thickness of the conductor slow the wave along it — the "end effect" — which is why the practical formula uses about 95% of a half wave, giving the 468-over-frequency rule in feet that generations of operators have memorised. Bending the legs down into an inverted V, running insulated wire or using fat tubing lowers the resonant frequency further and calls for another few percent off, and since trimming is easy and splicing is not, the rule is always to cut long and tune by measurement.

Worked example

For 7.1 MHz the wavelength is 42.2 m, so a flat-top wire dipole is 42.2 ÷ 2 × 0.95 = 20.06 m long (65′ 10″), two legs of 10.03 m (32′ 11″). As an inverted V it comes down to about 19.3 m. Cut the legs at 10.3 m, hang it, and trim to the lowest SWR; a quarter-wave vertical for the same band is 10.03 m tall.

Frequently asked questions